Genes encoding enzymes of the sulfate reduction pathway in Saccharomyces cerevisiae immediately downstream of sulfite reductase, MET6, MET17, and CYS4, were sequenced in 12 wine isolates of this yeast. The MET17 alleles were identical in sequence to each other and to the sequence of the standard laboratory strain, S288C. For one additional commercial strain, a disruption of one of the MET17 alleles was found, but the other allele was identical to the consensus sequence. All 12 strains showed the identical five neutral base pair changes in CYS4 sequence when compared to the sequence reported for S288C. One strain contained an additional base pair change that led to an amino acid change. Two neutral base pair changes were observed in the sequences of MET6 for one wine yeast strain and three other strains had changes in sequence that were not neutral and altered the amino acid sequence. Regulatory regions upstream of these genes were also sequenced for a subset of strains. Several differences in noncoding regions were found, a few of which resulted in changes in putative transcription factor binding sites. Genes encoding different alleles were used to transform a corresponding null mutation of S288C and enzyme activity and hydrogen sulfide production evaluated. The CYS4UCD932 allele resulted in faster fermentation rates and reduced hydrogen sulfide production when compared with the same strain transformed with CYS4S288C. The MET6 alleles showed no effects on sulfide formation in a null background.
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