Increased distal gene transcription by the elongation factor RfaH, a specialized homologue of NusG

The Escherichia coli RfaH protein is required for the expression of operons directing synthesis and export of the toxin haemolysin, the lipopolysaccharide core, and the F-factor sex pilus. Mutation of rfaH increases transcriptional polarity along all three operons. By demonstrating strong RfaH-dependent suppression of transcription polarity in vitro, we have established RfaH as a novel transcriptional activator, and we reveal that RfaH is a homologue of the essential protein NusG that modulates general transcriptional pausing and termination in prokaryotes. Full transcription of the distal genes from an upstream promoter required RfaH and the 5' cis-acting ops element, both in vivo and in vitro. In vivo the requirement for the ops element was suppressed by overexpressing RfaH, and in vitro the presence of ops lowered the concentration of RfaH required to stimulate transcript elongation. We suggest that RfaH directs transcript elongation in an analogous way to NusG, but does so in a subset of bacterial operons primarily engaged in the production of extracellular components required for virulence and fertility.