Characterization of a protein–protein interaction within the SigO–RsoA two-subunit σ factor: the σ70 region 2.3-like segment of RsoA mediates interaction with SigO Xue, Xiaowei and Davis, Maria C. and Steeves, Thomas and Bishop, Adam and Breen, Jillian and MacEacheron, Allison and Kesthely, Christopher A. and Hsu, FoSheng and MacLellan, Shawn R.,, 162, 1857-1869 (2016), doi = https://doi.org/10.1099/mic.0.000358, publicationName = Microbiology Society, issn = 1350-0872, abstract= σ factors are single subunit general transcription factors that reversibly bind core RNA polymerase and mediate gene-specific transcription in bacteria. Previously, an atypical two-subunit σ factor was identified that activates transcription from a group of related promoters in Bacillus subtilis. Both of the subunits, named SigO and RsoA, share primary sequence similarity with the canonical σ70 family of σ factors and interact with each other and with RNA polymerase subunits. Here we show that the σ70 region 2.3-like segment of RsoA is unexpectedly sufficient for interaction with the amino-terminus of SigO and the β′ subunit. A mutational analysis of RsoA identified aromatic residues conserved amongst all RsoA homologues, and often amongst canonical σ factors, that are particularly important for the SigO–RsoA interaction. In a canonical σ factor, region 2.3 amino acids bind non-template strand DNA, trapping the promoter in a single-stranded state required for initiation of transcription. Accordingly, we speculate that RsoA region 2.3 protein-binding activity likely arose from a motif that, at least in its ancestral protein, participated in DNA-binding interactions., language=, type=