Correlated conformational events in EF-G and the ribosome regulate translocation

In bacteria, the translocation of tRNA and mRNA with respect to the ribosome is catalyzed by the conserved GTPase, elongation factor-G (EF-G). In order to probe the rate determining features in this process, EF-G-catalyzed translocation was imaged from two unique structural perspectives using single-molecule fluorescence resonance energy transfer. The data reveal that the rate at which the ribosome spontaneously achieves a transient, “unlocked” state is closely correlated with the rate at which the tRNA-like, domain IV/V element of EF-G engages the A site. Following these structural transitions, translocation occurs comparatively fast, suggesting that conformational processes intrinsic to the ribosome determine the rate of translocation. Experiments performed in the presence of non-hydrolyzable GTP analogues and specific antibiotics further reveal that allosterically linked conformational events in EF-G and the ribosome mediate rapid, directional substrate movement and EF-G release.