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.