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How a circularized tmRNA moves through the ribosome

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Science  15 Feb 2019:
Vol. 363, Issue 6428, pp. 740-744
DOI: 10.1126/science.aav9370

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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

Abstract

During trans-translation, transfer-messenger RNA (tmRNA) and small protein B (SmpB) together rescue ribosomes stalled on a truncated mRNA and tag the nascent polypeptide for degradation. We used cryo–electron microscopy to determine the structures of three key states of the tmRNA-SmpB-ribosome complex during trans translation at resolutions of 3.7 to 4.4 angstroms. The results show how tmRNA and SmpB act specifically on stalled ribosomes and how the circularized complex moves through the ribosome, enabling translation to switch from the old defective message to the reading frame on tmRNA.

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