Transfer RNAs (tRNAs) are abundantly expressed RNA molecules required to bring amino acids to the translating ribosome for protein synthesis. Of the 506 known human tRNAs, 32 are encoded as pre-tRNAs, containing introns that are spliced during maturation (Lowe and Eddy, 1997; Phizicky and Hopper, 2010). The nuclear-localized tRNA splicing endonuclease (TSEN) complex recognizes the secondary structure of pre-tRNA molecules and cleaves at the exon-intron boundaries to yield a 2’,3’-cyclic phosphate (i.e. phosphodiester) and 5’-OH terminal at the splice site (Peebles et al., 1983). Although still the subject of some debate, there are at least two proposed mechanisms of re-ligating these “half” tRNAs in mammals (i.e. the 5’-and 3’-exons remaining after excision of the single intron). In the first, HSPC117 directly mediates the ligation of these ends, utilizing the phosphate from the phosphodiester bond in the linkage, and is inhibited by the presence of a 5’-phosphorylation (Popow et al., 2011). In the second, 5’-phosphorylation by CLP1, a component of the TSEN complex in mammals, is required (Paushkin et al., 2004; Weitzer and Martinez, 2007; Zillmann et al., 1991), but the
