Repression is relieved before attenuation in the trp operon of Escherichia coli as tryptophan starvation becomes increasingly severe

Expression of the tryptophan operon of Escherichia coli is regulated over about a 500- to 600-fold range by the combined action of repression and attenuation. Repression regulates transcription initiation in response to variation in the intracellular concentration of tryptophan. Attenuation regulates transcription termination at a site in the leader region of the operon in response to changes in the extent of charging of tRNATrp. We measured repression independently of attenuation to ascertain whether these regulatory mechanisms were used differentially by the bacterium as the severity of tryptophan starvation was increased. We found that repression regulated transcription of the operon over the range from growth with excess tryptophan to growth under moderate tryptophan starvation. By contrast, attenuation (termination control) was not relaxed until tryptophan starvation was in the moderate-to-severe range. Thus, attenuation and repression were used to regulate transcription in response to different degrees of tryptophan deprivation. Consistent with this conclusion is the observation that when tryptophan starvation was sufficient to relieve repression 50 to 60%, 65% of the tRNATrp of the bacterium was charged. These findings provide a possible explanation for the existence of only two tryptophan codons in the coding region for the trp leader peptide of Enterobacteriaceae.