Role of transcription pausing in the control of the pyrE attenuator in Escherichia coli.

Expression of the Escherichia coli pyrE gene is regulated by transcription attenuation in the intercistronic orfE-pyrE region and modulated by the distance between the transcribing RNA polymerase and the leading ribosome as a function of the supply of UTP and GTP. In this communication we show that pyrE expression is hyper-repressed in vivo following addition of uracil in strains carrying the nusAcs10 mutation. This phenotype, previously seen in rpsL1204 strains whose ribosomes are pseudodependent on streptomycin and work at suboptimal elongation rate, indicates that RNA polymerase escapes from the ribosomes in the pyrE attenuator region in the nusA mutant. In vitro transcription studies revealed that the build-up of the full-length attenuated orfE transcript occurred more slowly in the presence of the NusA protein than in its absence. Moreover, the NusA protein enhanced several transcription pauses through the orfE gene. These effects were more pronounced when low concentrations of either UTP or GTP were used than at low concentrations of either CTP or ATP. The results indicate that the NusA protein is required for proper regulation of pyrE gene expression and is involved, together with the NTP pools, in maintaining the coupling between transcription and translation in the pyrE attenuator region by inhibiting RNA chain elongation.