Fig. 6.

Cryo-electron microscopy (16 Å resolution) of mammalian 80S ribosomes paused at the IBV frameshift signal (Namy et al., 2006). (A) Overall structure of the complex with the large (60S) and small (40S) ribosomal subunits indicated as are the P-site tRNA (green), eukaryotic elongation factor-2 (eEF2; red) and the pseudoknot (PK, purple). (B) Close-up of the 40S subunit focusing on the mRNA entry channel and the electron density for the P-site tRNA in the pseudoknot complex vs. that bound to a non-frameshift-stimulating stem–loop RNA. Note the clear differences in the position of the P-site tRNA (see text for details). (C) A cartoon model of a mechanical model for −1 PRF, in which slippage of the P-site tRNA occurs during translocation mediated by eEF2-GTP hydrolysis. Reproduced with permission from Namy et al. (2006) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.).