Ribosome release modulates basal level expression of the trp operon of Escherichia coli

The leader peptide stop codon (UGA) of the Escherichia coli trp operon was replaced by UAA and UAG. The transcriptional behavior of the mutated leader regions in vitro and the extent of transcription termination observed with each in vivo were virtually identical to that of the wild type leader region. Introduction of a release factor 1 (UAA- and UAG-specific) mutation into strains with the different stop codons caused increased termination in strains with UAA and UAG, but not with UGA (in cells grown in the presence of tryptophan). This finding provides evidence for the view that ribosome release from the leader peptide stop codon is an important event in setting the basal level of transcription readthrough at the trp attenuator.