Schematic of SSU body rotation and head swiveling. (A) Simplified schematic of tRNA-mRNA translocation. (1) EF-G binding stabilizes the R/S state of the ribosome; (2) Pi release facilitates the backward movement of the SSU body domain toward the N state, whereas the head domain remains in the S state; the tRNAs move to the CHI state. (3) The SSU head domain starts to move backward and tRNAs translocate to the P and E sites. (4) The ribosome adopts an N state and EF-G and the E-site tRNA dissociate. (B, left) Positions of the S6–L9 FRET pair that reports on the SSU body rotation (blue) relative to the LSU (gray). (Middle) Directions of the principal movements of the SSU body and head domains. (Right) Positions of the S13–L33 FRET pair to monitor the swiveling of the SSU head (red) relative to the body. Ribosomal proteins are omitted for visual simplicity. Stars indicate the position of the fluorescence labels on the ribosomal proteins.
