Fig. 1.

Stepwise, RNA-driven displacement of the σ-finger: primary σ-factor, 5′-hydroxyl RNA. Crystal structures of (A) σ^A-RPo (PDB 4G7H; ref. 5), (B) σ^A-RPitc2 (PDB 4G7O; ref. 5), (C) σ^A-RPitc3 (this work; see SI Appendix, Table S2), (D) σ^A-RPitc4 (this work; see SI Appendix, Table S2), (E) σ^A-RPitc5 (this work; see SI Appendix, Table S2), (F) σ^A-RPitc6 (this work; see SI Appendix, Table S2), and (G) σ^A-RPitc7 (this work; see SI Appendix, Table S2). Left subpanels, electron-density map and atomic model. Right subpanels, atomic model. Green mesh, simulated-annealing Fo-Fc electron-density map contoured at 2.5 σ; gray surface, RNAP β′ subunit; yellow sticks, σ-finger; red sticks, template-strand DNA; pink sticks, RNA; purple sphere, catalytic Mg²⁺ ion.