The complete, functional and dynamic cycle of the bacterial Initiation Factor 3

Abstract

Initiation factor 3 (IF3) is an essential protein that enhances the fidelity and speed of bacterial initiation of mRNA translation. The dynamic interplay between the two independent IF3 domains, their alternative binding sites, and the mechanism that ensures translation initiation fidelity remains elusive. Here, we show that the functional positioning of IF3 domains occurs at velocities ranging over two orders of magnitude, driven by each 30S initiation ligand. IF1 and IF2 rapidly promote the accommodation of IF3 on the 30S platform with the C-terminal domain moving towards the P site. Reversion of this movement is triggered by decoding the mRNA start codon and rate limits translation initiation. Binding of the tRNA results in the concomitant accommodation of the N-terminal domain of IF3, largely dependent on the mRNA and initiator tRNA. 70S initiation complex formation promotes the closing and dissociation of IF3, recycling the factor for a new round of translation initiation. Altogether our results unveil the kinetic spectrum of IF3 conformations and highlight fundamental movements of the factor that ensure accurate translation initiation.