EF4 reveals the energy barrier for tRNA back-translocation in the peptidyl transferase center.

In the translating ribosomal complex, transfer RNA (tRNA) is stabilized in the ribosome by its anticodon stem-loop (ASL) and 3'-CCA end through base-pairing interactions with mRNA codon on the small subunit and rRNA in the peptidyl transferase center (PTC) of large subunit, respectively.Elongation factor 4 (EF4), a highly conserved translational GTPase, has been identified to trigger back-translocation. Early this year, we reported high resolution cryo-EM structures of EF4 in complex with Escherichia coli 70S ribosome in pre- and post-translocational states with direct observations that EF4 disrupts the base pairs between the 3'-end of peptidyl-tRNA and the P-loop of rRNA in PTC. Here, we focus on the novel molecular mechanism how EF4 catalyzes back-translocation, and discuss the common and specific energy barriers for forward- and back-translocation.