Abstract
SUMMARY: Salmonella typhimurium, like Escherichia coli, has a methionine-regulated β-aspartokinase and homoserine dehydrogenase in addition to threonine-sensitive and lysine-sensitive aspartokinase isoenzymes and a threonine-sensitive homoserine dehydrogenase. These methionine-regulated enzymes are not subject to feedback inhibition by methionine (or by methionine and S-adenosylmethionine), but the homoserine dehydrogenase is inhibited by the methionine precursor cysteine and to a lesser extent by homocysteine. Studies of enzyme concentrations in methionine analogue-resistant strains show that mutations which lead to de-repression of the later methionine-forming enzymes also result in de-repressed synthesis of the methionine-regulated β-aspartokinase and homoserine dehydrogenase, although the methionine-forming enzymes as a whole are not subject to co-ordinate control.
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