Crl activates transcription by stabilizing active conformation of the master stress transcription initiation factor.

sigma(S) is a master transcription initiation factor that protects bacterial cells from various harmful environmental stresses including antibiotic pressure. Although its mechanism remains unclear, it is known that full activation of sigma(S)-mediated transcription requires a sigma(S)-specific activator, Crl. In this study, we determined a 3.80 angstrom cryo-EM structure of an Escherichia coli transcription activation complex (E. coli Crl-TAC) comprising E. coli sigma(S)-RNA polymerase (sigma(S)-RNAP) holoenzyme, Crl, and a nucleic-acid scaffold. The structure reveals that Crl interacts with domain 2 of sigma(S) (sigma(S)(2)) and the RNAP core enzyme, but does not contact promoter DNA. Results from subsequent hydrogen-deuterium exchange mass spectrometry (HDX-MS) indicate that Crl stabilizes key structural motifs within sigma(S)(2) to promote the assembly of the sigma(S)-RNAP holoenzyme and also to facilitate formation of an RNA polymerase-promoter DNA open complex (RPo). Our study demonstrates a unique DNA contact-independent mechanism of transcription activation, thereby defining a previously unrecognized mode of transcription activation in cells.