INACTIVATION OF MET2 IN BREWERS-YEAST INCREASES THE LEVEL OF SULFITE IN BEER

Brewer's yeasts sometimes produce inadequate or excessive amounts of s ulfate, an antioxidant and flavour stabilizer, so means of controlling the sulfite production are desired. Understanding the physiology and regulation of the sulfur assimilation pathway of Saccharomyces yeasts is the key to change sulfate production. The MET2 gene of Saccharomyce s yeasts encodes homoserine O-acetyl transferase, which catalyzes the conversion of homoserine to O-acetyl homoserine which in turn combines with hydrogen sulfide to form homocysteine, the immediate precursor o f methionine. We expected that inactivation of MET2 would lead to accu mulation of sulfide and derepression of the entire sulfur assimilation pathway and, therefore, possibly also to sulfite accumulation. Brewer 's yeasts were constructed in which several of the four MET2 gene copi es were inactivated. Sulfite production was increased in strains with one remaining MET2 gene and even more so when no active MET2 was prese nt. In both cases, hydrogen sulfide production was also increased. To the extent that excess sulfide can be removed, this strategy may be ap plied to control sulfite accumulation by brewer's yeast in beer produc tion.