Biosynthesis of sulphur amino acids in Saccharomyces cerevisiae: regulatory roles of methionine and S-adenosylmethionine reassessed.

cys4-1, a mutation in the reverse trans-sulphuration pathway, relieves the sulphate assimilation pathway and homocysteine synthase from methionine-mediated repression. Since the mutation blocks the synthesis of cysteine from methionine downstream from homocysteine, this indicates that neither methionine nor S-adenosylmethionine serve as low-molecular-mass effectors in this regulatory system, contradicting earlier hypotheses.