(a) Icosahedral reconstruction of the polymerase complex particle at 4.8-Å resolution is shown along the icosahedral threefold axis of symmetry. The P1 monomers around fivefolds are coloured in blue and the P1 monomers around twofolds and threefolds in red. (b) Four three-dimensional sub-particle class averages (pink, resolution 12 Å) of the densities under the threefolds and the P1 shell, averaged from all of the sub-particles to provide a frame of reference (blue, resolution 12 Å), are shown. The view is from the inside of the particle. Three classes revealed clear P2 density in the three possible orientations relative to the symmetry axis (0°, 120° and 240°). For another class no P2 density was evident (asterisk). Incorrectly averaged P2 density in the original threefold symmetrized reconstruction is indicated with an arrowhead. (c) Localized reconstruction of the P2 density at 7.9-Å resolution with a fitted X-ray structure of P2 (PDB:1HHS) is shown from three different orthogonal orientations. Colouring of the X-ray structure follows the canonical polymerase domain architecture (fingers, red; palm, green; thumb, blue; C terminus, yellow; connecting chains, mauve). Two small densities, not accounted by the P2 X-ray crystallographic structures, are indicated with arrowheads. (d) P2 density, reconstructed together with the P1 shell, is shown in the same orientation as in c. Densities connecting the P2 polymerase to the surrounding P1 shell are marked with arrowheads, and correspond to the densities marked in c. (e–g) Possible contact sites between P2 (the coloured ribbon) and P1 (grey ribbon) are shown and the corresponding amino acid residues labelled.