Trans-translation: the tmRNA-mediated surveillance mechanism for ribosome rescue, directed protein degradation, and nonstop mRNA decay.

The accurate flow of genetic information from DNA to RNA to protein is essential for all living organisms. An astonishing array of quality-assurance mechanisms have evolved to ensure that high degree of fidelity is maintained at every stage of this process. One of the most fascinating quality-control mechanisms involves tmRNA, also known as SsrA or 10Sa RNA. tmRNA is a versatile and highly conserved bacterial molecule endowed with the combined structural and functional properties of both a tRNA and a mRNA. The tmRNA system orchestrates three key biological functions: (1) recognition and rescue of ribosomes stalled on aberrant mRNAs, (2) disposal of the causative defective mRNAs, and (3) addition of a degradation tag to ribosome-associated protein fragments for directed proteolysis. Although not essential in Escherichia coli, tmRNA activity is essential for bacterial survival under adverse conditions and for virulence in some, and perhaps all, pathogenic bacteria. Recent evidence suggests that in addition to its quality-control function the tmRNA system might also play a key regulatory role in certain physiological pathways. This review will focus on recent advances in our understanding of the structural properties, mechanistic details, and physiological significance of this unique RNA and its principal protein partners.