Energy-dependent inactivation and modification of a tryptophan biosynthetic enzyme in Escherichia coli

The bifunctional tryptophan biosynthetic enzyme N-(5'-phosphoribosyl)anthranilate (PRA) isomerase/indole-3-glycerol phosphate (InGP) synthetase was purified from Escherichia coli cultures incubated under several conditions, some of which result in inactivation of the enzyme (starvation for ammonium or sulfate, chloramphenicol inhibition). Recovery of enzyme activity during purification from cultures incubated under inactivating conditions suggested that activity was restored or that an inhibitor was removed. In these preparations, the enzymatic properties were altered slightly but not in a manner that could account for inactivation. Each preparation exhibited one major protein species (Mr approximately 50,000) when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis but revealed several species during isoelectric focusing in the range of pH 5.5 to 6.0. Comparison of the distribution of species indicated that modification to the more acidic forms had occurred in growing and nongrowing cells but not during stationary phase, glucose starvation, or energy depletion. The results suggested that inactivation and modification had occurred in vivo by an energy-dependent process. Several metabolites tested did not inhibit the purified enzyme. Attempts to detect an inhibitor in crude cell extracts were also not successful.