Are tRNAs imported into the mitochondria of kinetoplastid protozoa as 5'-extended precursors?

All mitochondrial tRNAs in kinetoplastid protozoa are encoded in nuclear DNA and transported into the mitochondrion (Simpson et al., Nucl Acids Res 1989;17:5427-5445; Hancock and Hajduk, J Biol Chem 1990;265:19208-19215). It has been proposed that tRNAs in these cells are imported into the mitochondrion as 5'-extended precursors which are processed by a mitochondrial RNase P-like activity (Hancock et al., J Biol Chem 1992;267:23963-23971). We have examined this hypothesis by cloning and sequencing primer extension products of mitochondrial tRNAs from Leishmania tarentolae and Trypanosoma brucei, and have found that these are derived from circularized mature tRNA molecules. We suggest that these molecules are produced by the endogenous RNA ligase activity (Bakalara et al., J Biol Chem 1989;264:18679-18686) either in vivo or during mitochondrial isolation. We did not obtain any evidence for the existence of high molecular weight precursors of mitochondrial tRNAs. This negative result is consistent with previous in vivo transfection studies with both L. tarentolae (Lima and Simpson, RNA 1996;2:429-440) and T. brucei (Hauser and Schneider, EMBO J 1995;14:4212-4220; Schneider et al., Mol Cell Biol 1994;14:2317-2322), in which mitochondrial targeting of plasmid-expressed tRNAs was independent of the presence of 5'-flanking sequences. We conclude that the hypothesis for 5'-extended tRNA precursors in kinetoplastid mitochondrial importation remains to be verified.