The yeast IRC7 gene encodes a β-lyase responsible for production of the varietal thiol 4-mercapto-4-methylpentan-2-one in wine

Three varietal thiols are key aroma compounds in Sauvignon Blanc wines: 4-mercapto-4-methylpentan-2-one (4MMP), 3-mercaptohexanol (3MH) and its acetylated derivative 3-mercaptohexyl acetate (3MHA). Screening of Saccharomyces cerevisiae strains identified a clinical isolate with elevated 4MMP production after fermentation. Bulked Segregant Analysis of a cross between this isolate and the laboratory strain revealed a single major locus for 4MMP production near the telomere of chromosome 6. Deletion of the IRC7 gene from this region in YJM450 reduced 4MMP production below detectable levels, but did not affect yields of 3MH, in Sauvignon Blanc wine. Sequencing revealed that the IRC7 gene in YJM450 had been introgressed from a strain of Saccharomyces paradoxus. Most strains of S. cerevisiae, including the laboratory strain S288C, have a 38-bp deletion that inactivates IRC7. Overexpression of a full-length S. cerevisiae allele of IRC7 in a wine yeast, Zymaflore F15, increased 4MMP production in Sauvignon Blanc wine from undetectable levels (<10 ng L−1) to concentrations of 1000 ng L−1, and also increased 3MH and 3MHA. Biochemical analysis of soluble protein extracts showed that both the cerevisiae and paradoxus IRC7 proteins show β-lyase activity, with a substrate preference for cys-4MMP over cys-3MH.

► Production of the varietal thiol 4MMP by a yeast strain maps to the IRC7 gene
► An IRC7 gene has been introgressed into Saccharomyces cerevisiae from S. paradoxus
► Deletion of IRC7 in yeast results in loss of detectable 4MMP in wine
► Overexpression of IRC7 in yeast gives elevated 4MMP levels in Sauvignon Blanc
► IRC7 exhibits β-lyase activity in vitro with a preference for cys-4MMP over cys-3MH.