Initial trp operon sequence in Escherichia coli is transcribed without coupling to translation

Abstract

The transcription of the "leader" region (Bronson et al., 1973) of the trp operon in Escherichia coli was studied in normal mutants which delete most of the operator-distal region of the operon [a deletion strain (trp OAEG) retaining only about one third of the "leader" region and two deletion strains (trpOAE14 and trpOAE2) retaining the whole "leader" region and an initial portion of the trpE], as well as in a strain with an intact trp operon, but with a temperature-sensitive lesion in ribosomal protein factor EFTs (strain HAK88). In these deletion mutants, mRNA molecules corresponding to the "leader" region were detected as most of the trp-specific mRNA. Less inhibition of transcription, of the promoter-proximal portion of the trp "leader" region than that of more distal genes of the operon, was found in chloramphenicol-treated cells of strain trpOAE14. It was also observed that transcription of the initial one third portion of the "leader" region was not repressed by tryptophan in strains trpOAE6 and trp OAE14. A similar effect of a translation block on transcription of the distal part of the "leader" region was observed with strain HAK88 at the nonpermissive temperature. In sedimentation analysis of polyribosomes containing the trp mRNA molecules from the deletion mutants, trp mRNA from strain trpOAE14 was found in monosomes and small polyribosomes, whereas the majority of the trp mRNA from strain trpOAE6 was found joined to a single ribosome or ribosomal subunit. These results suggest that ribosomes bind in vivo to a site(s) located in the middle of the "leader" mRNA sequence, and that the initial transcription of the trp operon does not require any connection to functional translational machinery, while continuation of RNA synthesis beyond a first ribosome binding site seems indispensably coupled to ribosome function.