Phenotypic characterization of the Saccharomyces cerevisiae Mediator subunit Med15 in fermentation and the role of glutamine bias in Med15 orthologs

Wine fermentation is a process in which yeast consumes glucose and fructose in grape juice and produces carbon dioxide and ethanol. Along with the production of ethanol, other toxic byproducts also get produced during fermentation, such as acetic acid and oxidative compounds. To survive in the fermentation environment, yeast cells change their gene expression and activate stress-response genes. One key protein complex involved in such gene regulation is the Mediator complex. All eukaryotes, including human, use the Mediator complex to regulate gene expression. Understanding how the Mediator complex works will provide important insight into the eukaryotic gene regulatory network. The Mediator complex consists of multiple subunits, with each subunit contributing part of Mediator functionality. One key subunit is Med15 which is important for regulating genes involved in carbon metabolism and yeast stress resistance. In my thesis work I investigated the role of yeast Med15 in grape juice fermentation and found that yeast cells with functional Med15 protein fermented faster and produced more ethanol and byproducts than the yeast without functional Med15. My results show that Med15 not only regulate stress resistance, it also regulates fermentation efficiency. the Med15 protein has an unusual amino acid composition, with a strong bias toward glutamine and other disorder promoter residues. In addition, the MED15 gene is fast evolving and the MED15 counterpart in other organisms is difficult to identify. I found that the Med15 proteins encoded by other eukaryotic genomes are also glutamine-enriched and highly disordered. Nonetheless, I was able to show that Med15 proteins from other organisms including human, are partially functional in yeast. These results suggest that the functionality of Med15 is at least partially based on the conservation of glutamine richness and disorder and not on specific amino acid composition.