Inactivating mutations in Irc7p are common in wine yeasts, attenuating carbon-sulfur β-lyase activity and volatile sulfur compound production

During alcoholic fermentation of grape sugars, wine yeast produce a range of secondary metabolites that play an important role in the aroma profile of wines. In this study, we have explored the ability of a large number of wine yeast strains to modulate wine aroma composition, focusing on the release of ‘fruity’ thiols 3-mercaptohexan-1-ol (3-MH) and 4-mercapto-4-methylpentan-2-one (4-MMP) from their respective cysteinylated non-volatile precursors. The role of yeast gene IRC7 in thiol release has been well established, and it has been shown that a 38bp deletion found in many wine strains cause them to express a truncated version of Irc7p which does not possess cysteine- S -conjugate β-lyase activity. In our data we find that IRC7 allele length alone does not fully explain the capacity of a strain to release thiols. Screening of a large number of strains coupled with analysis of genomic sequence data allowed us to identify several previously undescribed single nucleotide polymorphisms in IRC7 that, when coupled with allele length, more robustly explain the ability of a particular yeast strain to release thiols from their cysteinylated precursors. We also demonstrate that allelic variation of IRC7 not only affects the release of thiols, but modulates the formation of negative volatile sulfur compounds from the amino acid cysteine. The results of this study provide winemakers an improved understanding of the genetic determinants that affect wine aroma and flavour, which can be used to guide the choice of yeast strains that are fit-for-purpose. Importance Volatile sulfur compounds contribute to wine aromas that may be considered pleasant, such as ‘tropical’, ‘passionfruit’ and ‘guava’, as well as aromas that are considered undesirable, such as ‘rotten-eggs’, ‘onions’, and ‘sewer’. During fermentation, wine yeasts release some of these compounds from odourless precursor molecules, a process that is most efficient when performed by yeasts that express active forms of the protein Irc7p. We show that most wine yeasts carry mutations that reduce activity of this protein, affecting the formation of volatile sulfur compounds that impart both pleasant, and unpleasant aromas. The results provide winemakers guidance on the choice of yeasts that can emphasize or de-emphasize this particular contribution to wine quality.