Mitochondrial tRNA synthetases act in the mitochondrial matrix to catalyze the reactions of tRNAs encoded in the mitochondrial genome, their cognate amino acids, and ATP to form aminoacyl-tRNAs, AMP, and pyrophosphate (Schneider et al. 2000). The synthetase enzymes that catalyze these reactions are all encoded in the nuclear genome. In three cases, glycine, lysine, and glutamine, a single gene encodes two enzyme isoforms, one cytosolic and one mitochondrial. All other mitochondrial tRNA synthetases are encoded by genes different from the ones encoding the corresponding cytosolic enzymes.