a, Schematic domain organisation of subunit RPC10 and its position in the elongating human Pol III complex. Coloured bars indicate modelled regions and are labelled as ‘built’. N – N-terminal zinc ribbon; C – C-terminal zinc ribbon. b, Conformational switch of RPC10. The Pol III core is shown as transparent surface and RPC10 as cartoon in two conformations of its C-terminal zinc ribbon domain (C): outside (yellow) and inside (light red) the Pol III funnel. The black arrow illustrates the movement of the acidic hairpin (D88, E89), shown as dark red spheres, to engage the active site. c, Elongating human Pol III in two superimposed conformations derived from 3D Variability Analysis (3DVA) with RPC10 outside (yellow map) and inside (light red map). Positions of the C-ribbon are boxed. Conformational rearrangements are highlighted with white arrows and indicate the partial opening of the clamp domain. The framed close-up view shows the appearance of the connection between heterotrimer and stalk in the ‘open clamp’ conformation.