TRNT1 (CCA-adding enzyme) polymerizes the non-templated nucleotides CCA onto the 3' end (acceptor arm) of tRNAs (Reichert et al. 2001, Lizano et al. 2007, Lizano et al. 2008). In contrast to some eubacterial tRNA genes, eukaryotic tRNA genes do not encode CCA at the 3' end. The added CCA sequence is determined by the structural properties of the TRNT1 protein itself rather than by any nucleic acid template (Augustin et al. 2003, Ernst et al. 2015, Kuhn et al. 2015, see also the structure of the bacterial enzyme in Li et al. 2002). After polymerization of a single CCA motif, conformational change of the enzyme ejects the tRNA before additional nucleotides can be polymerized (Kuhn et al. 2015). Aberrant RNA with two CCA motifs is targeted for destruction.