Post-initiation control by the initiation factor sigma.

Bacterial core RNA polymerase can initiate transcription at promoters only if guided by a sigma subunit that directs the core enzyme to a subset of sigma-specific promoters. Specific and stable interactions between the promoter DNA elements and sigma are required for efficient promoter recognition. At the same time, persistent sigma-DNA contacts can hinder RNA polymerase escape from a promoter or halt the enzyme downstream from the transcription start site, thereby reducing transcription of the affected genes. This microcommentary reviews recent data arguing that sigma-dependent stalled transcription complexes form frequently in vivo, where they likely play important and diverse regulatory roles.