Cystathionine as a precursor of methionine in Escherichia coli and Aerobacter aerogenes.

Balish, Edward (Argonne National Laboratory, Argonne, Ill.), and Stanley K. Shapiro. Cystathionine as a precursor of methionine in Escherichai coli and Aerobacter aerogenes. J. Bacteriol. 92:1331-1336. 1966.-Cystathionine has been shown to be a precursor of methionine biosynthesis in Escherichia coli and Aerobacter aerogenes. A double enzyme assay was developed to show the formation of homocysteine from cystathionine. The results obtained support the concept that cystathionine serves as a precursor of methionine via the intermediate formation of homocysteine. The latter compound is methylated by the homocysteine methyltransferase of these microorganisms. Sulfhydryl and keto acid assays were used to demonstrate cystathionase activity. Methionine represses both homocysteine methyltransferase formation and cystathionase formation. However, the presence of methionine in reaction mixtures resulted in product inhibition of homocysteine methyltransferase activity, but not of cystathionase activity.