Structures of <i>Vibrio cholerae</i> transcription complexes reveal how ToxR and TcpP recruit the RNA polymerase and activate virulence genes

Activation of virulence in <i>Vibrio cholerae</i>, the etiological agent of cholera disease, is mediated by two transmembrane one-component signal-transduction proteins, ToxR and TcpP, which are also transcription factors. Using cryo-electron microscopy, we have solved five structures of the <i>ompU</i> and <i>toxT</i> transcription activation complexes, including the RNA polymerase (RNAP) holoenzyme, promoter DNAs, transcribed RNA, and their corresponding transcription factors, ToxR or TcpP and ToxR-TcpP, respectively. Activation is achieved through the interaction of ToxR or TcpP with the α-C-terminal repeat domain of RNAP where a single residue of the activator, a phenylalanine, appears to be the most critical contact, as confirmed by mutagenesis. No interactions of the transcription factors were observed with other subunits of the RNAP, i.e., the σ subunit as it occurs in the structurally related PhoB family of two-component transcription factors. The structures, and their comparison with our previously solved DNA promoter-ToxR x-ray structures, unveil the molecular mechanism of cholera virulence gene activation.