Splicing and 3'-processing of the tyrosine tRNA of Trypanosoma brucei.

Trypanosoma brucei belongs to a family of protozoa that is characterized as having diverged early in the evolution of eukaryotes. Many unusual forms of RNA processing have been discovered in these cells, one of which is trans-splicing, and, although a great number of genes have been sequenced, no evidence for cis-splicing has yet been found. This study shows that tRNA(Tyr) of T. brucei contains an 11-nucleotide intron. tRNA of the size predicted for unspliced precursor was detected by Northern analysis using an oligonucleotide probe complementary to the putative intervening region. Direct sequence analysis of mature tRNA(Tyr) showed that the predicted intron region is absent from this form of the molecule. Mutated versions of the gene encoding tRNA(Tyr) were introduced into trypanosomes by DNA transformation and shown to be expressed in vivo. Introduction into the genome of a tRNA(Tyr) gene containing an amber suppressor mutation led to a significant accumulation of unspliced precursor molecules. Analysis of the tRNA(Tyr) species from both the wild type and transformed cells revealed unspliced and spliced processing intermediates. The relative abundance of each of these intermediates suggests that 3'-processing and splicing occur independently and that, in the wild type, splicing tends to precede 3'-processing and CCA addition.