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Mechanisms of Ribosomal Translocation Along mRNA

Movement of EF-G

Movement of EF-G between compact and extended conformations detected by single-molecule FRET.
A single-molecule FRET trace for doubly-labeled EF-G bound to the ribosome shows fluorescence
intensities observed for donor (green) and acceptor (red). Decrease in energy transfer, which
is manifested by the decrease in acceptor fluorescence, indicates interdomain movement of EF-G
from compact to a more extended conformation (From Salsi et al (2015) J Mol Biol).

All ribosomes have a ribonucleoprotein structure made up of two unequal subunits, both of which contain three binding sites for tRNA: the A (aminoacyl), P (peptidyl) and E (exit) sites. Sequential movement of tRNAs from the A site to the P site to the E site is coupled with the movement of their associated codons in the mRNA. Translocation is induced by elongation factor G (EF-G) in bacteria, or by its eukaryotic homologue EF-2. We study how conformational changes in EF-G and the ribosome promote translocation of tRNA and mRNA. For instance, we have recently shown that ribosome-bound EF-G adopts distinct conformations corresponding to the pre- and post-translocation states of the ribosome and that upon ribosomal translocation, domain IV of EF-G moves towards the A site of the small ribosomal subunit while promoting tRNA translocation (Brilot et al (2013) PNAS; Salsi et al (2014) PNAS).

In eukaryotes, there is another type of ribosomal movement besides codon-by-codon translocation, namely “scanning” of the small ribosomal subunit along the 5’ untranslated region of the mRNA during the initiation of protein synthesis. We are expanding our research toward studying the mechanism of this process.

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