Role of regulatory features of the trp operon of Escherichia coli in mediating a response to a nutritional shift.

Physiological studies were performed under nutritional stress and nonstress conditions to assess the relative importance of the various regulatory mechanisms that Escherichia coli can use to alter its rate of tryptophan synthesis. Mutants were examined in which the trp repressor was inactive, transcription termination at the trp attenuator was altered, transcription initiation at the trp promoter was reduced, or feedback inhibition of anthranilate synthase was abolished. Strains were examined in media with and without tryptophan, phenylalanine and tyrosine, or acid-hydrolyzed casein and following shifts from one medium to another. Growth rates and anthranilate synthase levels were measured. In media lacking tryptophan, each of the mutants showed relief of repression and/or attenuation and maintained a near-normal growth rate. Following a shift from a medium containing tryptophan to a tryptophan-free medium containing phenylalanine and tyrosine or acid-hydrolyzed casein, mutants with abnormally low trp enzyme levels exhibited an appreciable growth lag before resuming growth. The wild-type strain displayed termination relief only under one extreme shift condition, upon transfer from a minimal medium containing tryptophan to minimal medium with only phenylalanine and tyrosine. A promoter down-mutant had difficulty adjusting to a shift from high tryptophan to low tryptophan levels in a medium containing acid-hydrolyzed casein. In all media tested, anthranilate synthase levels were lower in a feedback-resistant mutant than in the wild type. These studies demonstrate the capacity of E. coli to adjust its rate of tryptophan synthesis to maintain rapid growth following a shift to stressful nutritional conditions.