Biosynthesis of sulphur amoni acids in Saccharomyces cerevisiae. I. Genetic analysis of leaky mutants of sulphite reductase.

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- mutant resides in a leaky missense induced in the Sr gene.