Formation of the 30S initiation complex (30S IC) is an important checkpoint in regulation of gene expression. The selection of mRNA, correct start codon, and the initiator fMet-tRNAfMet requires the presence of three initiation factors (IF1, IF2, IF3) of which IF3 and IF1 control the fidelity of the process, while IF2 recruits fMet-tRNAfMet. Here we present a cryo-EM reconstruction of the complete 30S IC, containing mRNA, fMet-tRNAfMet, IF1, IF2, and IF3. In the 30S IC, IF2 contacts IF1, the 30S subunit shoulder, and the CCA end of fMet-tRNAfMet, which occupies a novel P/I position (P/I1). The N-terminal domain of IF3 contacts the tRNA, whereas the C-terminal domain is bound to the platform of the 30S subunit. Binding of initiation factors and fMet-tRNAfMet induces a rotation of the head relative to the body of the 30S subunit, which is likely to prevail through 50S subunit joining until GTP hydrolysis and dissociation of IF2 take place. The structure provides insights into the mechanism of mRNA selection during translation initiation.

During protein synthesis, tRNAs and mRNA move through the ribosome between aminoacyl (A), peptidyl (P), and exit (E) sites of the ribosome in a process called translocation. Translocation is accompanied by the displacement of the tRNAs on the large ribosomal subunit toward the hybrid A/P and P/E states and by a rotational movement (ratchet) of the ribosomal subunits relative to one another. So far, the structure of the ratcheted state has been observed only when translation factors were bound to the ribosome. Using cryo-electron microscopy and classification, we show here that ribosomes can spontaneously adopt a ratcheted conformation with tRNAs in their hybrid states. The peptidyl-tRNA molecule in the A/P state, which is visualized here, is not distorted compared with the A/A state except for slight adjustments of its acceptor end, suggesting that the displacement of the A-site tRNA on the 50S subunit is passive and is induced by the 30S subunit rotation. Simultaneous subunit ratchet and formation of the tRNA hybrid states precede and may promote the subsequent rapid and coordinated tRNA translocation on the 30S subunit catalyzed by elongation factor G.translocation ͉ elongation factor G ͉ cryo-electron microscopy D uring the elongation cycle, the tRNAs⅐mRNA complex is translocated to allow reading of the following codon on mRNA. Translocation is promoted by elongation factor G (EF-G) and accompanied by GTP hydrolysis. During translocation, the ribosome changes from the pretranslocation state with deacylated tRNA in the peptidyl site (P site) and peptidyltRNA in the aminoacyl site (A site) to the posttranslocation state where A-and P-site tRNAs have moved to P and exit (E) sites, respectively. The universal design of ribosomes from two subunits inspired early suggestions that translocation may involve a movement of the subunits relative to each other (1, 2). Such models imply the existence of intermediate states for the tRNAs that differ from the classic A, P, and E states. Chemical probing experiments with pretranslocation ribosomes indicated that after peptidyl transfer the acceptor ends of the tRNAs spontaneously moved on the large 50S subunit toward their posttranslocation positions (3), indicating that peptidyl-tRNA and deacylated tRNA entered hybrid A/P and P/E states, respectively. The transition was observed in the absence of EF-G, and the driving force for the movement was attributed to different affinities of the A, P, and E sites on the 50S subunit (4) for the chemically different acceptor ends of the tRNAs (5, 6). Nevertheless, cryo-electron microscopy (cryoEM) of ribosomes in the pretranslocation state showed the tRNAs in their classic A, P, and E states (7). Although this result was difficult to reconcile with the hybrid-state model, it is important to note that in that complex the occupancy of the E site by deacylated tRNA may have prevented the P-site tRNA to enter the P/E-site hybrid state.CryoEM of ribosomes in complex with EF-G revealed a relative rotation between subunits referred to as ratc...

No abstract

No abstract