The interaction of RNA polymerase and its initiation factors is central to
the process of transcription initiation. To dissect the role of this interf
ace, we undertook the identification of the contact sites between RNA polym
erase and sigma (70), the Escherichia coli initiation factor. We identified
nine mutationally verified interaction sites between sigma (70) and specif
ic domains of RNA polymerase and provide evidence that sigma (70) and RNA p
olymerase 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 assoc
iated with progression through the process of transcription initiation.