Role of the metF and metJ genes on the vitamin B₁₂ regulation of methionine gene expression: Involvement of N⁵-methyltetrahydrofolic acid

The repression of MetE synthesis in Escherichia coli by vitamin B₁₂ is known to require the MetH holoenzyme (B₁₂-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 N⁵-methyltetrahydrofolic acid (N⁵-methyl-H₄-folate), the product of the MetF enzymatic reaction is required. Since the methyl group from N⁵-methyl-H₄-folate is normally transferred to the MetH holoenzyme to form a methyl-B₁₂ enzyme, the present results suggest that a methyl-B₁₂ enzyme is involved in the vitamin B₁₂ repression of metE expression. Other results argue against the possibility that a methyl-B₁₂ 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 B₁₂ but is totally responsible for the regulation of metF expression by vitamin B₁₂.