EFFECT OF L-METHIONINE AND VITAMIN B12 ON METHIONINE BIOSYNTHESIS IN ESCHERICHIA COLI

The repression of several enzymes involved in methionine synthesis in Escherichia coli by either methionine or vitamin B12 has been investigated. l-Methionine, at 10-2 m in the growth medium, yielded the following results: 83% repression of the non-B12 transmethylase; 77% repression of N5,10-methylene-H4-folate reductase; and 75% repression of β-cystathionase. Vitamin B12, at 10-9 m, showed 99% repression of the non-B12 transmethylase, 78% repression of N5,10-methylene-H4-folate reductase, and no repression of β-cystathionase. The significantly higher repression of the non-B12 transmethylase by vitamin B12 when compared to methionine, in conjunction with the lack of β-cystathionase repression by the vitamin, suggested that repression by the vitamin was not due to an increase in intracellular methionine formed. This was corroborated by direct assay of intracellular methionine levels which showed no increase after growth of the organism in the presence of the vitamin. However, the studies with other cobamides suggested that repression of the non-B12 transmethylase by vitamin B12 was dependent on formation of the B12 transmethylase holoenzyme. These data indicate that the vitamin and methionine may cause repression of the non-B12 enzyme by different mechanisms. © 1969.