Modular Organization of the NusA- and NusG-Stimulated RNA Polymerase Pause Signal That Participates in the Bacillus subtilis trp Operon Attenuation Mechanism.

The operon is regulated by a transcription attenuation mechanism in which tryptophan-activated TRAP binds to the nascent transcript and blocks the formation of an antiterminator structure such that the formation of an overlapping intrinsic terminator causes termination in the 5' untranslated region (5' UTR). In the absence of bound TRAP, the antiterminator forms and transcription continues into the genes. RNA polymerase pauses at positions U107 and U144 in the 5' UTR. The general transcription elongation factors NusA and NusG stimulate pausing at both positions. NusG-stimulated pausing at U144 requires sequence-specific contacts with a T tract in the nontemplate DNA (ntDNA) strand within the paused transcription bubble. Pausing at U144 participates in a translation repression mechanism. Since U107 just precedes the critical overlap between the antiterminator and terminator structures, pausing at this position is thought to participate in attenuation. Here we carried out pausing and termination experiments to identify components of the U107 pause signal and to determine whether pausing affects the termination efficiency in the 5' UTR. We determined that the U107 and U144 pause signals are organized in a modular fashion containing distinct RNA hairpin, U-tract, and T-tract components. NusA-stimulated pausing was affected by hairpin strength and the U-tract sequence, whereas NusG-stimulated pausing was affected by hairpin strength and the T-tract sequence. We also determined that pausing at U107 results in increased TRAP-dependent termination in the 5' UTR, implying that NusA- and NusG-stimulated pausing participates in the operon attenuation mechanism by providing additional time for TRAP binding. The expression of several bacterial operons is controlled by regulated termination in the 5' untranslated region (5' UTR). Transcription attenuation is defined as situations in which the binding of a regulatory molecule promotes transcription termination in the 5' UTR, with the default being transcription readthrough into the downstream genes. RNA polymerase pausing is thought to participate in several attenuation mechanisms by synchronizing the position of RNA polymerase with RNA folding and/or regulatory factor binding, although this has only been shown in a few instances. We found that NusA- and NusG-stimulated pausing participates in the attenuation mechanism controlling the expression of the operon by increasing the TRAP-dependent termination efficiency. The pause signal is organized in a modular fashion containing RNA hairpin, U-tract, and T-tract components.