How a circularized tmRNA moves through the ribosome

Mechanism of ribosome rescue Bacterial ribosomes that stall on truncated or cleaved messenger RNA (mRNA) are rescued by trans-translation. Two factors, transfer-messenger RNA (tmRNA) and small protein B (SmpB), resolve the stalled complex by tagging the nascent polypeptide for degradation and facilitating release of the ribosome. Rae et al. determined structures of key trans-translation intermediates. The structures reveal how SmpB identifies stalled ribosomes; how the large, circularized tmRNA molecule moves through the ribosome; and how translation is shifted from the truncated mRNA to tmRNA. Science , this issue p. 740