Biosynthesis of sulphur amino acids in Saccharomyces cerevisiae

The hypothesis of an alternative pathway of sulphur amino acid synthesis as the basis of the prototrophy of sulphite reductase negative (Sr^-) strains of Saccharomyces cerevisiae has been rejected. Met^- mutants obtained after phenylmercuric nitrate treatment of Sr^- strains accumulate H₂S as the consequence of a metabolic block which leads to methionine auxotrophy. This mutation has been shown to be independent of the Sr locus. We assume that the molecular basis of the prototrophy of Sr^- strains resides in a leaky missense induced in the Sr gene.