Extended Data Fig. 6: Pol I-RRN3 and Pol I OC dataset processing strategy.

a, Details of the pipeline are described within the STAR Method Details section. A small initial dataset was collected using FEI Talos Arctica and processed in cryoSPARC to obtain a reference that was used for global 3D classification for the high-resolution dataset collected on Titan Krios. First global 3D classification was performed with the dataset split into 5 subsets and representative selected classes are shown. Transparent surfaces show masks used for the 3D classification (pink and cyan) or for focused refinement (orange). Shown are unsharpened cryo-EM maps with threshold level 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 curves for the Pol I-RRN3 and Pol I OC maps showing the final average resolution (FSC = 0.143). c, Local resolution estimation of Pol I-RRN3 and Pol I OC maps obtained with the RELION implementation. Maps are shown at lower threshold values, so that the lower resolution peripheral subunits become visible. Angular distribution plot of all particles contributing to the structures is shown to the left of each local resolution map.