DOI: 10.1126/science.1240318, 1533 (2013);340 Science

Jayne BelnapSome Like It Hot, Some Not

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www.sciencemag.org SCIENCE VOL 340 28 JUNE 2013 1533

PERSPECTIVES

Dryland ecosystems

cover over 40% of

Earth’s terrestrial

landmass ( 1). Biocrusts—soil

communities consisting of

cyanobacteria, mosses, and

lichens—can cover up to 70%

of the ground in these ecosys-

tems (see the fi gure, panel A)

( 2). The crucial role played

by these and other very small

organisms in nutrient, carbon,

and water cycles has become

increasingly clear in the past

few decades ( 2, 3). Soil sta-

bility and the composition

and performance of vascu-

lar plant communities also

depend on biocrust health

and activity. Yet, little is known about the

identity, biology, ecophysiology, or distribu-

tion of the microbial components that domi-

nate biocrusts ( 4, 5). Data are also needed to

understand how they will respond to climate

change. On page 1574 of this issue, Garcia-

Pichel et al. ( 6) take a fi rst step in fi lling this

data gap.

Using samples from western U.S. desert

sites across a range of climatic regimes, the

authors compared DNA signatures of known

cultivated cyanobacterial isolates to those in

their fi eld samples. They found a clear pat-

tern of biogeographic segregation: At most

sites, one of two cyanobacterial species

dominated, with Microcoleus vaginatus (see

the fi gure, panel B, and movie S1) prevalent

at colder sites and M. steenstrupii at warmer

sites. The authors then conducted physio-

logical studies to confi rm that growth of M.

vaginatus was, indeed, favored at lower tem-

peratures and M. steenstrupii at higher tem-

peratures. They predict that the latter will

likely replace the former as temperatures

increase in the future.

The study illustrates the need to know

what microorganisms are present at a site

and how they affect ecosystem function,

rather than exclusively focusing on the more

visible macroorganisms such as plants, ani-

mals, or even mosses and lichens ( 2, 4, 7).

Modern genetic studies of microbes can

quickly and cheaply produce overwhelming

amounts of data, but adding the functional

dimension to molecular diversity has been

exceedingly difficult. There is an urgent

need for usable, consistent, and agreed-

upon taxonomic and functional categories

for microbes.

As shown by Garcia-Pichel et al., eco-

logically meaningful insights into microor-

ganisms can be obtained by coupling new

techniques (such as high-throughput molec-

ular surveys of community DNA) with tradi-

tional techniques such as cultivation ( 7) and

ecophysiological characterization of rele-

vant isolates. (This approach, of course, is

limited to organisms that can be cultured.)

As it turns out, M. steenstrupii (and the

nitrogen-fi xing cyanobacterium Scytonema)

appear to be far more important in hotter

deserts than previously recognized.

Understanding the large difference in

microbial composition in biocrusts from dif-

ferent regions will be crucial for managing

these communities under future conditions.

At colder sites, a shift from the dominant M.

vaginatus to M. steenstrupii with warming

could have a large effect on the ecosystem

services provided by biocrusts. The extent of

Some Like It Hot, Some Not

MICROBIOLOGY

Jayne Belnap

The microorganism composition of dryland

soils depends on regional climate.

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U. S. Geological Survey, 2290 South Resource Boulevard, Moab, UT 84532, USA. E-mail: jayne_belnap@usgs.gov

25 µmA B

Hidden variation. (A) Soils covered by biological soil crusts near Moab, Utah. Because of low plant cover in drylands, biologicalsoil crusts can comprise 70% or more of total cover and mediate most inputs and outputs from the soils. (B) Microcoleus

vaginatus. Garcia-Pichel et al. show that biocrusts are dominated by M. vaginatus in colder deserts and M. steenstrupii in hotter deserts. As global temperatures increase, it is likely that M. steenstrupii will replace M. vaginatus in many deserts. Although the two species are morphologically similar, the ecological implication of this replacement is unknown. Movie S1 shows M. vaginatus fi laments moving within a common polysaccharide sheath.

clinical efficacy of ACTs across most of

the malaria-endemic regions of the world;

even in western Cambodia most infections

are reported to clear after ACT treatment.

Malaria elimination remains an achievable

goal, one that is critically dependent on an

expanded and unified vision coordinated

among funders, governments, health care

providers, scientists, and the pharmaceutical

industry. The issue of emerging artemisinin

resistance highlights the necessity to inter-

vene at all levels to prevent a looming disas-

ter, and an opportunity to bring about a truly

global accomplishment that directly or indi-

rectly benefi ts us all.

References

1. N. J. White, Science 320, 330 (2008). 2. A. M. Dondorp et al., N. Engl. J. Med. 365, 1073 (2011). 3. A. P. Phyo et al., Lancet 379, 1960 (2012). 4. C. Amaratunga et al., Lancet Infect. Dis. 12, 851 (2012). 5. P. M. O’Neill, V. E. Barton, S. A. Ward, Molecules 15,

1705 (2010). 6. Q. Cheng, D. E. Kyle, M. L. Gatton, Int. J. Parasitol. Drugs

Drug Resist. 2, 249 (2012). 7. B. Witkowski et al., Antimicrob. Agents Chemother. 57,

914 (2013).

8. I. H. Cheeseman et al., Science 336, 79 (2012). 9. S. Takala-Harrison et al., Proc. Natl. Acad. Sci. U.S.A.

110, 240 (2013). 10. O. Miotto et al., Nat. Genet. 45, 648 (2013). 11. S. Boiteux, S. Jinks-Robertson, Genetics 193, 1025

(2013). 12. P. K. Rathod, T. McErlean, P. C. Lee, Proc. Natl. Acad. Sci.

U.S.A. 94, 9389 (1997). 13. A. M. Vaughan et al., J. Clin. Invest. 122, 3618 (2012). 14. J. Straimer et al., Nat. Methods 9, 993 (2012). 15. B. Gold et al., Proc. Natl. Acad. Sci. U.S.A. 109, 16004

(2012). 16. P. W. Gething et al., Malar. J. 10, 378 (2011).

10.1126/science.1240539

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28 JUNE 2013 VOL 340 SCIENCE www.sciencemag.org 1534

PERSPECTIVES

Translocation in Action

BIOCHEMISTRY

Marina V. Rodnina

Structures of translocation intermediates reveal

how tRNA molecules move through the

ribosome during protein synthesis.

Ribosomes are macromolecular fac-

tories that translate the information

encoded in messenger RNA (mRNA)

into the amino acid sequence of proteins.

Each time an amino acid has been transferred

to the growing peptide chain, the mRNA and

two transfer RNAs (tRNAs) move through

the ribosome one codon at a time. This move-

ment—called translocation—is promoted by

elongation factor G (EF-G). Three papers in

this issue, by Tourigny et al. on page 1542

( 1), Pulk and Cate on page 1544 ( 2), and

Zhou et al. on page 1543 ( 3), present high-

resolution structures of translocation inter-

mediates and provide insights into the under-

lying mechanism.

The translocation process occurs within

milliseconds and entails a large number of

structural rearrangements. During translo-

cation, the small and large ribosomal sub-

units (SSU and LSU) rotate relative to

each other ( 4). The SSU undergoes internal

motions (collectively called swiveling) of its

head domain relative to the body ( 5). The

tRNAs move from the A (aminoacyl) to the

P (peptidyl), and from the P to E (exit) bind-

ing sites, and there are several intermediate

positions that the tRNAs can adopt sponta-

neously ( 4, 6, 7) (see the fi gure). All these

rearrangements are rapid and only loosely

coupled, making it extremely challenging to

obtain structural data on the trajectories of

the movements.

To trap EF-G on the ribosome, all three

groups ( 1– 3) used nonhydrolyzable ana-

logs of guanosine 5′-triphosphate (GTP) and

placed a single tRNA on the mRNA codon

in the P site of the ribosome. The structures

show that the ribosome is trapped in a chi-

meric intermediate state (see the fi gure, panel

B) that differs from the ground states before

(panel A) and after (panel C) translocation.

This means that the tRNAs move through a

series of intermediates not only on the LSU

( 7) but on the SSU as well ( 8), providing new

insight into the mechanics of tRNA translo-

cation. One interesting possibility is that GTP

hydrolysis by EF-G is required to promote the

backward rotation of the SSU head domain

and the movement of the tRNA and mRNA

into the posttranslocation state (see the fi g-

ure, panel C). The results of experiments with

a guanosine triphosphatase (GTPase)–defi -

cient EF-G mutant appear to be consistent

with this idea ( 9).

EF-G is a large, five-domain GTPase

that changes its conformation in response

to GTP hydrolysis (see the fi gure). Without

GTP hydrolysis, translocation is slow and

the release of EF-G from the ribosome is

blocked ( 10). Like all GTPases, EF-G has the

mobile switch 1 and 2 elements in its GTP-

binding domain I. The switch regions are dis-

ordered in the unbound EF-G and become

ordered in the complex with the ribosome.

This transition causes reorientation of the

EF-G domains, such that the tip of domain IV

moves and the intermediate state of the ribo-

some is stabilized (see the fi gure, panel B).

The ribosome on its own allows the

tRNAs to move in the forward or backward

direction. EF-G provides the directionality Max Planck Institute for Biophysical Chemistry, 37077 Goettingen, Germany. E-mail: rodnina@mpibpc.mpg.de

this effect depends on how the two species

differ in their ability to colonize bare soils,

stabilize soils, and affect nutrient, water, and

carbon cycles. In fact, given their genetic

differences, the two species would be bet-

ter placed in different families rather than in

the same genus. However, the little research

done has focused on M. vaginatus, with no

work beyond description on M. steenstrupii,

because its importance in biocrusts has not

been recognized ( 8). M. steenstrupii con-

stitutes a much more diverse phylogenetic

clade than M. vaginatus ( 9), and it is likely to

be much more genetically and functionally

diverse. Renewed efforts should be made to

characterize it in all its complexity.

There is also little information to date on

the ecological consequences of changing the

composition of the nitrogen-fi xing cyanobac-

teria in biocrusts. Garcia-Pichel et al. did not

directly address biogeographic patterns in

these species, but their data show that Scyto-

nema sp. appears favored at sites with higher

temperatures and Tolypothrix sp. at sites

with lower temperatures. Possible outcomes

of replacing Tolypothrix sp. with Scytonema

sp. include alteration of nitrogen, phospho-

rus, and carbon cycles. Again, most research

has focused on one species, the ubiquitous

Nostoc, with little information available for

either Scytonema or Tolypothrix.

This lack of research also hampers efforts

to actively restore disturbed biocrusts. Most

attempts to cultivate and inoculate soils with

cyanobacteria to “kickstart” soil stabiliza-

tion and restoration in areas degraded by

human impact use M. vaginatus and some-

times Nostoc. These efforts are surely at risk

of failure if the site should be inoculated

with M. steenstrupii and Scytonema (or pos-

sibly other species) instead, because culti-

vation, inoculation, and/or postinoculation

techniques could differ substantially among

various species. These situations thus call

for a better understanding of which species

are currently present at a site and their physi-

ological tolerances.

Chemolithotrophic bacteria and Archaea

involved in the nitrogen cycle ( 10) and bio-

crust fungi ( 11) are some other examples of

potentially important groups that we know

little about but that may also play pivotal

roles in the structure and function of bio-

crusts and many other ecosystems. It is time

to tackle the diffi cult job of identifying the

relevant microbes and their distributions

and of establishing their functional roles to

enable better management and restoration of

dryland ecosystems.

References 1. S. B. Pointing, J. Belnap, Nat. Rev. Microbiol. 10, 551

(2012).

2. J. Belnap, O. L. Lange, Eds., Biological Soil Crusts: Struc-

ture, Function, and Management, Ecological Studies

Series 150, series edited by I. T. Baldwin et al. (Springer,

Berlin, 2003).

3. W. Elbert et al., Nat. Geosci. 5, 459 (2012).

4. J. L. Green, B. J. Bohannan, R. J. Whitaker, Science 320,

1039 (2008).

5. J. B. H. Martiny et al., Nat. Rev. Microbiol. 4, 102 (2006).

6. F. Garcia-Pichel et al., Science 340, 1574 (2013).

7. C. Parmesan, Annu. Rev. Ecol. Evol. Syst. 37, 637 (2006).

8. J. Belnap, in Biological Soil Crusts: Structure, Function,

and Management, J. Belnap, O. L. Lange, Eds. (Springer,

Berlin, 2003), pp. 241–261.

9. F. Garcia-Pichel, M. F. Wojciechowski, PLoS ONE 4, e7801

(2009).

10. Y. Marusenko et al., Ecol. Processes 2, 9 (2013).

11. S. L. Collins et al., J. Ecol. 96, 413 (2008).

Supplementary Materials www.sciencemag.org/cgi/content/full/science.1240318/DC1

Movie S1

10.1126/science.1240318

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