EF-G and EF4: translocation and back-translocation on the bacterial ribosome.

Ribosomes translate the codon sequence of an mRNA into the amino acid sequence of the corresponding protein. One of the most crucial events is the translocation reaction, which involves movement of both the mRNA and the attached tRNAs by one codon length and is catalysed by the GTPase elongation factor G (EF-G). Interestingly, recent studies have identified a structurally related GTPase, EF4, that catalyses movement of the tRNA2-mRNA complex in the opposite direction when the ribosome stalls, which is known as back-translocation. In this Review, we describe recent insights into the mechanistic basis of both translocation and back-translocation.