Amino acid interrelationships in cysteine toxicity in Neurospora crassa

Abstract

L-Cysteine became toxic to the growth of Neurospora crassa (wild, Em 5297a), 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 N. crassa mutants, namely, methionineless mutant 38706 and cystathionineless mutant 9666.