The interaction between bacterial transcription factors and RNA polymerase during the transition from initiation to elongation

There are three stages of transcription: initiation, elongation and termination, and, traditionally, there has been a clear distinction between the stages. The specificity factor sigma is completely released from bacterial RNA polymerase after initiation and is then recycled for another round of transcription. Elongation factors then associate with the polymerase followed by termination factors (where necessary). These factors dissociate prior to initiation of a new round of transcription. However, there is growing evidence suggesting that sigma factors can be retained in the elongation complex. The structure of bacterial RNAP in complex with an essential elongation factor NusA has recently been published and suggested that, rather than competing for the major sigma binding site, NusA binds to a discrete region on RNAP. A model was proposed to help explain the way in which both factors could be associated with RNAP during the transition from transcription initiation to elongation.