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Spontaneous ribosomal translocation of mRNA and tRNAs into a chimeric hybrid state

Significance The crystal structure of a pretranslocation complex containing mRNA and two tRNAs shows spontaneous movement of the mRNA and tRNAs from the classical state to a chimeric hybrid state in the absence of elongation factor G (EF-G) or GTP. This finding suggests that many steps of translocation, including rotation of the 30S subunit head domain and movement of the A-site tRNA into the position of the classical-state P-site tRNA, are supported by the ribosome itself. However, movement of mRNA and tRNA has become uncoupled, resulting in negative slippage of the translational reading frame. These findings have implications for our current view of the role of EF-G, suggesting that its main function may be to stabilize codon–anticodon pairing during translocation. The elongation factor G (EF-G)–catalyzed translocation of mRNA and tRNA through the ribosome is essential for vacating the ribosomal A site for the next incoming aminoacyl-tRNA, while precisely maintaining the translational reading frame. Here, the 3.2-Å crystal structure of a ribosome translocation intermediate complex containing mRNA and two tRNAs, formed in the absence of EF-G or GTP, provides insight into the respective roles of EF-G and the ribosome in translocation. Unexpectedly, the head domain of the 30S subunit is rotated by 21°, creating a ribosomal conformation closely resembling the two-tRNA chimeric hybrid state that was previously observed only in the presence of bound EF-G. The two tRNAs have moved spontaneously from their A/A and P/P binding states into ap/P and pe/E states, in which their anticodon loops are bound between the 30S body domain and its rotated head domain, while their acceptor ends have moved fully into the 50S P and E sites, respectively. Remarkably, the A-site tRNA translocates fully into the classical P-site position. Although the mRNA also undergoes movement, codon–anticodon interaction is disrupted in the absence of EF-G, resulting in slippage of the translational reading frame. We conclude that, although movement of both tRNAs and mRNA (along with rotation of the 30S head domain) can occur in the absence of EF-G and GTP, EF-G is essential for enforcing coupled movement of the tRNAs and their mRNA codons to maintain the reading frame.