Bacterial transcription elongation factors: new insights into molecular mechanism of action.

Like transcription initiation, the elongation and termination stages of transcription cycle serve as important targets for regulatory factors in prokaryotic cells. In this review, we discuss the recent progress in structural and biochemical studies of three evolutionarily conserved elongation factors, GreA, NusA and Mfd. These factors affect RNA polymerase (RNAP) processivity by modulating transcription pausing, arrest, termination or anti-termination. With structural information now available for RNAP and models of ternary elongation complexes, the interaction between these factors and RNAP can be modelled, and possible molecular mechanisms of their action can be inferred. The models suggest that these factors interact with RNAP at or near its three major, nucleic acid-binding channels: Mfd near the upstream opening of the primary (DNA-binding) channel, NusA in the vicinity of both the primary channel and the RNA exit channel, and GreA within the secondary (backtracked RNA-binding) channel, and support the view that these channels are involved in the maintenance of RNAP processivity.