Thermolabile repression of cephalosporinase synthesis in Citrobacter freundii.

An unusual regulatory system of cephalosporinase synthesis in Citrobacter freundii has been found. When the bacteria are grown at 20 C, the cephalosporinase is synthesized as a typical inducible enzyme and benzylpenicillin acts as an effective inducer. The enzyme, however, is synthesized in the absence of the inducer at growth temperatures above 25 C. when the growth temperature is shifted from 20 C to 37 C, the induction of enzyme synthesis is observed after about one half of the organism doubling time, but it does not occur in the presence of chloramphenicol. The reverse control mutants, the enzyme constitutive synthesis of which is markedly depressed by benzylpenicillin, were isolated from the C. freundii wild strain. The possibility that the enzyme synthesis is governed by a regulatory system analogous to the its mutant of the lac operon in Escherichia coli was suggested.