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1 1 Engineering Plant Secondary Metabolism in Microbial Systems 2 3 Michael E. Pyne * , Lauren Narcross * , Vincent J.J. Martin *,1 4 5 6 * Department of Biology, Centre of Applied Synthetic Biology, Centre for Structural and 7 Functional Genomics, Concordia University, Montréal, Québec, Canada 8 1 Corresponding author: E-mail address: [email protected] 9 10 Short title: Producing plant secondary metabolites in microbes 11 12 One-sentence summary: An overview of common challenges and 13 strategies underlying efforts to reconstruct plant isoprenoid, alkaloid, 14 phenylpropanoid, and polyketide biosynthetic pathways in microbial 15 systems. 16 17 Author Contributions: M.E.P. and L.N. prepared the manuscript. 18 V.J.J.M supervised and edited the manuscript. All authors participated 19 in discussion and revision of the manuscript. 20 Plant Physiology Preview. Published on January 14, 2019, as DOI:10.1104/pp.18.01291 Copyright 2019 by the American Society of Plant Biologists www.plantphysiol.org on March 21, 2020 - Published by Downloaded from Copyright © 2019 American Society of Plant Biologists. All rights reserved.

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Page 1: Engineering Plant Secondary Metabolism in …...108 alkaloids, giving rise to more than 3,000 structures. The tryptophan precursor anthranilate 109 also serves as a precursor to several

1

1

Engineering Plant Secondary Metabolism in Microbial Systems 2

3

Michael E. Pyne*, Lauren Narcross

*, Vincent J.J. Martin

*,1 4

5

6

*Department of Biology, Centre of Applied Synthetic Biology, Centre for Structural and 7

Functional Genomics, Concordia University, Montréal, Québec, Canada 8

1 Corresponding author: E-mail address: [email protected] 9

10

Short title: Producing plant secondary metabolites in microbes 11

12

One-sentence summary: An overview of common challenges and 13

strategies underlying efforts to reconstruct plant isoprenoid, alkaloid, 14

phenylpropanoid, and polyketide biosynthetic pathways in microbial 15

systems. 16

17

Author Contributions: M.E.P. and L.N. prepared the manuscript. 18

V.J.J.M supervised and edited the manuscript. All authors participated 19

in discussion and revision of the manuscript. 20

Plant Physiology Preview. Published on January 14, 2019, as DOI:10.1104/pp.18.01291

Copyright 2019 by the American Society of Plant Biologists

www.plantphysiol.orgon March 21, 2020 - Published by Downloaded from Copyright © 2019 American Society of Plant Biologists. All rights reserved.

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Introduction 21

Secondary metabolites are broadly defined as natural products synthesized by an 22

organism that are not essential to support growth and life. The plant kingdom 23

manufactures over 200,000 distinct chemical compounds, most of which arise from 24

specialized metabolism. While these compounds play important roles in interspecies 25

competition and defense, many plant natural products have been exploited for use as 26

medicines, fragrances, flavors, nutrients, repellants, and colorants. 27

Despite this vast chemical diversity, many secondary metabolites are present at 28

very low concentrations in planta, eliminating crop-based manufacturing as a means of 29

attaining these important products. The structural and stereochemical complexity of 30

specialized metabolites hinders most attempts to access these compounds using chemical 31

synthesis. Although native plants can be engineered to accumulate target pathway 32

metabolites (Zhou et al., 2009; Glenn et al., 2013; Lange and Ahkami, 2013; Wilson and 33

Roberts, 2014; Tatsis and O’Connor, 2016), metabolic engineering is technically more 34

challenging in plants than in microbes. 35

Advancements in synthetic biology have stimulated the synthesis of valuable 36

natural products in tractable laboratory microbes by interfacing plant secondary pathways 37

with core host metabolism. Microbial synthesis overcomes many of the obstacles 38

hindering traditional chemical synthesis and plant metabolic engineering, thus providing 39

an alternative avenue for exploring plant specialized pathways. 40

This Update provides a brief overview of engineering plant secondary metabolism 41

in microbial systems. We briefly outline biosynthetic pathways mediating formation of 42

the major classes of natural products with an emphasis on high-value terpenoids, 43

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alkaloids, phenylpropanoids, and polyketides. We also highlight common themes, 44

strategies, and challenges underlying efforts to reconstruct and engineer these pathways 45

in microbial hosts. We focus chiefly on de novo biosynthetic approaches in which plant 46

specialized metabolites are synthesized directly from sugar feedstocks rather than 47

supplemented precursors or intermediates. Readers are directed to a selection of 48

pioneering supplementation studies within the context of microbially-sourced plant 49

natural products (Becker et al., 2003; Kaneko et al., 2003; Yan et al., 2005; Watts et al., 50

2006; Leonard et al., 2007; Hawkins and Smolke, 2008; Leonard et al., 2008; Fossati et 51

al., 2014; Fossati et al., 2015). 52

53

OVERVIEW OF KEY PLANT SECONDARY METABOLIC PATHWAYS 54

Terpenoids 55

Terpenoids (also called isoprenoids) are the largest class of plant secondary metabolites, 56

comprising more than 50,000 natural products (Connolly and Hill, 1991). Whereas the 57

terpene classification refers strictly to hydrocarbons, terpenoids possess a range of 58

chemical functionalities. The central precursors geranyl pyrophosphate (GPP; C10), 59

farnesyl pyrophosphate (FPP; C15), and geranylgeranylpyrophosphate (GGPP; C20) form 60

the structural basis of most higher-order terpenoids (Figure 1). Plant terpene synthases 61

convert these pyrophosphate intermediates to terpenes, which are then functionalized in 62

downstream reactions. 63

GPP forms the backbone of most monoterpenoids (C10), including linear (geraniol 64

and linalool) and cyclic (camphor and eucalyptol) terpenoids, as well as monoterpenes 65

(limonene and pinene). Geraniol can be modified to the pest repellant citronellol, while 66

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limonene gives rise to menthol, a flavoring agent and decongestant. GPP also supplies the 67

prenyl group in the biosynthesis of cannabinoids (Ahmed et al., 2015; Vickery et al., 68

2016), a class of natural products with promising pharmaceutical properties (Aizpurua-69

Olaizola et al., 2016). Sesquiterpenes (C15) are derived from FPP, itself generated through 70

condensation of GPP and isopentenyl pyrophosphate (IPP) in yeast or directly from IPP 71

and dimethylallyl pyrophosphate (DMAPP) units in plants. The sesquiterpene 72

amorphadiene gives rise to the antimalarial drug precursor artemisinic acid. The addition 73

of another IPP unit to FPP in yeast gives rise to GGPP, which forms the scaffold of the 74

diterpenes and diterpenoids (C20), such as taxadiene, a precursor to the taxane family of 75

chemotherapeutics. Condensation of two FPP units yields the linear triterpene squalene 76

(C30), which serves as the universal building block of all sterols, including ergosterol in 77

yeast. In plants, squalene gives rise to a number of pharmacologically active 78

triterpenoids, such as β-amyrin and lupeol. Tetraterpenes (C40) are produced through 79

condensation of two molecules of GGPP, yielding phytoene, the precursor to the 80

carotenoids, such as lycopene and β-carotene. Polyterpenes such as natural rubber (cis-81

polyisoprene) comprise thousands of isoprene units. 82

83

Alkaloids 84

In the broadest sense, alkaloids are defined as low-molecular-weight metabolites 85

containing heterocyclic (true alkaloids) or exocyclic (protoalkaloids, amines, and 86

polyamines) nitrogen atoms. Approximately 20,000 natural alkaloids are known, many of 87

which exhibit analgesic (morphine), stimulant (caffeine and ephedrine), psychotropic 88

(mescaline and cocaine), antibacterial (sanguinarine), anticancer (vinblastine and 89

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vincristine), antitussive (codeine), anti-inflammatory (berberine), antispasmodic 90

(papaverine), or antimalarial (quinine) activities. 91

Phenylalanine, tyrosine, and their derivatives (e.g., phenethylamine, tyramine, and 92

dopamine) are the source of a tremendous number of alkaloids, including the 93

benzylisoquinoline alkaloids (BIAs) and the phenethylisoquinoline alkaloids (Figure 2). 94

BIAs are a large class of roughly 2,500 metabolites that includes berberine, noscapine, 95

sanguinarine, and morphine. These important medicines are derived from the 96

condensation of dopamine and 4-hydroxyphenylacetaldehyde (4-HPAA), both of which 97

are synthesized from tyrosine. Dopamine can also condense with derivatives of 98

phenylalanine (4-hydroxydihydrocinnamaldehyde), yielding the phenethylisoquinoline 99

class of alkaloids. Phenylalanine and tyrosine form the basis of a number of simpler 100

protoalkaloids and catecholamines, including modified amphetamines (ephedrine and 101

cathinone), dopamine, mescaline, and adrenaline. Owing to the ubiquity of the indole 102

group in nature, tryptophan also forms the basis of many important alkaloids, such as 103

simple indoles, β-carbolines (serotonin and harmine), and the monoterpenoid indole 104

alkaloids (MIAs). 105

MIAs, derived from the condensation of the tryptophan analog tryptamine and a 106

monoterpenoid (secologanin), are one of the largest and most complex classes of 107

alkaloids, giving rise to more than 3,000 structures. The tryptophan precursor anthranilate 108

also serves as a precursor to several alkaloid sub-classes, including the quinazolines. 109

Beyond the aromatic amino acids, arginine and ornithine form the basis of the tropane, 110

pyrrolidine, pyrrolizidine, and pyridine alkaloids, whereas lysine is the precursor to the 111

piperidines and quinolizidines. Nucleosides also form the basis of some alkaloids, such as 112

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caffeine and theobromine. 113

114

Phenylpropanoids and polyketides 115

The phenylpropanoids represent a class of more than 8,000 plant phenolics derived from 116

tyrosine and phenylalanine via the phenylpropanoid pathway (Wu and Chappell, 2008). 117

The name phenylpropanoid refers to the distinctive C6-C3 structure of metabolites within 118

this pathway. The key phenylpropanoid branch point intermediate is p-coumaroyl-CoA 119

(Figure 3), which forms the basis of the flavonoids, stilbenoids, coumarins, lignans, 120

catechins, and aurones. In addition to the C6-C3 skeleton, the phenylpropanoid pathway 121

also diverts at the level of cinnamic or ferulic acid to yield an array of C6-C1 benzoates, 122

such as vanillin, benzaldehyde, and gallic acid (Vogt, 2010; Kallscheuer et al., 2019). 123

Although the stilbenoids and flavonoids originate from the phenylpropanoid pathway, 124

they are elongated by type III plant polyketide synthases (PKSs), underscoring the mixed 125

biosynthetic nature of these specialized metabolites (Box 1). PKSs accept CoA-bound 126

substrates (Yu et al., 2012), most often p-coumaroyl-CoA, although cinnamoyl- and 127

feruloyl-CoA form the basis of some phenylpropanoid polyketides, such as pinosylvin 128

and curcumin (Preisig-Müller et al., 1999; Kita et al., 2008). Once loaded with a starter 129

molecule, malonyl-CoA moieties are incorporated into the growing polyketide chain. 130

A number of different polyketide backbones can be produced, though the most 131

heavily targeted for metabolic engineering are the naringenin chalcone and resveratrol 132

scaffolds, which give rise to the respective flavonoid and stilbenoid subclasses (Lussier et 133

al., 2012). These compounds are functionalized in downstream reactions including 134

aromatic hydroxylation, nicotinamide adenine dinucleotide phosphate (NADPH)-135

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dependent reduction, O-methylation, and glycosylation. The flavonoids alone encompass 136

more than 6,000 natural products, including chalcones, aurones, catechins, flavanones, 137

flavanols, isoflavonoids, and anthocyanins (Yu et al., 2012). In contrast to the flavonoids 138

and stilbenoids, coumarins are not polyketides, as their lactone ring structure is derived 139

from hydroxylation, isomerization, and lactonization of phenolic acids (cinnamic, p-140

coumaric, caffeic, or ferulic acid) or the corresponding phenolic acyl-CoA esters (Kai et 141

al., 2008; Yao et al., 2017). 142

143

PIONEERING STUDIES IN THE MICROBIAL SYNTHESIS OF PLANT 144

METABOLITES 145

The most famous application of synthetic biology for the synthesis of valuable plant 146

products is semisynthetic artemisinin, an antimalarial terpenoid natively produced by 147

sweet wormwood (Artemisia annua). Over a period of more than a decade, artemisinic 148

acid production has reached titers of 25 g/L in yeast (Paddon et al., 2013), whereas 149

Escherichia coli has been engineered to produce 27 g/L of the amorphadiene precursor 150

(Tsuruta et al., 2009). The yeast artemisinic acid production strain has been repurposed as 151

a sesquiterpenoid platform, facilitating industrial-scale production of more than 130 g/L 152

farnesene (Meadows et al., 2016). Paclitaxel is another terpenoid medicine and microbes 153

have been engineered to produce up to 1 g/L of the taxadiene scaffold (Ajikumar et al., 154

2010; Ding et al., 2014; Zhou et al., 2015). Other notable terpenoid pathway 155

reconstructions include limonene, pinene, geraniol, eucalyptol, humulene, β-156

carophyllene, patchoulol, santalene, and bisabolene (Table 1). 157

Because geraniol forms half of the MIA scaffold, strategies successfully used to 158

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produce monoterpenoids were exploited to synthesize MIA intermediates in yeast, such 159

as nepetalactol (Campbell et al., 2016; Billingsley et al., 2017) and strictosidine (Brown 160

et al., 2015). These efforts have yet to be extended to the de novo synthesis of 161

functionalized MIAs in a microbial system, although impressive partial pathway 162

reconstructions have been reported (Qu et al., 2015). The BIA class has also recently 163

experienced several breakthrough microbial pathway reconstitutions. As a result of 164

successes in resolving upstream pathways involving conversion of tyrosine to dopamine, 165

de novo formation of (S)-reticuline was demonstrated for the first time (Nakagawa et al., 166

2011; DeLoache et al., 2015). This pathway was later extended for de novo biosynthesis 167

of morphinan alkaloids in E. coli and yeast (Galanie et al., 2015; Nakagawa et al., 2016), 168

as well as noscapine in yeast (Li et al., 2018). Titers of (S)-reticuline have recently been 169

increased to 160 mg/L in E. coli (Matsumura et al., 2018). Stilbenoids have been the 170

target of several microbial synthesis efforts and the de novo titer of resveratrol has 171

reached 5 g/L in yeast (Li et al., 2016; Katz et al., 2017). Other stilbenoids produced in 172

microbes include pinostilbene, pterostilbene, pinosylvin, and piceatannol (Table 1). 173

Similarly, the branch point flavonoid naringenin has been produced from glucose at titers 174

above 200 mg/L (Lehka et al., 2016). The flavonoid pathway has been diversified in E. 175

coli and yeast to yield the dihydrochalcone phloretin and its derivatives, the flavanone 176

liquiritigenin, and the flavonols kaempferol, quercetin, and fisetin, among others (Table 177

1). This pathway was recently extended to produce anthocyanin pigments using E. coli 178

polycultures (Jones et al., 2017) and a single engineered yeast strain (Eichenberger et al., 179

2018; Levisson et al., 2018). 180

Not all natural product syntheses involve straightforward maximization of product 181

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titer, as some applications require careful tuning of product concentrations. One such 182

study involved combinatorial engineering of industrial brewing yeasts (S. cerevisiae) to 183

generate strains with a diverse range of geraniol and linalool production profiles, 184

culminating in the formulation of beers with a more desirable hops flavor compared to 185

commercial varieties (Denby et al., 2018). Studies such as these highlight the wide range 186

of applications afforded through the manipulation of plant metabolic pathways in 187

microbial systems. 188

189

ELUCIDATION OF PLANT METABOLIC PATHWAYS 190

In planta 191

Traditionally, the elucidation of metabolic pathways in source plants has been the driving 192

force and often limiting factor of microbial biosynthesis. Owing to the complexity of 193

plant metabolism, gaps in pathways are common, particularly within downstream 194

reactions mediating the formation of functionalized products. Pathway gaps in the context 195

of microbial synthesis refer to a lack of enzyme identification, as the biochemical nature 196

of metabolic transformations is typically known. 197

To bypass gaps in enzyme elucidation, pathway intermediates are often supplied 198

to cells exogenously, granted they are available and an uptake system exists. Although 199

enzyme gaps have plagued many natural product pathways, a flurry of activity has 200

recently led to the completion of several important pathways. While it was known that 201

production of the nepetalactol backbone within the MIA pathway involved reductive 202

cyclization of 10-oxogeranial, the enzyme mediating this transformation was finally 203

discovered in 2012 (Geu-Flores et al., 2012). Critical to this success was the search of 204

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source plant transcriptomes for redox enzymes co-expressed with MIA pathway 205

transcripts. Other key enzymes within the early MIA pathway were discovered in 2014 206

(Miettinen et al., 2014), prompting the synthesis of the key branch point MIA 207

strictosidine in yeast soon thereafter (Brown et al., 2015). 208

The MIA pathway leading to vinblastine was fully elucidated in 2018 (Caputi et 209

al., 2018); thus, it is only a matter of time before the de novo synthesis of this potent 210

anticancer medicine is achieved in a microbial system. Similarly, the final enzymes 211

involved in the formation of substituted amphetamines, such as ephedrine and 212

pseudoephedrine (Hagel et al., 2012), were also identified as of 2018 (Morris et al., 213

2018), providing rationale for their synthesis in a microbial system. The opium poppy 214

(Papaver somniferum) enzyme responsible for the stereochemical inversion of (S)- to (R)-215

reticuline eluded identification for many years, limiting morphine pathway 216

reconstructions to upstream [glucose to (S)-reticuline] and downstream [(R)-reticuline to 217

morphine] modules. Eventual identification of the fusion enzyme by three research 218

groups led to the reconstruction of the de novo morphinan pathway within a microbial 219

host (Galanie et al., 2015). Candidate epimerase genes were identified using 220

transcriptome database mining and candidates were silenced in opium poppy plants, 221

sequenced from mutants that accumulate (S)-reticuline, or cloned in S. cerevisiae for 222

functional analysis (Farrow et al., 2015; Galanie et al., 2015; Winzer et al., 2015). While 223

this sequence of events permeates the field of microbial synthesis, in which pathway 224

elucidation in planta precedes pathway reconstruction in microbes, a recent paradigm has 225

arisen in response to this laborious and time-consuming process. 226

227

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In microbes 228

Due to the declining cost of DNA synthesis, rapidly advancing bioinformatics tools, and 229

expanding omics databases, it has become possible to resolve single and multi-enzyme 230

gaps within a heterologous microbial host. Using this synthetic biology approach, 231

libraries of candidate gap-filling enzymes are first compiled by querying plant 232

transcriptome databases, such as 1000 Plants (Matasci et al., 2014) or PhytoMetaSyn 233

(Xiao et al., 2013) Projects. The corresponding genes are then codon-optimized, 234

synthesized, and expressed in a laboratory microbe. This workflow facilitates the 235

resolution of enzyme gaps in plant metabolic pathways without having to obtain physical 236

plant material or cDNAs (Narcross et al., 2016). In this regard, synthetic biologists do not 237

need to wait for gap-filling enzymes to be isolated and characterized from natural sources 238

to reconstitute target pathways in a microbial species. One of the groups involved in 239

identifying the aforementioned reticuline epimerase gene employed this strategy by 240

querying plant transcriptome databases and integrating synthetic genes into a yeast strain 241

harboring a downstream module for thebaine synthesis (Galanie et al., 2015). An 242

ambitious combinatorial variation of this technique was attempted to fill a seven-enzyme 243

gap within the vinblastine MIA pathway (Casini et al., 2018). While the authors were 244

unable to resolve this intricate pathway, they succeeded in building 74 strains, each 245

possessing a distinct combination of the seven gene candidates. This study illustrates the 246

immense potential of synthetic biology for elucidating and reconstructing complex 247

biochemical pathways in microbial systems. 248

249

HOST SELECTION 250

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Host selection is a fundamental facet of microbial synthesis and a key determinant of 251

pathway performance. Since it is virtually impossible to forecast the ideal host for 252

synthesizing a target metabolite, a number of factors must be weighed, including access 253

to a complete genome sequence and the availability of genetic tools. Precursor 254

availability is perhaps the most important criterion of host selection, as an abundant 255

precursor pool, such as acetyl-CoA or shikimate, sidesteps the need to rewire core 256

metabolism. Host selection is further complicated by the existence of phenotypically 257

distinct strains within a given species, for instance the E. coli K and B lineages or the 258

S288C and CEN.PK strains of S. cerevisiae. In a dramatic instance, utilization of E. coli 259

DH1 improved oxidation of amorphadiene by a factor of 1,000 relative to related strains 260

(Chang et al., 2007). Screening of E. coli lineages for polyketide synthesis has revealed 261

not only strain-specific differences in baseline polyketide production but also differences 262

in response to the same genetic manipulations (Yang et al., 2018). Due to differences in 263

amino acid biosynthesis between S. cerevisiae (Canelas et al., 2010), production of 264

vanillin-β-glucoside was significantly higher in S288C than in CEN.PK (Strucko et al., 265

2015). Similar yeast-specific differences were observed for the synthesis of the 266

polyketide triacetic acid lactone (Saunders et al., 2015). These studies highlight the often 267

overlooked task of screening strains to identify optimal producers prior to 268

comprehensively engineering a production strain. 269

270

Workhorses and specialized hosts 271

To date, the overwhelming majority of microbial syntheses have employed E. coli or S. 272

cerevisiae owing to their ease of genetic and metabolic manipulation, a comprehensive 273

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understanding of their genetics, metabolism, and physiology, and their highly active 274

central metabolic pathways. Several critical differences between these industry-proven 275

hosts inform many microbial synthesis campaigns (Box 2). 276

Recently, the diversity of microbial hosts exploited for the synthesis of plant 277

natural products has expanded dramatically. Some microorganisms naturally synthesize 278

aromatic amino acids at levels well above those of E. coli and S. cerevisiae, indicating a 279

major potential of these hosts for the production of alkaloids and phenylpropanoids. Of 280

particular importance is Corynebacterium glutamicum, an industrial bacterium with a 281

tremendous capacity to synthesize glutamate and lysine, as well as aromatic amino acids 282

(Azuma et al., 1993; Ikeda et al., 1993). Lactococcus lactis is another industry-proven 283

bacterium with a demonstrated capacity to synthesize plant terpenoids and 284

phenylpropanoids (Gaspar et al., 2013; Dudnik et al., 2018). The yeast Scheffersomyces 285

stipitis has gained interest for its ability to utilize C5 sugars, suggesting that this organism 286

has a more active pentose phosphate pathway than does S. cerevisiae. Indeed, the highest 287

shikimate titer reported to date in yeast was achieved using S. stipites (Gao et al., 2016). 288

Within the polyketide class of natural products, bacteria from the genus 289

Streptomyces are major natural producers, providing rationale for diverting these native 290

pathways to plant-derived polyketides (Baltz, 2010). The oleaginous yeast Yarrowia 291

lipolytica has also been demonstrated to be a promising host for terpenoids, polyketides, 292

and other compounds derived from acetyl-CoA (Abdel-Mawgoud et al., 2018). Other 293

hosts are desirable for superior growth characteristics, such as Pichia pastoris, a yeast 294

that attains higher cell densities than S. cerevisiae (Wriessnegger et al., 2014). 295

296

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RECONSTITUTION AND OPTIMIZATION OF PLANT PATHWAYS IN 297

MICROBIAL SYSTEMS 298

Pathway assembly and delivery 299

Pathway assembly refers to the design and organization of genetic parts required to 300

reconstruct heterologous pathways in microbes. This endeavor begins by selecting and 301

synthesizing gene variants responsible for mediating the synthesis of the target product. 302

The selected candidates are placed within transcriptional units containing a promoter and 303

terminator. Traditionally, heterologous genes and metabolic pathways have been 304

delivered and expressed from plasmids. While this technique has certain merits, such as 305

facilitating the rapid screening of prospective hosts (Casini et al., 2018) and easily 306

assessing the performance of pathway modules (Fossati et al., 2014), access to efficient 307

genome editing tools has led to the adoption of chromosomal expression systems 308

(Horwitz et al., 2015; Jakočiūnas et al., 2015). Furthermore, plasmid-borne expression 309

leads to significant variability in gene expression and copy number between cells (Ryan 310

et al., 2014). 311

With respect to transcriptional control, most pathways are assembled by placing 312

all genes under control of constitutive promoters. For instance, amorphadiene and 313

artemisinic acid production strains were first constructed using galactose induction, 314

which was later converted to constitutive expression without affecting production or 315

fitness (Westfall et al., 2012; Paddon et al., 2013). Because even “constitutive” promoters 316

exhibit distinct transcriptional profiles, which vary based on carbon source or medium 317

formulation (Reider Apel et al., 2016), major efforts have focused on comprehensively 318

characterizing both promoter and terminator elements (Yamanishi et al., 2013; Lee et al., 319

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2015). These toolkits facilitate the exploration of vast pathway design spaces comprised 320

of a staggering number of possible gene and expression combinations. 321

Product titer, rate, and yield following initial pathway reconstitution are often 322

very poor, necessitating many design-build-test engineering cycles to reach 323

commercially-viable levels. Historically, this process lasts 6-8 years and costs more than 324

$50 million (Nielsen and Keasling, 2016). These metrics are declining at a dramatic rate 325

as a result of the standardization of synthetic biology and the advent of high-throughput 326

strain engineering facilities (‘biofoundries’), which aim to miniaturize and automate 327

strain construction workflows (Chao et al., 2017; Casini et al., 2018). 328

329

Expression of plant genes in microbial species 330

The functional expression of plant genes in microbial species poses a number of technical 331

challenges. Many plant enzymes possess localization signals for targeting to specific 332

organelles, such as the chloroplast. Implementation of these plant genes in microbial 333

hosts typically leads to poor gene expression or insolubility of the corresponding protein 334

(Williams et al., 1998). Most terpene synthases are plastid-localized and thus N-terminal 335

truncation is an effective strategy for improving activity (Alonso-Gutierrez et al., 2013; 336

Zhao et al., 2016). Production of BIAs is also improved through N-terminal truncation of 337

norcoclaurine synthase (Li et al., 2016). While software tools exist for predicting optimal 338

signal peptide cleavage sites (Petersen et al., 2011), assessing various-sized truncations 339

affords a range of improvements (Nishihachijo et al., 2014; Jiang et al., 2017). 340

Perhaps the most challenging facet of synthesizing plant-derived metabolites in 341

microbial hosts is the functional expression of cytochrome P450 (CYP) enzymes. This 342

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obstacle provides much of the rationale for using eukaryotic expression systems over 343

bacteria (Box 2). CYPs are membrane-bound enzymes that carry out many of the 344

complex functionalizations in secondary metabolic pathways, including hydroxylations, 345

double bond epoxidations, and dealkylations (Meunier et al., 2004). Because CYPs 346

mediate oxidation reactions, catalysis is coupled to reduction of NADPH, which in turn 347

requires a cytochrome P450 reductase (CPR) partner to shuttle electrons between 348

NADPH and CYP. Additional components can also be involved, such as cytochrome b5 349

and a cytochrome b5 reductase (Porter, 2002; Paddon et al., 2013; Li et al., 2016). 350

Some species of plant contain hundreds of CYPs and multiple CPRs, leading to 351

immense challenges in optimizing heterologous CYP-CPR pairings. Although yeast 352

contains a native CPR, most plant CYPs exhibit greater activity when paired with CPRs 353

from the same plant species (Fossati et al., 2014). Production of highly functionalized 354

secondary metabolites, such as paclitaxel and hydrocodone, involves numerous CYP-355

catalyzed reactions, requiring the coordinated expression of multiple CPR genes. To 356

overcome this hurdle, most microbial synthesis studies employ a “one-size-fits-all” 357

strategy by pairing multiple CYPs with a single CPR. CYP and CPR expression must be 358

carefully balanced to ensure efficient shuttling of electrons and avoid the production of 359

toxic reactive oxygen species. Optimization of CYP-CPR pairing was paramount to the 360

success of the semi-synthetic artemisinin process (Paddon et al., 2013) and enabled 361

production of 500 mg/L oxygenated taxadiene in E. coli (Biggs et al., 2016). 362

The most efficient natural CYP is a primitive bacterial CYP-CPR fusion from 363

Bacillus megaterium (Munro et al., 2002). This architecture has been exploited for the 364

expression of plant CYPs in E. coli, in which the N-terminal membrane-binding domain 365

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16

is cleaved or modified for more efficient insertion within bacterial membranes (Hausjell 366

et al., 2018). Conversely, it has been shown that a direct CYP-CPR fusion in E. coli was 367

less active than carefully balanced expression of both components for oxygenated 368

taxadiene synthesis (Biggs et al., 2016). The authors suggest that since plant CYPs and 369

CPRs do not interact in a 1:1 ratio, a direct fusion approach is not appropriate. 370

371

Precursor supply 372

Engineering microbial systems for the production of phenylpropanoids and most 373

alkaloids demands an abundant supply of aromatic amino acid precursors (Figure 2 and 374

Figure 3). Tremendous strides have been made in engineering platform strains producing 375

high levels of tyrosine, phenylalanine, tryptophan, and their derivatives (Table 1). 376

Deregulating host precursor pathways is regarded as the chief hurdle to the 377

production of heterologous metabolites (Nielsen and Keasling, 2016). The yeast 378

shikimate and aromatic amino acid pathways are subject to multiple layers of 379

transcriptional and post-transcriptional control. The entry point to the shikimate pathway 380

involving condensation of E4P and PEP is catalyzed by 3-deoxy-D-arabino-381

heptulosonate-7-phosphate (DAHP) synthase, for which two isoenzymes exist in yeast 382

(Aro3 and Aro4). Allosteric inhibition by tyrosine can be overcome in both enzymes 383

using analogous mutations (Aro3K222L

and Aro4K229L

) (Luttik et al., 2008; Brückner et al., 384

2018). Similarly, chorismate mutase (Aro7) is feedback-inhibited by tyrosine, prompting 385

discovery of the Aro7G141S

allele for deregulating tyrosine biosynthesis (Luttik et al., 386

2008). Implementation of Aro4K229L

and Aro7G141S

in the same host leads to a 200-fold 387

increase in extracellular levels of aromatic compounds. 388

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The terpenoid branch of secondary metabolism has also been the focal point of 389

engineering precursor supply. The rate-limiting step of the mevalonate pathway catalyzed 390

by HMG-CoA reductase (HMGR) can be improved by implementing a truncated mutant 391

(tHMGR). Owing to the importance of this enzymatic step, studies achieving high-level 392

terpenoid or MIA production have incorporated up to four gene copies of tHMGR 393

(Paddon et al., 2013; Brown et al., 2015). 394

The production of GPP-derived monoterpenoids poses a unique engineering 395

challenge pertaining to the dual nature of the yeast Erg20 protein. This enzyme catalyzes 396

the synthesis of GPP from IPP and DMAPP, as well as the subsequent condensation of 397

GPP and an additional unit of IPP to give FPP (Figure 1). Owing to the central role of 398

FPP in ergosterol biosynthesis, Erg20 is an essential enzyme, signifying that its FPP 399

synthase activity can not be abolished. Instead, Erg20 variants with reduced GPP-binding 400

properties have been described (Chambon et al., 1990) and exploited for the production 401

of various monoterpenoids and MIAs (Brown et al., 2015; Campbell et al., 2016). 402

Relatedly, the essential squalene synthase enzyme (Erg9) generates squalene from two 403

FPP units, thus placing constraints on the production of all terpenoids deriving from FPP 404

(Figure 1). 405

The most fundamental means of circumventing host regulation is replacing native 406

precursor pathways with a heterologous counterpart. This approach was utilized in early 407

efforts to produce amorphadiene in E. coli, wherein the native MEP pathway was 408

replaced by the yeast mevalonate pathway (Martin et al., 2003). Reconstructing the yeast 409

pathway within a bacterial operon eliminates both E. coli and S. cerevisiae modes of 410

regulation, facilitating constitutive production of terpenoids. 411

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Acetyl-CoA plays a pivotal role in the biosynthesis of terpenoids, polyketides, 412

phenylpropanoids, and cannabinoids (Figure 1 and Figure 3), and its overproduction has 413

also been a target for improvement. In yeast, multiple acetyl-CoA synthesis pathways 414

exist in different subcellular compartments, which acetyl-CoA can not directly cross 415

(Strijbis and Distel, 2010). This provides options for strain engineering based on the 416

pathway that is most suited for a particular heterologous product (van Rossum et al., 417

2016). Alternatively, heterologous cytosolic acetyl-CoA pathways are of great interest 418

(Kozak et al., 2014; Kozak et al., 2014; Zhang et al., 2015; Cardenas and Da Silva, 2016; 419

Kozak et al., 2016; Meadows et al., 2016; Rodriguez et al., 2016). The highest reported 420

titer of a heterologous product derived from cytosolic acetyl-CoA is the synthesis of 130 421

g/L of the sesquiterpene farnesene under fermentative conditions (Meadows et al., 2016). 422

This approach utilized multiple alternative cytosolic acetyl-CoA pathways to overproduce 423

farnesene. 424

In prokaryotes, all routes to acetyl-CoA are equally accessible, reducing the 425

necessity for alternative pathways, although they have been demonstrated (Wang et al., 426

2018). However, buildup of acetyl-CoA results in its conversion to acetate, which is an 427

unproductive carbon sink. In E. coli, native acetate assimilation enzymes can be 428

overexpressed to convert acetate back to acetyl-CoA, resulting in improved flavanone 429

biosynthesis (Leonard et al., 2007; Zha et al., 2009). Gene knockouts have been identified 430

that prevent acetate synthesis and improve heterologous production without reducing 431

cellular fitness (Zha et al., 2009; Liu et al., 2017). Eliminating pathway bottlenecks to 432

improve flux through acetyl-CoA can also reduce acetate overflow (Li et al., 2015; King 433

et al., 2017; Wang et al., 2018). Relatively little attention has been placed on the 434

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19

availability of free Coenzyme A to accept increased carbon flux. Supplementation of the 435

Coenzyme A precursor pantothenate can improve the yield of heterologous products 436

requiring acetyl-CoA. Several studies have demonstrated that overexpression of the rate-437

limiting step of Coenzyme A synthesis, pantothenate kinase, improves product synthesis 438

two-fold (Fowler et al., 2009; Schadeweg and Boles, 2016; Liu et al., 2017). 439

Phenylpropanoid, polyketide and cannabinoid synthesis require malonyl-CoA, 440

which is used for fatty acid biosynthesis in E. coli and S. cerevisiae. In these organisms, 441

fatty acid synthesis is tightly-regulated (Davis et al., 2000). Improving synthesis and 442

reducing off-target consumption of malonyl-CoA are the major areas for overproduction. 443

Acetyl-CoA is converted to malonyl-CoA by acetyl-CoA carboxylase (Acc) and 444

improvements in this activity have been achieved by overexpression (Zha et al., 2009; Xu 445

et al., 2011; Yang et al., 2018), ablating post-translational modification sites (Choi and 446

Da Silva, 2014; Shi et al., 2014; Chen et al., 2018), and improving the availability of its 447

biotin cofactor (Leonard et al., 2007). Acc-independent malonyl-CoA synthesis has also 448

been demonstrated (Leonard et al., 2008), though the pathway requires supplemented 449

malonate. Reducing the diversion of malonyl-CoA to essential pathways is a challenge, 450

because fatty acids are required for biomass production. In yeast, knockout of genes 451

regulating fatty acid biosynthesis improved titers of malonyl-CoA-derived products (Zha 452

et al., 2009; Chen et al., 2017), yet the strain was auxotrophic for inositol. More 453

promisingly, biomass accumulation prior to conditional gene knockdown has been 454

demonstrated to improve polyketide titers in E. coli (Yang et al., 2015; Liang et al., 2016; 455

Wu et al., 2016; Yang et al., 2018). 456

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As a ubiquitous cofactor and the source of reductant for plant CYPs, NADPH is 457

an important engineering target for the synthesis of natural products. A proven strategy 458

for enhancing NADPH availability is the inactivation of native NADPH-consuming 459

reactions. A caveat of this approach pertains to the targeting of enzymes involved in 460

central carbon metabolism, such as glucose-6-phosphate isomerase (Pgi) and 461

phosphoenolpyruvate carboxylase (Ppc) in E. coli (Chemler et al., 2010) or glucose-6-462

phosphate dehydrogenase (Zwf1) in S. cerevisiae (Gold et al., 2015). Glutamate 463

dehydrogenase (GldH or Gdh1) is a promising candidate in both species (Asadollahi et 464

al., 2009; Chemler et al., 2010). Inactivation of these central targets often has deleterious 465

effects on strain fitness, including the generation of amino acid auxotrophy (Gold et al., 466

2015), demanding a delicate balancing of core pathways and sophisticated metabolic 467

models for predicting combinatorial modifications (Chemler et al., 2010). 468

The counter approach of upregulating NADPH-generating enzymes is a more 469

viable avenue for engineering NADPH supply. In yeast, these enzymes include 470

prephenate dehydrogenase (Tyr1) and cytosolic aldehyde dehydrogenase (Ald6) (Li et al., 471

2018). Curiously, both overexpression and deletion of ZWF1 have been shown to 472

enhance formation of tyrosine products (Gold et al., 2015; Li et al., 2018), underscoring 473

the complexity of yeast central metabolism and redox homeostasis. 474

475

Improving flux to heterologous pathways 476

Flux improvement involves iterative rounds of strain engineering with the aim of 477

elevating product titer, rate, and yield. Efforts to improve flux through a target pathway 478

begin with the identification of rate-limiting enzymatic conversions, which is diagnosed 479

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by monitoring levels of pathway intermediates. The most fundamental means of relieving 480

metabolic bottlenecks is to increase levels of rate-limiting enzymes by increasing gene 481

expression or copy number. In addition to high-level expression of the heterologous 482

pathway, the entire eight-gene yeast pathway from acetyl-CoA to FPP was overexpressed 483

in the industrial artemisinic acid strain (Paddon et al., 2013). 484

Poor pathway flux can also arise from the diversion of pathway intermediates to 485

off-target routes, either by host activities or promiscuous plant enzymes. Because 486

phenylpropanoid biosynthesis derives from phenylalanine or tyrosine, the yeast pathway 487

for aromatic amino acid degradation (Ehrlich pathway) is typically inactivated through 488

deletion of amino acid decarboxylase genes (ARO10 and PDC5) (Rodriguez et al., 2015). 489

Yeast BIA biosynthesis involves a more intricate balancing of tyrosine pathways, as the 490

dopamine and 4-HPAA precursors are derived from its biosynthesis and degradation, 491

respectively (Narcross et al., 2016). The physiological fate of 4-HPAA in yeast involves 492

conversion to tyrosol or 4-hydroxyphenylacetate, which can be mediated by more than 20 493

candidate enzymes (Hazelwood et al., 2008; DeLoache et al., 2015). These enzymes have 494

also been implicated in the irreversible reduction of aldehyde intermediates within the 495

MIA pathway (Billingsley et al., 2017), suggesting that any pathway possessing carbonyl 496

intermediates is subject to diversion by host activities. In a similar scenario, a yeast 497

reductase was shown to divert p-coumaroyl-CoA within the phenylpropanoid pathway to 498

an unwanted side product (Lehka et al., 2016). 499

Pathway flux can also be diverted to off-target routes by heterologous pathway 500

enzymes. O- and N-methyltransferases within the core BIA pathway have been shown to 501

exhibit a wide substrate range (Frick et al., 2001; Ounaroon et al., 2003; Fossati et al., 502

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22

2014), which was circumvented by screening of plant homologues for variants exhibiting 503

a more stringent substrate specificity (Narcross et al., 2016). In addition to catalyzing the 504

reductive cyclization of 10-oxogeranial, iridoid synthase within the MIA pathway was 505

found to exhibit activity on other intermediates (Campbell et al., 2016). In this case, 506

enzyme activity was higher on the unwanted substrates than the physiological substrate. 507

Enzyme promiscuity may be avoided in co-cultures, in which a pathway is split 508

between strains such that promiscuous enzymes are sequestered from pathway 509

intermediates (Nakagawa et al., 2014). Loss of intermediates is also mitigated through 510

enzyme fusion, as in the cases of geraniol synthase or patchoulol synthase through fusion 511

to yeast farnesyl diphosphate synthase (Erg20) (Albertsen et al., 2011; Jiang et al., 2017). 512

Fusions have the potential to facilitate more efficient substrate channeling by maintaining 513

close physical proximity of sequential pathway enzymes, thus limiting competition from 514

host enzymes. In addition to terpenoid systems, fusion of resveratrol biosynthetic 515

enzymes facilitated a 15-fold improvement in titer in engineered S. cerevisiae (Zhang et 516

al., 2006). Enzyme fusions have the added potential to improve protein stability or 517

solubility, as demonstrated through fusions of taxadiene synthase with the soluble E. coli 518

maltose binding protein, yielding a 25-fold improvement in taxadiene titer (Reider Apel 519

et al., 2016). Fusions with maltose-binding protein also improved production of α-ionone 520

up to 50-fold (Zhang et al., 2018). 521

522

EXPANDING THE PLANT SECONDARY METABOLIC SPACE IN 523

MICROBIAL SYSTEMS 524

An emerging application of synthetic biology aims to exploit microbial systems for drug 525

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discovery. In this manner, plant enzyme promiscuity and combinatorial biochemistry are 526

exploited to synthesize new structural scaffolds and functionalizations not found in 527

nature. 528

A common chemical modification within the pharmaceutical industry is 529

halogenation, in which fluoro or chloro moieties are introduced to drug candidates to 530

modify the physicochemical properties of the compound. Because many natural products 531

derive from amino acids, halogenated analogs can be supplemented to engineered 532

microbes leading to the formation of halogenated products. This strategy was used to feed 533

yeast fluoro- or chloro-tyrosine, resulting in the synthesis of halogenated BIA pathway 534

intermediates (Li et al., 2018). Halogenated end products were not detected, suggesting a 535

more stringent substrate specificity of downstream BIA enzymes. Moreover, this method 536

relies on the supplementation of costly amino acid analogs and could be overcome by 537

engineering de novo production of halogenated BIAs. 538

O-sulphated reticuline has been produced de novo by implementing a human 539

sulphotransferase into E. coli engineered for reticuline biosynthesis (Matsumura et al., 540

2018). Promiscuity of BIA pathway enzymes has also been exploited for the production 541

of novel scaffolds, such as disubstituted- and spiro-tetrahydroisoquinolines, by harnessing 542

the capacity of norcoclaurine synthase to accept an exceptional range of carbonyl 543

substrates (Lichman et al., 2017). Again, this strategy relies on the supplementation of 544

costly substrates, in this case carbonyl species that are not synthesized by 545

microorganisms. Moving these in vitro assays to robust de novo biosynthetic processes 546

presents a formidable challenge. 547

548

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CONCLUDING REMARKS 549

Microbial synthesis provides a new avenue for the production of natural plant 550

pharmaceuticals, fragrances, nutrients, and colorants. Although this field is in its infancy 551

and faces many challenges (see Outstanding Questions), the wealth of successes in recent 552

years has solidified microbial biosynthetic production as a viable alternative to natural 553

product extraction and total chemical synthesis. The elucidation of increasingly complex 554

plant secondary pathways continues to drive the reconstruction of longer and more 555

challenging routes in microbial systems. Indeed, the discovery of missing enzymes in 556

several highly sought-after plant specialized pathways has primed a number of untapped 557

natural products for de novo biosynthesis in a microbial species. Several microbially-558

derived natural products have reached commercialization and these successes will 559

continue to pave the way for new opportunities. The exploitation of microbial systems for 560

drug discovery promises to deliver important new structural scaffolds and chemical 561

functionalizations that have hitherto not been observed in nature. 562

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563

ACKNOWLEDGMENTS 564

This work was supported financially by a NSERC-Industrial Biocatalysis Network (IBN) grant, 565

and a NSERC Discovery grant. M.E.P. is supported by a NSERC postdoctoral fellowship, and 566

L.N. is supported by a FQRNT DE Doctoral Research Scholarship for Foreign Students. V.J.J.M. 567

is supported by a Concordia University Research Chair. 568

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Table 1 – Selection of de novo plant secondary pathway reconstructions in microbial systems 569

Titer (mg/L)

Metabolite class Metabolite S. cerevisiae E. coli Reference(s)

PRECURSORS tyrosine 1,930a 55,000 (Patnaik et al., 2008)

p-coumaric acid 1,930 2,510 (Rodriguez et al., 2015; Jones et al., 2017)

dopamine 24 2,150 (Nakagawa et al., 2014; DeLoache et al., 2015)

TERPENOIDS Monoterpenoids

geraniol 1,680 2,000 (Liu et al., 2016; Jiang et al., 2017)

pinene ND 970 (Yang et al., 2013)

limonene 0.49 2,700 (Willrodt et al., 2014; Jongedijk et al., 2015)

eucalyptol 400 653 (Ignea et al., 2011; Mendez‐Perez et al., 2017)

linalool 0.75 505 (Mendez‐Perez et al., 2017; Camesasca et al.,

2018)

Sesquiterpenoids

β-farnesene 130,000 8,700 (Meadows et al., 2016; You et al., 2017)

bisabolene 5,200 912 (Peralta-Yahya et al., 2011; Özaydın et al., 2013)

α-humulene 1,300 ND (Zhang et al., 2018)

β-caryophyllene ND 1,520 (Yang et al., 2016)

amorphadiene 37,000 27,400 (Tsuruta et al., 2009; Westfall et al., 2012)

artemisinic acid 25,000 ND (Paddon et al., 2013)

santalene 163 ND (Tippmann et al., 2016)

patchoulol 40.9 ND (Albertsen et al., 2011)

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Diterpenoids

jolkinol C 800 ND (Wong et al., 2018)

taxadiene 72.8 1,020 (Ajikumar et al., 2010; Ding et al., 2014)

Triterpenoids

β-amyrin 108.1 NR (Takemura et al., 2017; Zhu et al., 2018)

Tetraterpenoids

β-carotene 162.1 3,200 (Yang and Guo, 2014; Wang et al., 2016)

lycopene 1,650 3,520 (Sun et al., 2014; Chen et al., 2016)

ALKALOIDS Benzylisoquinoline

alkaloids

(S)-reticuline 0.082 160 (DeLoache et al., 2015; Matsumura et al., 2018)

thebaine 0.0064 2.1 (Galanie et al., 2015; Nakagawa et al., 2016)

hydrocodone 0.0003 0.36 (Galanie et al., 2015; Nakagawa et al., 2016)

noscapine 2.2 ND (Li et al., 2018)

Monoterpene indole

alkaloids

strictosidine 0.5 ND (Brown et al., 2015)

PHENYLPROPANOIDS Stilbenoids

resveratrol > 5,000 51.8 (Katz et al., 2017; Yang et al., 2018)

pinostilbene 5.52 ~2.5 (Kang et al., 2014; Li et al., 2016)

pterostilbene 34.93 33.6 (Li et al., 2016; Heo et al., 2017)

pinosylvin 130.02 281 (Katz et al., 2017; Wu et al., 2017)

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Flavonoids

naringenin >200 103.8 (Lehka et al., 2016; Yang et al., 2018)

scutellarin 108 ND (Liu et al., 2018)

genistein 7.7 ND (Trantas et al., 2009)

pinocembrin 2.6 432.4 (Wu et al., 2016; Eichenberger et al., 2017)

quercetin 20.38 ND (Rodriguez et al., 2017)

kaempferol 66.29 57b (Yang et al., 2014; Duan et al., 2017)

dihydrokaempferol 44 ND (Levisson et al., 2018)

pelargonidin-3-O-

glucoside 0.02 9.5 (Jones et al., 2017; Levisson et al., 2018)

phloretin 42.7 ND (Eichenberger et al., 2017)

Coumarins

umbelliferone ND 66.1 (Yang et al., 2015)

esculetin ND 61.4 (Yang et al., 2015)

4-hydroxycoumarin ND 483.1 (Lin et al., 2013) atiter reported for the downstream metabolite p-coumaric acid 570

btiter reported for kaempferol 3-O-rhamnoside 571

ND: no data available 572 NR: titer not reported 573

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Figure legends 574

Figure 1. Interfacing plant terpenoid secondary metabolic pathways with microbial 575

metabolism. Plant secondary metabolic reactions and pathways (green) are shown linked 576

to core microbial metabolism (beige shading; black font). S. cerevisiae is shown as the 577

prospective host species. Key yeast enzymes discussed in the main text are shown. 578

Abbreviations: Bts1, geranylgeranyl diphosphate synthase; DMAPP, dimethylallyl 579

pyrophosphate; Erg9, squalene synthase; Erg20, farnesyl pyrophosphate synthetase; FPP, 580

farnesyl pyrophosphate; GPP, geranyl pyrophosphate; GGPP, geranylgeranyl 581

pyrophosphate; IPP, isopentenyl pyrophosphate; MEV, mevalonate pathway. 582

583

Figure 2. Interfacing plant alkaloid secondary metabolic pathways with microbial 584

metabolism. S. cerevisiae is shown as the prospective host species. Refer to Figure 1 for 585

abbreviations and color coding. Additional abbreviations: AAA path, aromatic amino 586

acid pathway; Aro1, pentafunctional arom protein; Aro2, chorismate synthase and Flavin 587

reductase; Aro3+Aro4, DAHP synthase isoenzymes; Aro7, chorismate mutase; E4P, 588

erythrose 4-phosphate; 4-HPAA, 4-hydroxyphenylacetaldehyde; R5P, ribose 5-589

phosphate; PPP, pentose phosphate pathway; PEP, phosphoenolpyruvate; TCA cycle, 590

tricarboxylic acid cycle. 591

592

Figure 3. Interfacing plant phenylpropanoid and polyketide secondary metabolic 593

pathways with microbial metabolism. A selection of natural plant pathways in a 594

prospective S. cerevisiae host is shown. More extensive networks of natural and synthetic 595

phenylpropanoid routes have been shown previously (Wang et al., 2015; Zhao et al., 596

2015). Refer to Figure 1 and Figure 2 for abbreviations and color coding. Additional 597

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abbreviations: Acc1, acetyl-CoA carboxylase; PAL, phenylalanine ammonia-lyase; TAL, 598

tyrosine ammonia-lyase; PKS, polyketide synthase. 599

600

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ADVANCES

• Microbial synthesis of plant secondary metabolites is in the midst of a renaissance.

• The elucidation of major plant biosynthetic routes has stimulated the production of high-value natural products in microbial systems.

• Efforts are expanding beyond precursors and branch point intermediates to functionalized end products.

• Although baker’s yeast (Saccharomyces cerevisiae) remains the host of choice for reconstructing plant secondary pathways, the range of alternative hosts has diversified.

• Synthetic biology has enabled the expansion of the plant secondary metabolic space in microbial species.

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OUTSTANDING QUESTIONS

• Can microbial production of plant metabolites compete with traditional methods of chemical synthesis and crop-based manufacturing?

• Pathways for several untapped sub-classes of plant specialized metabolites have been elucidated recently (e.g., modified amphetamines and vinblastine). Can these pathways be reconstituted in a microbial species?

• Other functionalized metabolites of immense pharmaceutical value (e.g., paclitaxel) have proven inaccessible to date. What are the limitations and how can they be overcome to realize the microbial synthesis of these important drugs?

• To what extent will emerging synthetic biology workflows reduce the time and cost of advancing bioprocesses from proof-of-concept to large-scale production?

• Will microbial systems provide a viable avenue for drug discovery through the exploitation of enzyme promiscuity and combinatorial biochemistry?

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BOX 1. Metabolites of mixed biosynthetic

origin

A number of important plant natural products are derived from mixed biosynthetic origins, obscuring the boundaries between secondary metabolite classes (Box 1 Figure). Within the alkaloid class, MIAs arise through condensation of the tryptophan analog tryptamine and a secoiridoid monoterpene (secologanin). Secologanin can also condense with dopamine, giving rise to emetine and cephaeline, the active alkaloids of the emetic drug ipecac. Nicotine is formed through coupling of two heterocyclic rings of distinct biosynthetic origin. Whereas the pyrrolidine ring derives from ornithine via the tropane alkaloid pathway (Figure 1), the pyridine ring is formed from niacin, itself synthesized from aspartate. Several other alkaloids are formed from products of the phenylpropanoid pathway, such as substituted amphetamines through condensation of pyruvate and benzoic acid, a C6-C1 analog of cinnamic acid (Hagel et al., 2012). Modified amphetamines are also called phenylpropylamino alkaloids to reflect their dual biosynthetic origin. Similarly, the autumnaline scaffold of the phenethylisoquinoline alkaloids is derived through condensation of dopamine and 4-hydroxydihydrocinnamaldehyde, another cinnamic acid derivative. The capsaicinoids produced by chili peppers have a unique tripartite classification, as they are exocyclic alkaloids that arise in part from the phenylpropanoid pathway and polyketide synthesis.

Specifically, coupling of ferulic-acid-derived vanillylamine and a fatty acyl-CoA (8-methyl-6-nonenoyl CoA) derived from valine or leucine gives rise to the capsaicin scaffold. Many other functionalized phenylpropanoids, notably the flavonoids and stilbenoids, are also polyketides involving chain elongation by malonyl-CoA (Figure 3). The curcuminoids and gingerols, responsible for the color and pungency of turmeric and ginger, respectively, are two additional subclasses comprising phenylpropanoid CoA esters linked by a polyketide synthase. Aromatic plant secondary metabolites are also frequently prenylated, which requires a terpenoid donor (Vickery et al., 2016). One such class of natural products is the cannabinoids, of which more than 100 distinct compounds are produced by the cannabis plant (Ahmed et al., 2015). A polyketide synthase catalyzes the decarboxylative condensation of hexanoyl-CoA with three molar equivalents of malonyl-CoA to generate olivetolic acid, which is then prenylated by GPP, generating cannabigerolic acid (CBGA). CBGA is the precursor to tetrahydrocannabinol (THC), cannabidiol, and their derivatives. These bioactive natural products have the potential to treat various medical conditions, including cancer, anorexia, epilepsy, and Alzheimer’s and Huntington’s diseases (Aizpurua-Olaizola et al., 2016).

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BOX 2. E. coli versus S. cerevisiae

Most microbial synthesis projects start with the question: E. coli or S. cerevisiae? Yeast alcohol production exemplifies the most ancient industrial bioprocess and both hosts have been exploited extensively in industrial biotechnology. Although many plant CYPs and CPRs can be functionally expressed in bacteria, often following extensive engineering of N-terminal membrane domains, CYPs are typically functional in yeast without modification. Because yeast produces only three native CYPs, interference from heterologous CYP and CPR proteins is generally not problematic. Therefore, we surmise that yeast is the superior host to support the biosynthesis of highly functionalized plant secondary metabolites, particularly those arising from multi-CYP pathways. The occurrence of eight CYP-catalyzed reactions in the downstream taxane pathway precludes the use of bacterial systems for producing these important products. E. coli has been engineered to synthesize the highly functionalized morphinan BIA hydrocodone (Nakagawa et al., 2016), yet the full de novo pathway was split between four engineered strains. The same pathway has been reconstructed within a single S. cerevisiae strain

(Galanie et al., 2015), albeit at lower titers, again highlighting the potential advantages of eukaryotic hosts for synthesizing complex plant secondary metabolites. Analogously, de novo anthocyanin synthesis in E. coli employed four engineered strains (Jones et al., 2017), which again contrasts strategies using S. cerevisiae in which a single strain hosts the full anthocyanin pathway (Eichenberger et al., 2018; Levisson et al., 2018). Although yeast is clearly favored for harboring downstream pathways, precursor titers are often higher in E. coli (e.g., for production of tyrosine, p-coumaric acid, dopamine, reticuline, and taxadiene; Table 1). For instance, E. coli has been engineered to produce 1 g/L of taxadiene (Ajikumar et al., 2010), which contrasts yeast titers of only 72.8 mg/L (Ding et al., 2014). This disparity in precursor production and downstream functionalization points to co-culture strategies in which natural product pathways are divided between E. coli and S. cerevisiae hosts. This approach proved highly effective for the synthesis of functionalized taxanes, in which the taxadiene precursor was secreted by E. coli and imported and derivatized by S. cerevisiae harboring multiple CYP enzymes (Zhou et al., 2015).

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Figure 1. Interfacing plant terpenoid secondary metabolic pathways with microbial

metabolism. Plant secondary metabolic reactions and pathways (green) are shown linked to core

microbial metabolism (beige shading; black font). S. cerevisiae is shown as prospective host

species. Key yeast enzymes discussed in the main text are shown. Abbreviations: Bts1,

geranylgeranyl diphosphate synthase; DMAPP, dimethylallyl pyrophosphate; Erg9, squalene

synthase; Erg20, farnesyl pyrophosphate synthetase; FPP, farnesyl pyrophosphate; GPP, geranyl

pyrophosphate; GGPP, geranylgeranyl pyrophosphate; IPP, isopentenyl pyrophosphate; MEV,

mevalonate pathway.

Sugars

Pyruvate

Acetyl-CoA

IPP

MONO-

TERPENOIDS

SESQUI-

TERPENOIDS

DI-

TERPENOIDS

GPPGeraniol

Taxadiene

Squalene

TETRA-TERPENOIDS

FPP

GGPP

Phytoene

Citronellol

OH

OH

β-carotene

TRI-TERPENOIDS

α-Pinene

HH

OH

Menthol

Isoprene Prenol

HEMI-

TERPENOIDS

Lycopene

Bisabolene

POLY-

TERPENOIDS

IPP

Farnesol

Erg20

Amorphadiene Artemisinic acid

Limonene

FerruginolGGPP

Natural rubber

(polyisoprene)

n IPP

n IPP

β-Amyrin Lupeol

Erg20

Erg9

Bts1

Ergosterol

Farnesene

OH

OH

OH

O

Camphor

OH

Thymol

OH

Linalool

OH

n>5,000

DMAPP

FPP

OH

MEV

IPP

OH

O

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Figure 2. Interfacing plant alkaloid secondary metabolic pathways with microbial metabolism. S. cerevisiae is shown as

prospective host species. Refer to Figure 1 for abbreviations and color coding. Additional abbreviations: AAA path, aromatic amino

acid pathway; Aro1, pentafunctional arom protein; Aro2, chorismate synthase and Flavin reductase; Aro3+Aro4, DAHP synthase

isoenzymes; Aro7, chorismate mutase; E4P, erythrose 4-phosphate; 4-HPAA, 4-hydroxyphenylacetaldehyde; R5P, ribose 5-

phosphate; PPP, pentose phosphate pathway; PEP, phosphoenolpyruvate; TCA cycle, tricarboxylic acid cycle.

Sugars

PEP

Pyruvate

Acetyl-CoA

G6P

R5P

PPP Glycolysis

NucleosidesPURINE ALKALOIDS

Caffeine

BENZYLISOQUINOLINE

ALKALOIDS

MONOTERPENOID INDOLE

ALKALOIDS

Noscapine Morphine Berberine Sanguinarine

TROPANE, PYRROLIDINE, PYRROLIZIDINE,

PYRIDINE ALKALOIDS

PIPERIDINE, QUINOLIZIDINE ALKALOIDS

E4P

Chorismate

Vasicine

QUINAZOLINE

ALKALOIDS

Anthranilate

Prephenate

Nicotine Cocaine Atropine

GPPTryptophan

AMPHETAMINE ANALOGS

Phenylalanine

Tyrosine

NH2

OH

O

NH2

OH

O

OH

Shikimate

path

MEV

Piperine

AAA path

Ephedrine Cathinone

Secologanin

NH2

O

OH

NH

Tryptamine

Dopamine 4-HPAA

H

H

OH

N

O

OH

H

H

OO

O

OO

NO

O

O

O

N

N

N

N

PHENETHYL-

ISOQUINOLINE

ALKALOIDS

Colchicine

L-DOPA

OH

N

N

O

NH

O

O

O

O

O

O

NH2

Vinblastine Quinine Camptothecin

OO

N+

O

O

OH

O

O

OO

OO

O

OH

N

NNH

N

H

H

N

NOH

O

OOH

O

N

N

OOO

N+

O

O

Sparteine

Pyruvate

N

N

O

O O

O

N

H

H

OO

OHN

H

H

O

O

O

N

SIMPLE INDOLES +

β-CARBOLINES

Serotonin

α-Ketoglutarate Glutamate

Arginine

Lysine

Cadaverine

Putrescine

Ornithine

TCA cycle

Harmine

OH

NH2

NH

O NH

N

N

NH

H

Aro3+Aro4

Aro1

Aro2

Aro7

OH

NH

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Figure 3. Interfacing plant phenylpropanoid and polyketide secondary metabolic pathways with

microbial metabolism. A selection of natural plant pathways in a prospective S. cerevisiae host is

shown. More extensive networks of natural and synthetic phenylpropanoid routes have been shown

previously (Wang et al., 2015; Zhao et al., 2015). Refer to Figure 1 and Figure 2 for abbreviations and

color coding. Additional abbreviations: Acc1, acetyl-CoA carboxylase; PAL, phenylalanine ammonia-

lyase; TAL, tyrosine ammonia-lyase; PKS, polyketide synthase.

Sugars

PEP E4P

Prephenate

Shikimate

path

Malonyl-CoA

Phenylalanine Tyrosine

NH2

OH

O

NH2

OH

O

OH

OH

O

Cinnamic acidOH

OH

O

p-Coumaric acid

PAL TAL

p-Coumaroyl-CoA

Acc1

COUMARINS

FLAVONOIDS

STILBENOIDS

Naringenin

(flavanone)

OH O

OH O

OH

3 Malonyl-CoA

3 Malonyl-

CoA

OH

OH

OH

Resveratrol

Umbelliferone

Pelargonidin 3-glucoside

(anthocyanin)

Quercetin

(flavonol)

Genistein

(isoflavonoid)

Pinosylvin

Pterostilbene

Pinostilbene

OOH O

O

O

OH

OH

OH

OH

O

OH

PPPGlycolysis

AAA

path

PKS

PKS

Caffeoyl-CoA Feruloyl-CoA

OH

S-CoA

O

Scopoletin

OH

OH

S-CoA

O

Acetyl-CoACoumarin

OO

Esculetin

OH

OOH O

O

OOH O

4-Hydroxycoumarin

OH

OO

Cinnamoyl-CoA

S-CoA

O

PKS3 Malonyl-CoA

OH

OH

OH O

OH O

OH

OH

OH O

OH O GlcO

OH O+

OH

OH

Caffeic acid Ferulic acid

OH

OH

OH

O

OH

O

S-CoA

O

OHOH

O

O

OTHER

PHENOLS

Vanillin

Angelicin

OOO

O

OH

O

Dhydrokaempferol

(dihydroflavonol)

OH

OH O

OH O

OH

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