Trans-translation model for tmRNA recognition and accommodation. A ribosome stalls on an incomplete message (A), leading to the recruitment of a quaternary “AlatmRNA–SmpB–EF-Tu·GTP” complex to the A-site of the stalled ribosomes. At this step, two molecules of SmpB, one per subunit, are recovered. (B) Then, EF-Tu hydrolyzes the GTP and dissociates from the TLD and from the ribosome; the acceptor arm of TLD moves into the A-site of the 50S subunit (accommodation). The SmpB protein interacting with the 30S decoding site stays in place while the one interacting with the 50S subunit moves away. A movement of H2 triggered by the accommodation prepares the shift on the tmRNA internal ORF.
