Binding of the initiation factor σ70 to core RNA polymerase is a multistep process

The interaction of RNA polymerase and its initiation factors is central to the process of transcription initiation. To dissect the role of this interface, we undertook the identification of the contact sites between RNA polymerase and sigma (70), the Escherichia coli initiation factor. We identified nine mutationally verified interaction sites between sigma (70) and specific domains of RNA polymerase and provide evidence that sigma (70) and RNA polymerase interact in at least a two-step process. We propose that a cycle of changes in the interface of sigma (70) with core RNA polymerase is associated with progression through the process of transcription initiation.