The biological roles of trans-translation

The unique transfer-messenger RNA (tmRNA) molecule has been identified in all bacterial species examined, suggesting that its action confers an important survival advantage to bacteria. Acting both as a tRNA and an mRNA, in a process known as trans-translation, tmRNA adds a short peptide tag to undesirable proteins. Trans-translation plays at least two physiological roles: removing ribosomes stalled upon mRNA, and targeting the resulting truncated proteins for degradation by proteases. The first of these roles is required for all known activities of tmRNA, whereas the second may be dispensed with in most cases with little biological effect. However, tmRNA-targeted proteolysis may be important for fine-tuning expression of certain genes by altering the concentration of regulatory proteins. Here, we review recent literature that addresses the biological functions of tmRNA.