Role of the metF and metJ genes on the vitamin B 12 regulation of methionine gene expression: Involvement of N 5-methyltetrahydrofolic acid

Abstract

The repression of MetE synthesis in Escherichia coli by vitamin B 12 is known to require the MetH holoenzyme (B 12-dependent methyltransferase) and the metF gene product. Experiments using trimethoprim, an inhibitor of dihydrofolate reductase, show that the MetF protein is not directly involved in the repression, but that N 5-methyltetrahydrofolic acid (N 5-methyl-H 4-folate), the product of the MetF enzymatic reaction is required. Since the methyl group from N 5-methyl-H 4-folate is normally transferred to the MetH holoenzyme to form a methyl-B 12 enzyme, the present results suggest that a methyl-B 12 enzyme is involved in the vitamin B 12 repression of metE expression. Other results argue against the possibility that a methyl-B 12 enzyme functions in this repression solely by decreasing the cellular level of homocysteine, which is required for MetR activation of metE expression. Experiments with metJ mutants show that the MetJ protein mediates about 50% of the repression of metE expression by B 12 but is totally responsible for the regulation of metF expression by vitamin B 12.