Synthesis of tryptophanase in Escherichia coli: Isolation and characterization of a structural-gene mutant and two regulatory mutants

Summary A mutant of E. coli has been isolated that is temperature-sensitive in respect of tryptophanase. When incubated at 60°C, cell-free extracts of the mutant suffer inactivation of enzyme activity much more rapidly than similar extracts of the wild type. After lysogeny with a specialized transducing phage carrying the wild-type tryptophanase gene, the mutant is able to synthesize tryptophanase that is wild-type in its response to treatment at 60°C. It is concluded that the mutation lies in the structural gene for the enzyme.Two further mutants have been isolated that synthesize tryptophanase constitutively. One mutation renders synthesis of the enzyme indifferent to the presence of inducer; the other mutation allows synthesis of the enzyme in the absence of inducer at about 35% of the fully induced wild-type rate. Neither mutation alleviates catabolite repression. Genetic mapping shows that the constitutive mutations lie very close to the structural-gene mutation, on the side of the structural gene distant from bglR.