Structures of tmRNA and SmpB as they transit through the ribosome

In bacteria, trans-translation is the main rescue system, freeing ribosomes stalled on defective messenger RNAs. This mechanism is driven by small protein B (SmpB) and transfer-messenger RNA (tmRNA), a hybrid RNA known to have both a tRNA-like and an mRNA-like domain. Here we present four cryo-EM structures of the ribosome during trans-translation at resolutions from 3.0 to 3.4 angstrom. These include the high-resolution structure of the whole pre-accommodated state, as well as structures of the accommodated state, the translocated state, and a translocation intermediate. Together, they shed light on the movements of the tmRNA-SmpB complex in the ribosome, from its delivery by the elongation factor EF-Tu to its passage through the ribosomal A and P sites after the opening of the B1 bridges. Additionally, we describe the interactions between the tmRNA-SmpB complex and the ribosome. These explain why the process does not interfere with canonical translation. Trans-translation, mediated by small protein B (SmpB) and transfer-messenger RNA (tmRNA), enables recycling of the ribosomes stalled on defective mRNAs in bacteria. Here, the authors report structures of the ribosome during trans-translation that reveal a translocation intermediate and elucidate the movements of the tmRNA-SmpB complex in the ribosome.

Automated summary

Trans-translation, mediated by SmpB and tmRNA, enables recycling of ribosomes stalled on defective mRNAs in bacteria. Structures of the ribosome during trans-translation reveal a translocation intermediate and elucidate the movements of the tmRNA-SmpB complex in the ribosome.