In bacteria, a primary σ-factor associates with the core RNA polymerase (RNAP) to control most transcription initiation, while alternative σ-factors are used to coordinate expression of additional regulons in response to environmental conditions. Many alternative σ-factors are negatively regulated by anti-σ-factors. In , , and many other γ-proteobacteria, the transcription factor Crl positively regulates the alternative σ-regulon by promoting the association of σ with RNAP without interacting with promoter DNA. The molecular mechanism for Crl activity is unknown. Here, we determined a single-particle cryo-electron microscopy structure of Crl-σ-RNAP in an open promoter complex with a σ-regulon promoter. In addition to previously predicted interactions between Crl and domain 2 of σ (σ), the structure, along with -benzoylphenylalanine cross-linking, reveals that Crl interacts with a structural element of the RNAP β'-subunit that we call the β'-clamp-toe (β'CT). Deletion of the β'CT decreases activation by Crl without affecting basal transcription, highlighting the functional importance of the Crl-β'CT interaction. We conclude that Crl activates σ-dependent transcription in part through stabilizing σ-RNAP by tethering σ and the β'CT. We propose that Crl, and other transcription activators that may use similar mechanisms, be designated σ-activators.