A nuclear tRNA gene cluster in the protozoan Leishmania tarentolae and differential distribution of nuclear-encoded tRNAs between the cytosol and mitochondria

Abstract

All mitochondrial tRNAs in the protozoan Leishmania are believed to be encoded in the nuclear genome and imported selectively into the mitochondria by an as yet unknown mechanism. Previously, we reported that two tRNAs whose genes are tightly linked were imported by mitochondria. In contrast, a tRNA encoded by a lone tRNA gene was not detectable in mitochondria. The lone tRNA gene had flanking sequences that were different from the linked genes. These studies implied a possible correlation between tRNA gene organization and gene flanking sequence, and selective tRNA import into mitochondria. Here, we report the identification of a cluster of 10 tRNA genes and show the distribution of the corresponding tRNAs in cytosolic and mitochondrial fractions. tRNA leu(CAG) and tRNA 2 arg(TCG) are abundant in the cytosol, but relatively scarce in mitochondria. Conversely, tRNA ile(TAT) and tRNA 1 lys(TTT) are abundant in mitochondria, but relatively scarce in the cytosol, tRNA val(TAC) and tRNA 2 thr(TGT) are barely detectable in either cellular compartment, while tRNA gln(TTG), tRNA 1 arg(ACG), tRNA gly(TCC), and tRNA trp(CCA) are detected in approximately equal levels in both compartments. Sequencing of the 2600 bp that comprise the tRNA gene cluster also encoding the genes for 5S RNA and URNAB RNA indicates that nucleotide composition, length, and location of genes within the cluster do not clearly correlate with import characteristics. The unexpected presence of the tRNA trp(CCA)-gene transcript in mitochondria is also reported. Evidence suggests that this tRNA may have unidentified base modifications at the anticodon triplet.