This Is the End: Processing, Editing and Repair at the tRNA 3-Terminus

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Abstract
The generation of a mature tRNA 3end is an important step in the processing pathways leading to functional tRNA molecules. While 5end processing by RNase P is similar in all organisms, generation of the mature 3terminus seems to be more variable and complex. The first step in this reaction is the removal of 3trailer sequences. In bacteria, this is a multistep process performed by endo and exonucleases. In contrast, the majority of eukaryotes generate the mature tRNA 3end in a single step reaction, which consists of an endonucleolytic cut at the tRNA terminus. After removal of the 3trailer, a terminal CCA triplet has to be added to allow charging of the tRNA with its cognate amino acid. The enzyme catalyzing this reaction is tRNA nucleotidyltransferase, homologs of which have been found in representatives of all three kingdoms. Furthermore, in metazoan mitochondria, some genes encode 3terminally truncated tRNAs, which are restored in an editing reaction in order to yield functional tRNAs. Interestingly, this reaction is not restricted to distinct tRNAs, but seems to act on a variety of tRNA molecules and represents therefore a more general tRNA repair mechanism than a specialized editing reaction. In this review, the current knowledge about these crucial reactions is summarized.