FIGURE 3. Stereo views of RNA and protein elements in the ribosome–PCC junction.

Real-space refined models showing E. coli ribosomal proteins rendered as ribbons, and rRNA regions interacting with the PCC as thick, light-grey backbone snake-like structures (rattlers). The PCC is coloured and rendered as in Fig. 2d with the cryo-EM density in cyan mesh. a–c, The ribosome–PCC junction at the polypeptide exit site (nascent chain density semi-transparent yellow surface) is shown in the front (a) and back (b) views. Ribosomal elements near the polypeptide exit site and parts of the PCC in the three connection regions are also illustrated (c), with the line of view perpendicular to the membrane plane. Connection regions between ribosome and PCC are circled in orange and labelled. d, The non-translocating PCC uses its CFADs to interact with hairpins in the mRNA (mRNA shown as semi-transparent purple surface with non-interacting mRNA rattler regions in yellow). h, helix.