Possible Role for Allelic Variation in Yeast MED15 in Ecological Adaptation

Abstract

The propensity for Saccharomyces cerevisiae yeast to ferment sugars into ethanol and CO2 has long been useful in the production of a wide range of food and drink. In the production of alcoholic beverages, the yeast strain selected for fermentation is crucial because not all strains are equally proficient in tolerating fermentation stresses. One potential mechanism by which domesticated yeast may have adapted to fermentation stresses is through changes in the expression of stress response genes. MED15 is a general transcriptional regulator and RNA Pol II Mediator complex subunit which modulates the expression of many metabolic and stress response genes. In this study, we explore the role of MED15 in alcoholic fermentation. In addition, we ask whether MED15 alleles from wine, sake or palm wine yeast improve fermentation activity and grape juice fermentation stress responses. And last, we investigate to what extent any differences in activity are due to allelic differences in the lengths of three polyglutamine tracts in MED15. We find that strains lacking MED15 are deficient in fermentation and fermentation stress responses and that MED15 alleles from alcoholic beverage yeast strains can improve both the fermentation capacity and the response to ethanol stresses when transplanted into a standard laboratory strain. Finally, we find that polyglutamine tract length in the Med15 protein is one determinant in the efficiency of the alcoholic fermentation process. These data lead to a working model in which polyglutamine tract length and other types of variability within transcriptional hubs like the Mediator subunit, Med15, may contribute to a reservoir of transcriptional profiles that may provide a fitness benefit in the face of environmental fluctuations.

The role of Med15 sequence features in transcription factor interactions

“…While shorter stretches of glutamine are present in Med15, Q1, Q2 and Q3 stand out because they vary in length (number of consecutive Q or QA residues) in MED15 alleles from different strains of S. cerevisiae [21]. Recent studies have addressed the functional implications of naturally occurring tract length variant alleles of MED15 on resistance to the coal-cleaning chemical, 4-methylcyclohexane methanol [32], and grape juice fermentation [33]. Here we investigate which additional activities of Med15 require these glutamine-rich regions, and the extent to which variations to the polyglutamine tracts in Med15 influence these protein activities.…”