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Debaryomyces (Torulaspora) hansenii is a cryotolerant, marine yeast, which can tolerate salinity levels up to 24%, whereas Saccharomyces cerevisiae growth is inhibited when salinity reaches 10%. Cryo- and osmotolerance account for its important role in several agro-food processes. Indeed, D. hansenii is the most common species found in all types of cheeses, unlike other yeast species, the prevalence of which is cheese dependent [1]. D. hansenii is also common in dairies and in brine [2] because it is able to grow in the presence of salt at low temperature and to metabolize lactic and citric acids. D. hansenii also provides proteolytic and lipolytic activities during cheese ripening. In addition, D. hansenii is one of the most frequent yeast species to be associated with chilled food [3]. Although D. hansenii is considered as non-pathogenic, one case of bone infection [4] associated with this yeast was reported and several clinical samples were identified as D. hansenii (and its anamorph Candida famata) in superficial infections [5]. C. famata is able to overproduce riboflavin (vitamin B2) and riboflavin-overproducing mutants of this species are used for industrial riboflavin production. D. hansenii is also an alkane-assimilating yeast.

Although a transformation system has been described, molecular genetic studies are still in their infancy in this species: excluding rDNA sequences, 46 gene entries corresponding to 28 different proteins existed in Genbank before release of the whole genome data, mostly related to stress responses and carbon metabolism.

Most D. hansenii strains appear haploid, mate very rarely and diploidize transiently by somatogamous autogamy to form asci containing generally a single spore [6],[7]. D. hansenii carries two types of linear DNA plasmids whose stability depends for one of them on high osmotic pressure [8],[9]. Recent phylogenetic analysis has shown that the Debaryomyces genus is no longer monophyletic and D. hansenii defines now one of the four clades which constitute this genus [10]. The species contains two varieties var. hansenii and var. fabryi, the second of them is not very often found and is poorly characterized. D. hansenii belongs to the large clade of yeasts species comprising most of the Candida and Pichia species. From a phylogenetic viewpoint, D. hansenii is closely related to the pathogenic Candida albicans, with which it shares several characteristics, and closest to yeasts formerly classified as Schwanniomyces, like Debaryomyces (Schwanniomyces) occidentalis.

The sequence of the genome of the type strain D. hansenii var. hansenii CBS767 revealed that the genome is made of seven chromosomes ranging from 1.25 Mb to 2.33 Mb with a total size of 12.2 Mb not including ribosomal DNA (rDNA). Three types of rDNA units were found that have an internal location within the chromosomes.

D. hansenii uses an alternative genetic code in which the CUG codon (leucine) is used as a serine codon and is read by the special, single copy tRNA-Ser (CAG), as in Candida albicans. With 205 tRNA genes corresponding to 43 types, it has the largest number of potentially co-transcribed tRNA gene pairs of the yeasts studied in this project.

D. hansenii seems to have the highest coding capacity among yeasts, amounting to 79.2% of the genome with a putative number of 6906 detected CDS. D. hansenii is also the yeast with the most redundant genome with an overall redundancy of 49.2%. Tandem gene duplications are 5 to 10 times more frequent than in any other yeast, whereas few duplicated blocks are detectable. As already detected in Génolevures project [11], allantoate transporters define the most over represented class of proteins.

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10. Kurtzman, C.P. and Robnett, C.J. (1998) Antonie Van Leeuwenhoek 73, 331-71.
11. Lepingle, A., Casaregola, S., Neuveglise, C., Bon, E., Nguyen, H., Artiguenave, F., Wincker, P. and Gaillardin, C. (2000) FEBS Lett 487, 82-6.