Figure 7. Model for multi-faceted activation of gene expression by RfaH.

In RfaH-controlled operons, the ops element is located within 100 nt upstream of a GTG predicted (based on protein sequence analysis) to serve as a translation start codon. In absence of RfaH (left), NusG-NTD binds to the β’-clamp helices (dark gray cylinder) and NusG-CTD interacts with Rho (purple) to terminate transcription by RNAP (gray). In the rfb operon, Rho decreases expression of distal genes by ~800-fold (Sevostyanova et al., 2011). When present (right), RfaH binds to elongating RNAP at the ops site and reduces Rho effect to 2-fold. This strong anti-polar activity depends on the coordinated action of both RfaH domains becoming separated during recruitment. Unaltered RfaH-NTD binds to the β’-clamp helices to reduce transcriptional pausing and exclude NusG-NTD from binding to RNAP; both activities inhibit Rho-dependent termination. The refolded RfaH-CTD recruits the 30S subunit (bound to the initiator tRNA) via direct contacts with S10. Thus, the tethered translation initiation complex scans the mRNA lacking a strong Shine-Dalgarno (SD) element for another, yet unknown, start signal. Recruitment of the ribosome directly increases translation and indirectly decreases Rho-dependent termination by shielding mRNA from Rho.