Leishmania tarentolae Contains Distinct Cytosolic and Mitochondrial Glutaminyl--tRNA Synthetase Activities

The intracellular distribution of glutaminyl-tRNA synthetases and their role in mitochondrial tRNA import were evaluated in the ancient eukaryote Leishmania tarentolae. The following results were obtained: (i) Glutaminyl-tRNA synthetase was detected in leishmanial mitochondria. This was unexpected because it has been postulated that, in organelles, Gln-tRNA^Gln is not formed by direct acylation of tRNA^Gln but by enzymatic transamidation of misacylated Glu-tRNA^Gln. (ii) Whereas the cytosolic extract is able to charge cytosolic and mitochondrial tRNAs^Gln, the mitochondrial matrix extract does not aminoacylate the cytosol-specific tRNA^Gln. This indicates that mitochondrial and cytosolic glutaminyl-tRNA synthetases are distinct. (iii) Seven of the 11 nucleotides that differ between the cytosolic and the mitochondrial tRNA^Gln are sufficient to convert the cytosol-specific tRNA^Gln into an optimal substrate for the mitochondrial enzyme. These nucleotides are arranged in three groups consisting of the nucleotides flanking the anticodon stem, the 5' nucleotide of the anticodon, and four nucleotides within the acceptor stem. And (iv), it was shown that the identity elements for recognition by the mitochondrial glutaminyl-tRNA synthetase do not overlap with a previously identified sequence segment required for mitochondrial import of the tRNA^Gln.