Abstract
Spliced leader RNA transcription is essential for cell viability in trypanosomes. The SL RNA genes are expressed from the only defined RNA polymerase II-dependent promoter identified to date in the trypanosome genome. The SL RNA gene promoter has been shown by in vitro and in vivo analyses to have a tripartite architecture. The upstream most cis-acting element, called PBP-1E, is located between 70 and 60 bp upstream from the transcription start site. This essential element functions along with two downstream elements to direct efficient and proper initiation of transcription. Electrophoretic mobility-shift studies detected a 122-kDa protein, called PBP-1, which interacts with PBP-1E. This protein is the first sequence-specific, double-stranded DNA-binding protein isolated in trypanosomes. Three polypeptides copurify with PBP-1 activity, suggesting that PBP-1 is composed of 57-, 46-, and 36-kDa subunits. We have cloned the genes that encode the 57- and 46-kDa subunits. The 46-kDa protein is a previously uncharacterized protein and may be unique to trypanosomes. Its predicted tertiary structure suggests it binds DNA as part of a complex. The 57-kDa subunit is orthologous to the human small nuclear RNA-activating protein (SNAP)50, which is an essential subunit of the SNAP complex (SNAPc). In human cells, SNAPc binds to the proximal sequence element in both RNA polymerase II- and III-dependent small nuclear RNA gene promoters. These findings identify a surprising link in the transcriptional machinery across a large evolutionary distance in the regulation of small nuclear RNA genes in eukaryotes.