Cysteine and growth inhibition of Escherichia coli: threonine deaminase as the target enzyme

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Abstract
Cysteine inhibits growth in E. coli strains C6 and HfrH 72, but not M108A. Growth inhibition was overcome by inclusion of isoleucine, leucine and valine in the medium. Isoleucine biosynthesis was apparently affected, since addition of this amino acid alone could alter the inhibitory effects of cysteine. Homocysteine, mercaptoethylamine and mercaptoethanol inhibited growth to varying degrees in some strains, these effects also being prevented by addition of branched-chain amino acids. Cysteine, mercaptoethylamine and homocysteine were inhibitors of threonine deaminase but not transaminase B, 2 enzymes of the ilvEDA operon. Cysteine inhibition of threonine deaminase was reversed by threonine, although the pattern of inhibition was mixed. A relationship between the growth-inhibitory effects of cysteine and other sulfur compounds and the inhibition of isoleucine synthesis at the level of threonine deaminase is suggested.