SUMMARY: Nineteen mutants of responding to either cysteine or methionine (cym) have been identified amongst cysteine () and methionine () auxotrophs. Their growth responses to known intermediates in the related pathways of cysteine and methionine biosynthesis and complementation patterns in abortive transduction tests divided the mutants into six groups. Results of conjugation, cotransduction and deletion mapping experiments substantiated these groups, each of which carried a lesion within known genes. Enzyme assays on cym mutants from five of the six groups confirmed their gene deficiencies. Growth response and enzyme assay data were not consistent with cym mutants being leaky mutants (spared by methionine). None of eight cym mutants tested were able to convert [S]methionine into [S]cysteine. Selenate specifically inhibits the early enzymes of cysteine synthesis. In cym mutants this inhibition was relieved by cysteine but not by methionine, indicating that cym mutants require active enzymes for growth on methionine. There was evidence that methionine stimulated activity of enzymes in a cym mutant. Resistance to inhibition by 1,2,4-triazole results in reduced levels of the -acetyl serine sulphydrylase. In cym mutants triazole resistance gave unstable suppression of the cym phenotype. Cym mutants may result from mutation in regulatory regions common to each of the genes, with the precise role of methionine as yet unknown.


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