SUMMARY: Two non-allelic suppressors have been isolated which suppress non-allelic, leaky, methionine-requiring mutants blocked either between cysteine and cystathionine or between cystathionine and homocysteine. Strains carrying either suppressor gene in the presence of the allele were stimulated by either cysteine or methionine, whereas the suppressed methionine-requiring mutants () although stimulated by methionine were inhibited by cysteine. S-methylcysteine supported the growth of leaky methionine-requiring mutants when it was present as the sole sulphur source and it also stimulated the growth of suppressed methionine-requiring mutants inhibited by cysteine. Sulphate or cysteine inhibited the growth response of certain methionine-requiring mutants to S-methylcysteine. The incorporation of radioactive sulphate into protein methionine was inhibited to a greater extent by S-methylcysteine than was its incorporation into protein cysteine. The results suggest that the sulphur of S-methylcysteine is converted to methionine without prior conversion to cysteine and that the suppressors act by retarding the formation of an inhibitor which accumulates as a result of the primary mutation to methionine requirement.


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