Processive transcription over long operons is essential for bacterial gene expression. Escherichia coli RNA polymerase (RNAP) routinely transcribes operons longer than 10+ kbp, yet this process is threatened by backtrack-induced long-lived pauses that can lead to premature termination. Multiple transcription factors modulate RNAP behavior: Gre factors stimulate transcript cleavage to rescue backtracked RNAP; NusA can stimulate pausing; and NusG generally suppresses pausing and helps connect RNAP to the ribosome by bridging the expressome. How these factors cooperate to secure full-length transcription of long operons, and how the NusG paralog RfaH contributes to that process, remained unclear prior to this study.
This work investigates the mechanistic role of RfaH in enabling RNAP to overcome pausing and maintain long-range processivity, with implications for expression of long virulence operons that depend on RfaH recruitment at specific DNA sequences (ops).
The authors reconstituted ops-induced pausing of RNAP in vitro and monitored transcription dynamics with high-throughput magnetic tweezers. This single-molecule approach allowed continuous observation of transcription runs and pausing behavior under defined conditions. Using this platform, the investigators characterized pause occupancy and the effects of different transcription factors on pause escape and run lengths.
The magnetic-tweezers assay forms the experimental foundation of the study, enabling dissection of pause entry and recovery pathways at the level of individual RNAP complexes paused at ops elements.
From the reconstituted system, the authors found that the sequence context surrounding the ops element sets the occupancy of ops-induced pauses. In other words, primary DNA/RNA sequence features influence how frequently RNAP becomes trapped in a long-lived pause at ops, and therefore the demand for rescue or anti-pausing activities from transcription factors.
This finding emphasizes that factor-dependent control of processivity is layered on top of an intrinsic, sequence-encoded propensity for RNAP to pause or backtrack at particular loci.
A central result is that RfaH is loaded onto RNAP paused at ops by two distinct pathways. One pathway permits immediate escape from the ops-induced pause upon RfaH recruitment. The other pathway requires prior or concurrent action by the transcript cleavage factor GreA to rescue a backtracked RNAP before RfaH can complete loading and permit productive elongation.
Thus, RfaH can function both in contexts where RNAP is paused but not deeply backtracked, and in contexts where rescue by GreA-type cleavage is necessary to restore a transcriptionally competent complex that RfaH can then license for continued processive elongation.
The study compared the effects of NusA, NusG, and RfaH on ops-paused RNAP. The authors report that both NusG and RfaH nullify the pause-stimulating activity of NusA in the assay. Functionally, NusG and RfaH act as anti-pausing factors in this context, opposing NusA’s tendency to stabilize paused states.
Moreover, the requirement for GreA rescue on one RfaH-loading pathway highlights cooperation between cleavage factors and anti-pausing factors: GreA can convert backtracked, stalled complexes into states competent for RfaH recruitment and subsequent long-range transcription.
In the magnetic-tweezers assays, RfaH sustained transcription runs that were reported to be approximately four times longer than those maintained by NusG under the same experimental conditions. This extended run length indicates a potent ability of RfaH to license RNAP for extended, pause-resistant elongation suitable for long operons.
The in vitro observations were tied to in vivo relevance: RfaH recapitulated NusG-like roles in cells, and depletion of RfaH led to failure of conjugation, consistent with impaired expression of long virulence or transfer operons that depend on RfaH recruitment at ops sequences.
By defining distinct pathways for RfaH recruitment and demonstrating its capacity to override NusA-induced pausing and to sustain long transcription runs, this work provides a mechanistic framework for how bacteria express very long operons. The findings create a foundation for further dissection of virulence operon regulation and identify RfaH-dependent steps as potential points for therapeutic intervention.
Note: this report is based on a preprint that has not been certified by peer review. Details beyond those reported in the preprint (for example, quantitative parameters not stated in the abstract) were not provided in the source and are not introduced here.