a, b Cryo-EM electron density maps (α2ββω subunits only) of RNAP EC (a) and RNAP–HelD complex (b). HelD density has been removed for clarity, and nucleic acids from the E. coli EC (PDB ID 6ALF) superimposed over the B. subtilis EC nucleic acids as a visual aid for the scale of conformational changes that occur in the primary DNA-binding and RNA exit channels upon binding HelD. Nucleic acids are coloured the same as in Fig. 1. The longer RNA from the E. coli EC helps illustrate the increase in aperture of the RNA exit channel. c left side EC, right side RNAP-HelD complex with HelD removed for clarity, with β (cyan) and βʹ (purple) residues shown to illustrate the change in aperture of the RNA exit channel β R800 and βʹD245 and DNA binding channel β P242 and βʹN283. Black arrow in left panel indicates the region of the βʹ clamp that is contacted by the HelD clamp arm and the double-ended arrow in the right panel indicates the movement of subunits away from each other on HelD binding.