Extended Data Fig. 1: Pol I EC data processing strategy.

a, Processing pipeline for the Pol I EC dataset. Details are described within the STAR Method Details section. Transparent surfaces show masks used for the 3D classification (violet and cyan) or multi-body refinement (yellow and pink)⁸⁴. Shown are unsharpened cryo-EM maps with threshold levels individually adjusted to show relevant features. Reported resolutions were obtained via RELION post-processing. Annotated maps correspond to the maps reported in Table 1 and used for model building. b, FSC curve for the Pol I EC showing the final average resolution for the map A (2.7 Å) and B (3.0 Å) (FSC = 0.143). c,d, Local resolution estimation of the Pol I EC for c Map A and d Map B obtained with the script implemented in RELION. Angular distribution plot of all particles contributing to the structures is shown next to the local resolution maps. c, The map at a higher threshold (left) shows the high resolution core, while at lower threshold values (right), lower resolution peripheral subunits become visible.