Different ribosome–ribosome orientations in HMW polysomes. (a) Example of area of cryo-tomograms that has been fitted with a low-resolution 80S reconstruction (yellow, small subunit; blue, large subunit). (b) Close-up of fitted ribosomes with relative orientations between the i^th and i+1 ribosome similar to that described for the t-t class previously (equivalent to cluster 118 in Brandt et al., 2010). (c) Close-up of fitted ribosomes with relative arrangements of the i^th and i+1 ribosomes similar to the alternative (cluster 30 in Brandt et al., 2010) arrangement seen previously. Note in addition that these are all mixed assemblies, the i^th and i+1 ribosomes are followed by an i+2 ribosome in various relative arrangements: in three cases a basis inversion compared with the other two in the cluster (left and bottom right) and in the other at 90° to them (top right). (d) Close-up of fitted ribosomes with relative orientations between the i^th and i−1 ribosome similar to that described for the t-t orientation (see cluster 42 in Brandt et al., 2010). (e) Fitted ribosomes with i to i−1 arrangements apparently similar to an alternative arrangement seen in a previous study (cluster 125 in Brandt et al., 2010). (f) Pairwise arrangements of ribosomes within polysomes that do not appear to correlate to one of the previously described major clusters. (g) Extended, mixed close assemblies of ribosomes identified within the polysomal tomograms.