The RNA antiterminator causes transcription pausing in the leader region of the tryptophan operon.

In vitro transcription studies with a trp leader DNA template derived from a double deletion mutant of Serratia marcescens revealed that the transcription complex pauses synthesis of part of the RNA antiterminator, structure 2:3. Pausing was enhanced by NusA protein and was dependent on the concentration of UTP in the transcription reaction mixture. A weak antiterminator pause also was detected during transcription of the wild-type S. marcescens trp leader template in the presence of NusA protein and 1 microM UTP. Transcription pausing following synthesis of the antiterminator also was observed in a cell-free transcription-translation system. Antiterminator-induced pausing may play an important role in vivo by delaying synthesis of RNA segment 4. This delay may influence basal level control in cells with an excess of tryptophan. In addition, formation of the antiterminator pause structure may introduce a more stringent tryptophan starvation requirement for RNA polymerase to read through the attenuator.