Biosynthesis of sulphur amino acids in Saccharomyces cerevisiae II. Analysis of suplhite-producing strains.

A number of strains of Saccharomyces which produce sulphite by sulphate reduction were examined from an enzymatic and genetic point of view. There are a number of mechanisms that regulate this activity. All of these mechanisms involve the sulphite-reducing activity. In the strains examined, reduced function as a result of mutation in the Sr-locus (affecting H2S-NADP oxidoreductase EC 1.8.1.2), repression of biosynthesis of the enzyme because of a mutation below the specific locus, and inhibition of the enzyme by endogenous factors were found to be responsible. The production of sulphite can also be connected with a complex state of heterozygosity. It is probably this multiplicity of biochemical and genetic mechanisms that accounts for the frequency with which the production of sulphite is observed in wild strains in nature.