The SsrA-SmpB system for protein tagging, directed degradation and ribosome rescue.

Bacteria contain a remarkable RNA molecule - known alternatively as SsrA RNA, tmRNA, or 10Sa RNA - that acts both as a tRNA and as an mRNA to direct the modification of proteins whose biosynthesis has stalled or has been interrupted. These incomplete proteins are marked for degradation by cotranslational addition of peptide tags to their C-termini in a reaction that is mediated by ribosome-bound SsrA RNA and an associated protein factor, SmpB. This system plays a key role in intracellular protein quality control and also provides a mechanism to clear jammed or obstructed ribosomes. Here the structural, functional and phylogenetic properties of this unique RNA and its associated factors are reviewed, and the intracellular proteases that act to degrade the proteins tagged by this system are also discussed.