Trypanosomal TBP functions with the multisubunit transcription factor tSNAP to direct spliced-leader RNA gene expression.

Protein-coding genes of trypanosomes are mainly transcribed polycistronically and cleaved into functional mRNAs in a process that requires trans splicing of a capped 39-nucleotide RNA derived from a short transcript, the spliced-leader (SL) RNA. SL RNA genes are individually transcribed from the only identified trypanosome RNA polymerase II promoter. We have purified and characterized a sequence-specific SL RNA promoter-binding complex, tSNAP(c), from the pathogenic parasite Trypanosoma brucei, which induces robust transcriptional activity within the SL RNA gene. Two tSNAP(c) subunits resemble essential components of the metazoan transcription factor SNAP(c), which directs small nuclear RNA transcription. A third subunit is unrelated to any eukaryotic protein and identifies tSNAP(c) as a unique trypanosomal transcription factor. Intriguingly, the unusual trypanosome TATA-binding protein (TBP) tightly associates with tSNAPc and is essential for SL RNA gene transcription. These findings provide the first view of the architecture of a transcriptional complex that assembles at an RNA polymerase II-dependent gene promoter in a highly divergent eukaryote.