Studies on the involvement of the UGA readthrough process in the mechanism of attenuation of the tryptophan operon of Escherichia coli

We asked if UGA suppression by charged tRNA^Trp, a process called UGA readthrough, is involved in the mechanism of attenuation of the tryptophan (trp) operon in Escherichia coli. For this purpose we used two mutations: strA (LD1) which causes restriction of UGA readthrough, and revA which partially overcomes the restriction of UGA readthrough caused by strA (LD1) (Engelberg-Kulka et al. 1982). trp attenuation was monitored by the regulation of the synthesis of the trp operon enzyme anthranilate synthetase (ASase) in trpR^- strains. We showed that the strA (LD1) mutation causes a significant increase in the level of synthesis of ASase in the presence of an excess of tryptophan, while the revA mutation reverses this effect, indicating that transcription termination at the trp attenuator site is relieved by restriction of UGA readthrough. Based on our results and the sequence data of the trp leader RNA of E. coli (Oxender et al. 1979), we offer a model for the involvement of the UGA readthrough process in trp attenuation. We suggest that the UGA readthrough process permits trp attenuation to respond to slight changes in the cellular concentration of charged tRNA^Trp.