SUMMARY: L-Cysteine became toxic to the growth of (wild, Em 5297), in the range 1·0–2·0 mM in the culture medium. The specificity of cysteine toxicity was shown by absence of toxicity with other sulphydryl compounds (β-mercaptoethanol, thioglycollic acid, reduced glutathione) and with cysteine metabolites, L-cysteic acid and taurine, under similar conditions. The toxicity of L-cysteine was completely overcome by supplements of S-methyl-L-cysteine and to a marked extent by DL-methionine and DL-homocysteine; partial counteraction of cysteine toxicity was observed with L-serine, DL-tryptophan, DL-alanine, DL-valine, DL-homoserine or DL-threonine. DL-Methionine and S-methyl-L-cysteine counteracted the inhibitory effect of L-cysteine in two mutants, namely, methionineless mutant 38706 and cystathionineless mutant 9666.


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