Regulation of the Escherichia coli tryptophan operon by readthrough of UGA termination codons

The regulation of the synthesis of trp operon enzymes was studied in streptomycin-resistant Escherichia coli mutants temperature-sensitive for UGA suppression by normal tRNA^Trp. Our mutants carry a trpR⁺ allele that when transferred to a different genetic background causes repression of trp operon enzyme synthesis at both low (35°C) and high (42°C) temperatures; however, in our mutants with an excess of tryptophan and at increased temperatures trp enzyme synthesis is derepressed. Based on our results and the sequence data of the trpR gene [Singleton et al. (1980) Nucleic Acids Res., 8, 1551-1560], we offer a model for the involvement of the limited misreading of UGA codons by normal charged tRNA^Trp in the autogenous regulation of the trpR gene expression. The UGA readthrough process may be a regulatory amplifier of the effect of tryptophan starvation.