Trans-translation
Coding by tmRNA was discovered in 1995 when Simpson and coworkers overexpressed a mouse cytokine in E. coli and found several truncated cytokine-derived peptides each tagged at the carboxyl termini with the same 11-amino acid residue extension (A)ANDENYALAA. With the exception of the N-terminal alanine, which comes from the 3' end of tmRNA itself, this tag sequence was traced to a short open reading frame in E. coli tmRNA. Recognizing that the tag peptide confers proteolysis, the trans-translation model for tmRNA action was proposed.
While details of the trans-translation mechanism are under investigation it is generally agreed that tmRNA first occupies the empty A site of the stalled ribosome. Subsequently, the ribosome moves from the 3' end of the truncated messenger RNA onto the resume codon of the MLR, followed by a slippage-prone stage from where translation continues normally until the in-frame tmRNA stop codon is encountered. Trans-translation is essential in some bacterial species, whereas other bacteria require tmRNA to survive when subjected to stressful growth conditions. Depending on the organism, the tag peptide may be recognized by a variety of proteases or protease adapters.
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