The RfaH protein controls the transcription of a specialized group of Escherichia coli and Salmonella operons that direct the synthesis, assembly and export of the lipopolysaccharide core, exopolysaccharide, F conjugation pilus and haemolysin toxin. RfaH is a specific regulator of transcript elongation; its loss increases transcription polarity in these operons without affecting initiation from the operon promoters. The operons of the RfaH-dependent regulon contain a short conserved 5' sequence, the ops element, deletion of which increases operon polarity to an extent similar to that caused by loss of RfaH. The ops element is also present upstream of polysaccharide gene clusters of Shigella flexneri, Yersinia enterocolitica, Vibrio cholerae and Klebsiella pneumoniae and the RP4 fertility operon of Pseudomonas aeruginosa, suggesting that this is a widely spread control system. The mechanistic coupling of RfaH and the ops element has been demonstrated in vitro and in vivo, and we suggest that the ops element recruits RfaH and potentially other factors to the RNA polymerase complex, modifying the complex to increase its processivity and allowing transcription to proceed over long distances.