Release of transcript and template during transcription termination at the trp operon attenuator.

We studied release of trp leader RNA and trp template DNA from RNA polymerase during transcription termination at the attenuator of the trp operon of Escherichia coli. Preliminary evidence had suggested that a stable ternary complex was formed at the trp attentuator. We observed that the complexes between RNA polymerase and trp leader RNA and the DNA template produced during transcription were labile at high salt concentrations and were undetectable when transcription was performed in the presence of heparin. These characteristics are atypical of the stable transcription termination complexes described by others (Richardson, J. P., and Conaway, R. (1980) Biochemistry 19, 4293-4299; Shigesada, K., and Wu, C. (1980) Nucleic Acids Res. 8, 3355-3369). We successfully reconstituted polymerase-trp leader RNA complexes in simple mixing experiments; these and other studies indicated that it is core polymerase that binds the leader transcript and the DNA template. In agreement with this conclusion, it was observed that sigma factor inhibited binding of RNA polymerase to the trp leader transcript and the DNA template and displaced leader RNA from RNA polymerase during transcription. It seems likely that small amounts of core polymerase present in the holoenzyme preparation, or generated during transcription, are responsible for the nonspecific binding of RNA transcript and DNA template. Our findings, therefore, suggest that the transcription termination event at the trp attenuator normally involves spontaneous dissociation of polymerase, template, and RNA transcript.