Gene Interactions Affecting Methionine Biosynthesis and the Response to S-methylcysteine by Mutants of Neurospora crassa

Abstract

SUMMARYTwo non-allelic suppressors have been isolated which suppress nonallelic, leaky, met hionine-requiring mu tan t s blocked either be tween cysteine and cystathionine or between cystathionine and homocysteine. Strains carrying either suppressor gene in the presence of the me+ allele were stimulated by either cysteine or methionine, whereas the suppressed methionine-requiring mutants (me su) 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 methioninerequiring mutants inhibited by cysteine. Sulphate or cysteine inhibited the growth response of certain methionine-requiring mutants to Smethylcysteine. 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.