The synthesis of methionine by suspensions of Escherichia coli

Abstract

Washed suspensions of a number of normal and mutant strains of E. coli converted homocysteine into methionine. The latter was determined by microbiological assay with Leuconostoc mesenteroides P 60. By the use of auxotrophic mutants unable to synthesize p-aminobenzoic acid and cobalamin respectively, it was shown that both these vitamins are essential for the reaction. Synthetic forms of folic acid did not replace p-aminobenzoic acid. Sulphathiazole, however, inhibited both the synthesis of methionine and the conversion of p-aminobenzoic acid into folic acid. Serine is the probable ultimate source of the methyl group. Methionine is not formed by mutants requiring serine or glycine for growth unless serine (or, under limited conditions, glycine) is added. Glycine is formed concurrently with methionine whether or not it is essential to add serine. Glycine, threonine and homoserine also act as one-C donors, though less efficiently than serine. There is evidence that threonine and homoserine must first be converted into glycine.