The attenuator of the tryptophan operon in E.coli: rho-mediated release of RNA polymerase from a transcription termination complex in vitro.

In vivo, termination of transcription at the attenuator site of the tryptophan (trp) operon of E. coli is influenced by the protein termination factor rho. In vitro, termination does not depend on rho factor, and is very efficient in a purified system consisting only of RNA polymerase, the DNA template, nucleoside triphosphates, and buffer. The extent of termination in this system is unaffected over a wide range of salt and nucleoside triphosphate concentration. However, there is a 10-fold stimulation of trp leader mRNA synthesis if rho factor is present during the transcription reaction. This stimulation occurs only at low molar ratios of polymerase to template, and can be blocked by rifampicin. It is thus most likely due to the recycling of RNA polymerase molecules that have been released from the attenuator site by rho factor. In fact, transcription of the trp leader region in vitro results in the fomration of a stable termination complex which can be observed on sucrose gradients or by binding to nitrocellulose filters. These data indicate that a major function of rho at the trp attenuator is to release completed transcripts from a pre-formed termination complex, rather than to cause the cessation of elongation.