Fig. 1.

Rotated states of the ribosome. (A) View of the bacterial 70S ribosome, composed of the small (30S) ribosomal subunit and the large (50S) ribosomal subunit. The small subunit of the ribosome (blue) can rotate from a starting conformation seen in post-initiation and termination states (12, 13, 18) (state R₀, black outline) to a fully rotated conformation seen in elongation, termination and recycling steps of translation (state R_F, red outline) (1, 15–17). 30S features include: Head, Platform, Body. The 50S subunit is shown in grey. Letters indicate the positions of the aminoacyl (A), peptidyl (P), and exit (E) tRNA binding sites. (B) Schematic of tRNA binding states on the ribosome. In the transition of the ribosome to the fully rotated state, tRNAs shift from binding in the A/A and P/P sites (30S subunit/50S subunit, respectively) to occupy hybrid binding sites (A/P and P/E for 30S/50S sites). The view of the ribosome is rotated 90° from that in A. (C) Rotations of the head domain of the small ribosomal subunit. Letters indicate the locations of the aminoacyl (A), peptidyl (P), and exit (E) tRNA binding sites on the large subunit. In state R₀ (black), the head domain is centered over the P site (~0° rotation). Rotations of the head domain towards the E site of up to 14° (red) have been observed (1, 6, 7). The 5′ to 3′ direction of mRNA, which threads around the neck region of the 30S subunit, is also indicated.