Mechanisms of σ-Dependent Transcription Initiation and Regulation.

Cellular RNA polymerase is a multi-subunit macromolecular assembly responsible for gene transcription, a highly regulated process conserved from bacteria to humans. In bacteria, sigma factors are employed to mediate gene-specific expression in response to a variety of environmental conditions. The major variant σ factor, σ, has a specific role in stress responses. Unlike σ-dependent transcription, which often can spontaneously proceed to initiation, σ-dependent transcription requires an additional ATPase protein for activation. As a result, structures of a number of distinct functional states during the dynamic process of transcription initiation have been captured using the σ system with both x-ray crystallography and cryo electron microscopy, furthering our understanding of σ-dependent transcription initiation and DNA opening. Comparisons with σ and eukaryotic polymerases reveal unique and common features during transcription initiation.