Summary
5-methyltetrahydrofolate, the product of a reaction catalysed by 5-methyltetrahydrofolate: FAD oxidoreductase (metF), is the methyl donor in the transmethylation of homocysteine in Salmonella typhimurium either via a vitamin B12 dependent (metH) or independent (metE) pathway. Both the metF and H enzymes were shown to be repressible by methionine.
B12 was found to repress synthesis of the metF enzyme in some metH mutants but not in others although all lacked B12-dependent 5-methyltetrahydrofolate homocysteine transmethylase. This suggested a dual enzymatic and regulatory role for the metH gene but no complementation was detected between any metH mutants.
The levels of metF and H enzymes were elevated in mutants blocked at early stages in methionine synthesis. Also the metH enzyme level in a metF mutant was increased by the addition to the medium of known precursors unable to support its growth, suggesting precursor induction of the enzymes. This increase did not occur in the presence of chloramphenicol.
The different regulatory systems involved in the methylation of homocysteine could reflect the importance of this step in the inter-relationship of different metabolic pathways.
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Whitehouse, J.M., Smith, D.A. Methionine and vitamin B12 repression and precursor induction in the regulation of homocysteine methylation in Salmonella typhimurium . Molec. Gen. Genet. 120, 341–353 (1973). https://doi.org/10.1007/BF00268148
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DOI: https://doi.org/10.1007/BF00268148