Repression by Methionine of Cystathionase Formation in Escherichia coli

Cystathionase catalyses the formation of homocysteine from cystathionine; its formation in cultures of Escherichia coli is repressed by the presence of methionine in the growth medium, and to a similar extent to that shown with homocysteine methylase (the enzyme complex which catalyses the conversion homocysteine → methionine). Cystathionase, again like homocysteine methylase, is formed rapidly without concomitant growth when repressed organisms are transferred to a medium free from methionine; such enzyme formation is prevented by chloramphenicol, suggesting that de novo synthesis of protein is required. The co-repression by methionine of the two enzyme systems fortifies the evidence that cystathionase is a component of the normal pathway of methionine synthesis by E. coli and consequently that its substrate, cystathionine, is a normal intermediate. Cystathionase preparations also formed pyruvate from cysteine; this activity paralleled that of cystathionase itself when the methionine status of the medium was changed, and it is concluded that the same protein is responsible for both activities.