New insights into the enzymatic role of EF-G in ribosome recycling.

During translation, elongation factor G (EF-G) plays a catalytic role in tRNA translocation and a facilitative role in ribosome recycling. By stabilizing the rotated ribosome and interacting with ribosome recycling factor (RRF), EF-G was hypothesized to induce the domain rotations of RRF, which subsequently performs the function of splitting the major intersubunit bridges and thus separates the ribosome into subunits for recycling. Here, with systematic mutagenesis, FRET analysis and cryo-EM single particle approach, we analyzed the interplay between EF-G/RRF and post termination complex (PoTC). Our data reveal that the two conserved loops (loop I and II) at the tip region of EF-G domain IV possess distinct roles in tRNA translocation and ribosome recycling. Specifically, loop II might be directly involved in disrupting the main intersubunit bridge B2a between helix 44 (h44 from the 30S subunit) and helix 69 (H69 from the 50S subunit) in PoTC. Therefore, our data suggest a new ribosome recycling mechanism which requires an active involvement of EF-G. In addition to supporting RRF, EF-G plays an enzymatic role in destabilizing B2a via its loop II.