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

σ 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 σ-mediated transcription requires a σ-specific activator, Crl. In this study, we determined a 3.80 Å cryo-EM structure of an transcription activation complex ( Crl-TAC) comprising σ-RNA polymerase (σ-RNAP) holoenzyme, Crl, and a nucleic-acid scaffold. The structure reveals that Crl interacts with domain 2 of σ (σ) 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 σ to promote the assembly of the σ-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.