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Press Info item.
24/11/2009
How the yeasts responsible for vinification have adapted to their environment
Three INRA research teams, working in collaboration with the French National Sequencing Centre, have sequenced the genome of a wine yeast, Saccharomyces cerevisiae, which is also well known for its use in bread-making to make dough rise. This study revealed how Saccharomyces cerevisiae has adapted to the drastic conditions of grape must and fermentation; indeed, its genome has evolved in spectacular fashion, and this has included the recuperation of genes from other yeasts, once of which (a common wine contaminant) has been identified. Adaptation to wine has required refashioning of the Saccharomyces cerevisiae genome, in which transfers of genes between the yeasts present in wine have played a determining role.
This work has revealed a hitherto unsuspected capacity for evolution in Saccharomyces cerevisiae, because it is the first time that the acquisition of eukaryote genes belonging to distant species has been demonstrated in this species. A clearer understanding of these molecular transfer mechanisms, and their resulting adaptation, should facilitate the selection and improvement of vinification yeasts.
This work was published in the "Proceedings of the National Academy of Sciences" journal on September 9, 2009.
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Saccharomyces cerevisiae yeasts play an essential role in the development of wines. They are responsible for transforming the sugars in grape must into alcohol, and are the source of a variety of molecules that impact the sensory properties of a wine. The addition of Saccharomyces cerevisiae strains selected from spontaneous fermentations in grape must (seeding) is a widely-employed operation that helps to ensure the satisfactory course of fermentation.
However, grape must is not a medium that is conducive to the proliferation of these micro-organisms. Indeed, it is a stressful medium because it is very acid, excessively rich in sugars and alcohol, containing sulphites and very low levels of essential compounds (nitrogen, lipids and vitamins), and is devoid of oxygen. The yeasts used for vinification result from stringent selection by the environment and patient selection by man. But how do they manage to adapt to this hostile environment and produce the aromas sought by wine-makers?
To answer this question, three INRA research teams in Montpellier, Versailles-Grignon and Colmar, working in partnership with the National Sequencing Centre (Centre national de séquençage, at the Génoscope in Evry), sequenced and analysed the genome of Saccharomyces cerevisiae EC1118, one of the strains most widely used to inoculate grape must. They then compared its genome with that of a laboratory reference strain.
Comparison of these genomes showed that the EC1118 genome differed in numerous ways from that of the reference Saccharomyces cerevisiae strain. First of all, several dozen mutations separated them. The chromosomes of EC1118 displayed several rearrangements compared with the reference strain, and particularly one translocation which allowed enhanced resistance to sulphites by modifying the expression of a key gene. The most surprising difference was the acquisition of three large regions comprising several dozen new genes that play a role during grape must fermentation as they are active during this phase. The predicted functions of these genes may endow them with an ecological advantage in grape must: for example, one of the "new" genes allows wine yeasts to utilise fructose, which is the most abundant sugar present at the end of fermentation.
Although the first two types of modification were predictable, the acquisition of new genes was totally unexpected as an evolutionary mode in a yeast.
The fact that these "new" genes were found in many other wine yeast strains, but not in laboratory strains of S. cerevisiae, and that they have been conserved over time, suggests that they contribute major advantages.
The scientists also determined clues to an explanation for the origin of these "new genes" in EC1118: they arose from other yeast species, some of them genetically distant from Saccharomyces cerevisiae. One of the donors is a yeast belonging to a close, as yet unknown species, while the others are more distant from Saccharomyces. It was possible to identify one of these yeasts as Zygosaccharomyces bailii, which is frequently encountered as a wine contaminant. Thus cohabitation in wine might have facilitated the acquisition of genes, probably through cellular fusion.
Knowledge of the gene repertoire and specificities of EC1118 will enable identification of the mechanisms that have participated in the evolution and adaptation of these strains during the winemaking process.
The challenge of current studies is to link sequence variations with the properties of different strains. From an industrial point of view, this knowledge is essential to identifying genes crucial to technological properties, and will constitute a valuable aid in the selection of strains.
This work also offers new prospects regarding the improvement of strain performance and the organoleptic qualities of wine.
For more information:
Eukaryote-to-eukaryote gene transfer events revealed by the genome sequence of the wine yeast Saccharomyces cerevisiae EC1118
PNAS 2009 106:16333-16338
Published on-line: http://www.pnas.org/content/106/38/16333.full.pdf+html?sid=711b244b-59fd-457c-9b36-57df3b5061c7
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