The human small nuclear RNA (snRNA) genes provide a particularly good system to study ft~nciamenta1 mechanisms of transcription such as the mechanisms that allow core promoters to recruit the correct RNA polymerase (pol), and the mechanisms by which activators of transcription perform their function. Indeed, the pol II and III snRNA promoters are very similar to each other, suggesting that any existing difference is relevant to the determination of RNA polymerase specificity. In addition, the snRNA promoters are relatively simple in structure. The core pol II snRNA promoters consist of just one essential element, the proximal sequence element or PSE, which recruits a multisubunit factor called SNAPc. The core pol Ill snRNA promoters contain both a PSE, which also recruits SNAPc, and a TATA box, which recruits the TATA box binding protein TBP. The regulatory region of both the pol II and III snRNA promoters consists of a distal sequence element or DSE, which activates transcription. The DSE can contain several binding sites, but one of them is almost invariably an octanier sequence that recruits the transcription activator Oct-1. We propose to pursue our characterization of the mechanisms by which snRNA promoters specifically assemble pol II and III transcription initiation complexes, in particular we propose to l) continue the characterization of SNAPc, specifically to determine how the complex recognizes the PSE, how it down-regulates its own binding to DNA, and how it cooperates with TBP for efficient binding to the U6 snRNA promoter; 2) continue the identification of the minimal set of factors required to assemble functional pol II and III initiation complexes on snRNA promoters; and 3) study the role of chromatin for pol II and III transcription of snRNA genes.
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