SELECTIVE IMPORT OF NUCLEAR-ENCODED TRANSFER-RNAS INTO MITOCHONDRIA OF THE PROTOZOAN LEISHMANIA-TARENTOLAE

The trypanosomatid mitochondrial genome does not encode tRNA genes at all and experimental evidence obtained with Leishmania tarentolae shows that tRNAs in mitochondria represent a selected set of imported nuclear-encoded tRNAs. In this paper we present the data showing that tRNAs derived from the clustered genomic tRNA genes are invariably imported into mitochondria, while tRNA from the solitary gene is not. By sequencing a cosmid DNA clone of L. tarentolae genomic DNA, we have identified a 1.5-kb subclone encoding a duplicate set of the closely linked tRNA(Tyr) (GTA) and tRNA(Thr) (AGT) genes. Northern analysis shows that these tRNAs are imported into mitochondria. In contrast, when the tRNA gene [tRNAG(Gln) (CUG)] located alone in a 40-kb DNA fragment was examined, the corresponding tRNA was not detected in the mitochondrion. This ''loner'' tRNA gene is highly unusual since the 3'-flanking putative RNA polymerase III transcription termination signal sequence is characterized by a long string of 8 Ts followed by an A and a stretch of 7 Cs, while all other trypanosomatid tRNA genes whose tRNA transcripts are imported are terminated by a possible transcription termination signal of only 4-6 Ts. Whether the correlation found between the gene organization and tRNA-import characteristics is of general significance needs to be investigated further. A simple computer analysis presented in this paper rules out the possibility that tRNAs found in the trypanosomatid mitochondrion are the products of the U-addition type 'RNA editing' of maxicircle DNA.