MET17 and Hydrogen Sulfide Formation in Saccharomyces cerevisiae

Abstract

Commercial isolates of Saccharomyces cerevisiae differ in the production of hydrogen sulfide (H 2 S) during fermentation, which has been attributed to variation in the ability to incorporate reduced sulfur into organic compounds. We transformed two commercial strains (UCD522 and UCD713) with a plasmid overexpressing the MET17 gene, which encodes the bifunctional O-acetylserine/O-acetylhomoserine sulfhydrylase (OAS/OAH SHLase), to test the hypothesis that the level of activity of this enzyme limits reduced sulfur incorporation, leading to H 2 S release. Overexpression of MET17 resulted in a 10- to 70-fold increase in OAS/OAH SHLase activity in UCD522 but had no impact on the level of H 2 S produced. In contrast, OAS/OAH SHLase activity was not as highly expressed in transformants of UCD713 (0.5- to 10-fold) but resulted in greatly reduced H 2 S formation. Overexpression of OAS/OAH SHLase activity was greater in UCD713 when grown under low-nitrogen conditions, but the impact on reduction of H 2 S was greater under high-nitrogen conditions. Thus, there was not a good correlation between the level of enzyme activity and H 2 S production. We measured cellular levels of cysteine to determine the impact of overexpression of OAS/OAH SHLase activity on sulfur incorporation. While Met17p activity was not correlated with increased cysteine production, conditions that led to elevated cytoplasmic levels of cysteine also reduced H 2 S formation. Our data do not support the simple hypothesis that variation in OAS/OAH SHLase activity is correlated with H 2 S production and release.