tRNA processing

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Homo sapiens
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Genes encoding transfer RNAs (tRNAs) are transcribed by RNA polymerase III in the nucleus and by mitochondrial RNA polymerase in the mitochondrion.
In the nucleus transcription reactions produce precursor tRNAs (pre-tRNAs) that have extra 5' leaders, 3' trailers, and, in some cases, introns which are removed by enzymes and enzyme complexes: RNase P cleaves the 5' leader, RNase Z cleaves the 3' trailer, TRNT1 polymerizes CCA onto the resulting 3' end, the TSEN complex cleaves at each end of the intron, and the tRNA ligase complex ligates the resulting exons (reviewed in Rossmanith et al. 1995, Phizicky and Hopper 2010, Suzuki et al. 2011, Abbott et al. 2014, Li and Mason 2014). The nucleotides within tRNAs undergo further chemical modifications such as methylation and deamination by a diverse set of enzymes (reviewed in Helm and Alfonzo 2014, Boschi-Muller and Motorin 2013). The order of events for each tRNA is not fully known and the understanding of the overall process is complicated by the retrograde (cytosol to nucleus) transport of tRNAs.
In the mitochondrial matrix transcription produces long precursor RNAs, H strand transcripts and an L strand transcript, that are cleaved by mitochondrial RNase P (an entirely proteinaceous complex), ELAC2, and other nucleases to yield 12S rRNA, 16S rRNA, mRNAs, and pre-tRNAs lacking 3' CCA sequences (reviewed in Van Haute et al. 2015). TRNT1 polymerizes an untemplated CCA sequence onto the 3' ends of the pre-tRNAs and chemical modifications are made to several nucleotides in the tRNAs.

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