A Coiled-Coil from the RNA Polymerase β′ Subunit Allosterically Induces Selective Nontemplate Strand Binding by σ70

For transcription to initiate, RNA polymerase must recognize and melt promoters. Selective binding to the nontemplate strand of the −10 region of the promoter is central to this process. We show that a 48 amino acid (aa) coiled-coil from the β′ subunit (aa 262–309) induces σ70 to perform this function almost as efficiently as core RNA polymerase itself. We provide evidence that interaction between the β′ coiled-coil and region 2.2 of σ70 promotes an allosteric transition that allows σ70 to selectively recognize the nontemplate strand. As the β′ 262–309 peptide can function with the previously crystallized portion of σ70, nontemplate recognition can be reconstituted with only 47 kDa, or 1/10 of holoenzyme.