a View of the active site region of the EC with the bridge-helix in teal, trigger-loop in yellow, template DNA strand in purple and RNA in orange. b The same view of the active site region of the RNAP-HelD complex with the SCA of HelD shown in red with the acidic D56 and D57 residues shown as sticks. c An overlay of the regions shown in a and b with the insertion of the SCA into the secondary channel indicated by the red arrow. EC elements are shown semi-transparent with nucleic acids coloured according to the scheme in Fig. 1. The bridge-helix and trigger-loop elements that are distorted by the SCA are shown, with the direction of movement from EC to HelD complex indicated by the black arrow. The catalytic Mg2+ ion is shown as a green sphere. d Hydrogen-bond and salt-bridge interactions (dashed blue lines) between the tip of the SCA (red semi-transparent cartoon with purple sticks) and RNAP. Subunit colouring is the same as in Fig. 1. The conserved active site Asp residues that chelate the catalytic Mg2+ (green sphere, grey dashed lines) are also shown to illustrate how the tip of the SCA cages but does not interact directly with residues involved in RNAP catalysis. The enlarged box on the right corresponds to the boxed region shown for the whole complex on the left.