S HO R T COMMUN I C AT I ON

Kluyveromyces lactis ^ a retrospectiveHiroshi Fukuhara

Institut Curie, Section de Recherche, UMR2027, Centre Universitaire Paris XI, Orsay, France.

Tel.: 133 1 69 86 3063; fax: 133 1 69 86 9429; e-mail: hiroshi.fukuhara@curie.u-psud.fr

Received 27 April 2005; accepted 31 May 2005.

First published online 28 November 2005.

doi:10.1111/j.1567-1364.2005.00012.x

Editor: Lex Scheffers

Keywords

Kluyveromyces lactis; model yeast; nonconventional yeast.

The use of Kluyveromyces lactis for research started in

early 1960s. In contrast to most cases of yeast research, the

study of this particular species was initially motivated by a

purely academic question, that is, possible adaptive regula-

tion of sugar metabolism in a lower eukaryote. Biotechno-

logical interest in K. lactis came much later. Until about

1980, K. lactis research was barely visible in the shadow of

the formidable development of the Saccharomyces cerevisiae

system.

The early 1960s were the era of lactose regulation in

Escherichia coli, which led to the birth of molecular biology.

Following the achievements of the investigators of the E. coli

system, a number of laboratories were trying to find an

inducible enzyme system in a eukaryotic organism in order

to evaluate the general significance of the operon concept.

Harlyn O. Halvorson at Madison, Wisconsin, was one of

those people. He contacted the great taxonomist L. J.

Wickerham (USDA, Peoria) who knew how different yeast

species assimilated various sugars. Apparently, it was he who

suggested the use of K. lactis, a species that assimilated b-glucosides in an adaptive mode. Halvorson and his collea-

gues have thus started to work on this yeast (then called

Saccharomyces lactis), using two isolates obtained from

Peoria, NRRL Y-1140 (CBS 2359 [Mat a]) and Y-1118 (CBS

6315 [Mat a]). A mating system for genetic analysis waselaborated. The b-glucosidase system of K. lactis turned outto be complicated by the fact that it was paraconstitutive

(half-inducible, half-constitutive). After this pioneer work

of the Madison group, only a few laboratories continued to

use K. lactis, essentially in the field of mitochondrial genetics

and biogenesis, in comparison with the S. cerevisiae system.

After all, yeast species available for formal genetic analysis

were rare and are still few even now: apart from the fission

yeast Schizosaccharomyces pombe and the nonfermenting

Yarrowia lipolytica, we have practically only K. lactis.

In the early 1980s, two lines of study, specific to K. lactis,

revived interest in this yeast. One was the series of works on

the regulation of lactose metabolism, and the other was the

discovery of the new killer system involving DNA plasmids.

The studies on the lactose regulon in K. lactis illustrated how

the regulatory system of this unicellular eukaryote differed

from the bacterial lactose operon. The lactose regulon, a

close variation of the galactose regulon of S. cerevisiae, was

shown to involve many genetically unlinked positive and

negative regulatory genes. In parallel, the works of the killer

system involving the linear DNA plasmids pGKL1 and 2 had

also played an important role in the development of K. lactis

biology, because at that time the only plasmids known in

yeast were the 2 mm circular DNA and the double-strandedkiller RNA of S. cerevisiae. In addition to their new killing

mechanism and their linear mode of replication, pGKL

plasmids were interesting in many aspects. The fact that K.

lactis secreted a high-molecular weight killer toxin retained

the attention of a few people who were looking for an

efficient system to produce recombinant proteins in a

secreted form. Indeed the 1980s were years that were full of

enthusiasm for gene engineering for biotechnology. Thus,

the milk-coagulating enzyme chymosin was produced in-

dustrially from K. lactis. Such achievements by a few

industrial companies encouraged research on this particular

yeast system. The successive biotechnology programs

funded by the European Commission were a timely support,

and it was through these programs that the first network of

K. lactis research was set up in 1988. Since then the work-

shop, Biology of Kluyveromyces, continues to be an im-

portant instrument of communication and collaboration of

the research community.

Collaboration between K. lactis workers has been facili-

tated by two circumstances. First, the researchers have used,

from the beginning, only a very small number of K. lactis

isolates, including those used by the Madison group, thus a

relatively homogeneous genetic system has rapidly emerged

and has been shared by most laboratories, allowing the

efficient exchange of strains. Second, after a deliberate

FEMS Yeast Res 6 (2006) 323324 c 2005 Federation of European Microbiological SocietiesPublished by Blackwell Publishing Ltd. All rights reserved

search, a 2 mm type circular plasmid pKD1 was discovered inthe species Kluyveromyces drosophilarum (hence the name of

the plasmid), which was capable of replicating in K. lactis,

offering a replicating vector system equivalent to the 2 mmvectors. Kluyveromyces drosophilarumwas later incorporated

into K. lactis.

The yeast species that assimilate lactose aerobically are

widespread, but those that ferment lactose are rather rare.

Beside K. lactis, Kluyveromyces fragilis is one of such lactose-

fermenting yeasts, and is well known in industry. This

species is now incorporated into K. marxianus (E. C.

Hansen) van der Walt (1971), distinct from K. lactis. At

present the taxonomists recognize two varieties in K. lactis.

One is K. lactis (Dombrowski) van der Walt var. lactis

(1986), the other is K. lactis (Dombrowski) van der Walt

var. drosophilarum (1986). Whereas the former is hetero-

thallic and ferments lactose, the latter is homothallic and

does not assimilate lactose (Lachance, 1998). Nearly all the

published works on K. lactis concern the variety lactis.

If S. cerevisiae stands in a very exceptional position among

yeasts because of its fermentation-oriented Crabtree-posi-

tive physiology, K. lactis appears to be a good model of the

large number of more aerobic species that are used in todays

yeast biotechnology. At the opposite extreme, the fermenta-

tion-less Yarrrowia lipolytica may be a model for highly

aerobic species, with its well-established genetic system.

While continuously stimulated by industrial interests, the

present research in K. lactis mostly focuses on fundamental

aspects of physiology and gene regulation. The themes cover

a wide range of topics, and the articles gathered in this issue

do not represent the diversity of the ongoing research in this

area.

The total DNA sequence of the K. lactis genome has been

established in 2004 through the Genolevures project. Thus,

K. lactis has become, after S. cerevisiae, a most useful

instrument of yeast study, combining both genetic and

genomic information. In view of the distinctive physiologies

of the two species, and their relatively recent common origin

suggested by genomic sequence analyses, the comparison of

these two species will be particularly useful to unveil details

of the process of evolution of gene regulation and genome

organisation. Such a comparative approach is a basic

practice of K. lactis workers, as most of them are active in S.

cerevisiae research and also exploring the biology of other

nonconventional species.

Reference

Lachance MA (1998) The Yeasts, ATaxonomic Study. 4th edn

(Kurtzman CP & Fell JW, eds), pp. 227247. Elsevier Science B.

V., Amsterdam.

FEMS Yeast Res 6 (2006) 323324c 2005 Federation of European Microbiological SocietiesPublished by Blackwell Publishing Ltd. All rights reserved

324 H. Fukuhara