Characterization of a protein-protein interaction within the SigO-RsoA two-subunit sigma factor: the sigma(70) region 2.3-like segment of RsoA mediates interaction with SigO

摘要

sigma 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 s 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 sigma(70) family of sigma factors and interact with each other and with RNA polymerase subunits. Here we show that the sigma(70) region 2.3-like segment of RsoA is unexpectedly sufficient for interaction with the amino-terminus of SigO and the beta' subunit. A mutational analysis of RsoA identified aromatic residues conserved amongst all RsoA homologues, and often amongst canonical sigma factors, that are particularly important for the SigO-RsoA interaction. In a canonical sigma 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.