Quality control of the elongation step of protein synthesis by tmRNP

Trans-translation is a quality-control process, activated upon premature te rmination of protein elongation, which recycles stalled ribosomes and degra des incomplete polypeptides. These functions are facilitated by transfer-me ssenger RNA (tmRNA, also called 10Sa RNA or SsrA RNA), a small stable RNA m olecule encoded by the SsrA gene found in bacteria, chloroplasts and mitoch ondria. Most tmRNAs consist of a tRNA- and an mRNA-like domain connected by up to four pseudoknots. Comparative sequence analysis provided the first i nsight into tmRNA secondary and three-dimensional structure. Studies of the E. coli tmRNA in vitro and in vivo demonstrated that tmRNA functions as a ribonucleoprotein (RNP) complex with elongation factor Tu (EF-Tu), protein SmpB and ribosomal protein S1. The tRNA-like and mRNA-like activities of tm RNA mark prematurely terminated proteins for degradation by attaching to th eir C-termini peptide tags, which are recognized by numerous proteases. Stu dies aimed at understanding the details of the molecular mechanisms of tran s-translation are ongoing.