Large-Scale Movements of IF3 and tRNA during Bacterial Translation Initiation

Summary

In bacterial translational initiation, three initiation factors (IFs 1–3) enable the selection of initiator tRNA and the start codon in the P site of the 30S ribosomal subunit. Here, we report 11 single-particle cryo-electron microscopy (cryoEM) reconstructions of the complex of bacterial 30S subunit with initiator tRNA, mRNA, and IFs 1–3, representing different steps along the initiation pathway. IF1 provides key anchoring points for IF2 and IF3, thereby enhancing their activities. IF2 positions a domain in an extended conformation appropriate for capturing the formylmethionyl moiety charged on tRNA. IF3 and tRNA undergo large conformational changes to facilitate the accommodation of the formylmethionyl-tRNA (fMet-tRNA ^fMet) into the P site for start codon recognition.

Graphical Abstract

Highlights

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  Structures of the 30S ribosomal subunit with initiation factors, tRNA and mRNA

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  IF3 helps to position the correct start codon in the P site before binding of tRNA

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  Large-scale conformational changes of IF3 and tRNA are observed

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  IF3 movements facilitate the accommodation of initiator tRNA in P site

Abstract

Bacterial ribosomes can start scanning for start codons only after undergoing large scale conformational changes governed by three key initiation factors.