The 22 tRNAs encoded by the mitochondrial genome are modified in the mitochondrial matrix by enzymes encoded in the nucleus and imported into mitochondria (reviewed in Suzuki et al. 2011, Salinas-Giege et al. 2015). Some enzymes such as PUS1 and TRIT1 are located in more than one compartment and modify both mitochondrial tRNAs and cytosolic tRNAs. Other enzymes such as MTO1, TRMU, and TRMT61B are exclusively mitochondrial.
Modifications near the anticodon and near the 3' end of tRNAs tend to affect interaction of the tRNA with mRNA within ribosomes and with tRNA synthetases, respectively. Modifications in other regions, typically in the "core" of the tRNA tend to affect folding and stability of the tRNA (reviewed in Hou et al. 2015). The unusual modification 5-taurinomethyl-2-thiouridine-34 in the anticodon of at least 3 tRNAs is found only in mammalian mitochondria and mutations that affect the responsible biosynthetic enzymes (GTPBP3, MTO1, TRMU) cause mitochondrial dysfunction and disease (reviewed in Torres et al. 2014).