Role of the metF and metJ genes on the vitamin B12 regulation of methionine gene expression : involvement of N5-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 met J mutants show that the MetJ protein mediates about 50% of the repression of met E expression by B 12 but is totally responsible for the regulation of met F expression by vitamin B 12 .