Figure 1. Structural description of translocation on the 30S subunit.

(a) Structure of the 70S ribosome with two tRNAs (red and yellow) and EF-G (orange). The 23S mRNA and 50S proteins are shown in white and ice blue. The 16S mRNA and 30S proteins are in blue and dark blue. (b) Schematic of the ribosome with mRNA (green) bound on the 30S subunit, and the P- and A-site tRNAs in hybrid P/E (red) and A/P (yellow) conformations. The 50S subunit is depicted in grey. The head and body domains of the 30S subunit are shown in blue and cyan. The region in which mRNA–tRNA movement occurs on the 30S is demarcated by a dashed box. (c) Schematic of tRNAs in classical E/E and P/P conformations, after translocation on the 30S subunit. (d) Subunit rotations are described by: φ_body (body rotation), φ_head (head rotation) and θ_head (magnitude of head tilting). Counterclockwise rotation of the body (from the perspective shown) and counterclockwise rotation of the head (viewed from above) are defined as positive. All structural representations were prepared using visual molecular dynamics (VMD)⁶⁵.