Identification of Yeast Genes Responsible for Low H2S production in Wine

Abstract

The unpleasant, rotten-egg odour of hydrogen sulfide (H2S) produced by yeast during fermentation is a major problem for the global wine industry. In this project, 192 progeny of two crosses between commercial wine yeast strains, F15xM2 and E4xF15, were used to map genes that affect H2S production. Genetic analysis identified three loci, centred on the MET2, MET5 and MET10 genes, which are responsible for low H2S production. Yeast progeny with a MET2 allele from M2 (or E4) produced less H2S during fermentation and normal levels of SO2 (~36 ppm). The yeast MET2 gene encodes homoserine O-acetyl transferase (HTA), which catalyses the conversion of homoserine to O-acetyl homoserine. The single amino acid mutation – MET2-R301G in M2 (and E4) – is suspected to increase the activity of HTA and lower H2S formation. Yeast strains that also carried the MET5 allele (MET5-V288del) from M2 (or E4) reduced H2S even further. Yeast strains inheriting the MET10 allele from E4 produced no detectable H2S during fermentation, and less methionol, carbon disulfide, methyl thioacetate in finished wine. But these strains produced high levels of SO2 (~115 ppm) and grew slowly in synthetic grape media with sulfate as the sulfur source. The yeast MET10 gene encodes the α catalytic subunit of sulfite reductase, which reduce sulfite to sulfide. The single amino acid mutation – MET10-G687D in E4 is suspected to decrease enzyme activity and eliminate H2S formation in wine. Yeast strains that overexpress yeast IRC7 alleles also produced more H2S. It is suggested that the yeast IRC7 protein, a β-lyase that is important for thiols release, can cleave cysteine into H2S, pyruvate and ammonia. Of the two sulfur-containing amino acids, added cysteine induced H2S and SO2 production in wine; in contrast, methionine supressed both H2S and SO2. Mutations in both MET5 and MET10 affected BiGGY colour scores, but alleles of MET2 that affect H2S production did not. This result is consistent with previous conclusions that BiGGY plate scores primarily reflect the sulfite reductase activity in yeast strains. The new alleles of MET2, MET5 and MET10 identified here offer significant advantages for developing improved H2S-preventing wine yeast strains.