trans-Translation is an extreme version of recoding in which the translating ribosome is diverted onto a specialized RNA, producing a protein encoded in two distinct RNA molecules. The specialized RNA that is used in trans, called tmRNA or SsrA, has properties of both a tRNA and an mRNA. tmRNA bound to a small protein, SmpB, can enter the A-site of substrate ribosomes and accept the nascent polypeptide, acting like a tRNA. The mRNA is removed from the ribosome, and an open reading frame within tmRNA is inserted in the decoding center and translated. The product of trans-translation is a protein encoded in part from the original mRNA and in part from tmRNA. This reaction is the only known example of translation from two physically distinct messages. One use of trans-translation is to release ribosomes that are stalled at the end of damaged mRNAs. However, trans-translation can also be induced in response to signals in the mRNA or nascent polypeptide, and by specific cleavage of the mRNA, suggesting that trans-translation can be used for regulation as well as quality control.
KeywordsStop Codon Rare Codon Nascent Polypeptide mRNA Cleavage lacI Gene
We thank S. Yokoyama and Y. Bessho for providing us with the coordinates for the model of tRNASer. We apologize to authors whose work we were not able to cite due to space constraints. The authors were supported by National Institutes of Health grant GM068720.
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