REDUCTION OF HYDROGEN-SULFIDE PRODUCTION IN BREWING YEAST BY CONSTITUTIVE EXPRESSION OF MET25 GENE

Hydrogen sulfide (H2S) is produced by yeast during fermentation and is known as one of the young flavors of beer. The MET25 gene product [i. e., O-acetylhomoserine (OAH), O-acetylserine (GAS) sulfhydrylase], whi ch utilizes H2S in the course of methianine synthesis, was assumed to play a key role in the production of excess HIS. We therefore attempte d to diminish HIS production in brewing by controlling the MET25 gene expression. In the present study, the MET25 gene was brought under con trol of a constitutive promoter and was introduced into a brewing yeas t strain. Resulting transformants showed a level of MET25 expression s everal times higher than that of the parental strain. In pilot-scale e xperiments, these transformants were confirmed to produce 10X less H2S than the parental strain did. This result wits attributable to the de crease in HIS production by yeast cells at the early stage of fermenta tion. Control of the MET25 gene may be one way to shorten the maturati on period.