Analysis of the requirements for transcription pausing in the tryptophan operon.

RNA polymerase pausing during transcription of the tryptophan (trp) operon leader region is postulated to be the key event that synchronizes transcription of this region with translation of the coding region for the trp leader peptide. Coupling of transcription to translation enables the cell to monitor the intracellular concentration of charged tRNATrp and determine whether polymerase should terminate transcription at the attenuator or proceed into the structural genes of the operon. We used mutant templates containing deletions of DNA segments corresponding to sequences that are predicted to form alternative RNA secondary structures to show that formation of an RNA hairpin in the leader transcript, and the concentration of the next nucleoside triphosphate to be added to the paused transcript, both markedly affect the kinetics of pausing in vitro. A model is presented that accounts for many of the findings obtained in this and other pausing studies.