tRNAs of Trypanosoma brucei. Unusual gene organization and mitochondrial importation

In Trypanosoma brucei the U snRNA B gene (J. C. Mottram, K. L. Perry, P. M. Lizardi, R. Lührmann, N. Agabian, and R. G. Nelson (1989) Mol. Cell. Biol. 9, 1212-1223) is very tightly linked with other small RNA genes coding for tRNA(ACGArg), tRNA(CUULys), and a approximately 275-nucleotide RNA (RNA X) of unknown function. A similar genomic organization is found at the U6 snRNA locus, where the U6 gene is linked to tRNA(CGUThr) and tRNA(GUATyr) genes. The tRNA(Lys) and tRNA(Arg) genes are members of a multigene family, whereas the tRNA(Thr) and tRNA(Tyr) genes are single copy. Two additional tRNA(CUULys) genes and one tRNA(UUULys) gene were also isolated and sequenced and, together with a sequence previously published (D. A. Campbell (1989) Nucleic Acids Res. 17, 9479), appear to represent the entire gene family. Probes for tRNA(Lys), tRNA(Arg), tRNA(Thr), and tRNA(Tyr) were found to hybridize with mitochondrial and cytoplasmic tRNAs but not with mitochondrial DNA. This supports the hypothesis that mitochondrial tRNAs may be nuclear-encoded and imported from the cytosol into the mitochondrion.