Photocross-linking of the RNA polymerase I preinitiation and immediate postinitiation complexes - Implications for promoter recruitment

The architecture of eukaryotic rRNA transcription complexes was analyzed, revealing facts significant to the RNA polymerase (pol) I initiation process. Functional initiation and elongation complexes were mapped by site-specific photocross-linking to template DNA. Polymerase I is recruited to the promoter via protein-protein interactions with DNA-bound transcription initiation factor-IB. The latter's TATA-binding protein (TBP) and TAFs photocross-link to the promoter from - 78 to + 10 relative to the tis ( + 1). Although TBP does not bind DNA using its TATA-binding saddle, it does photocross-link to a 22-bp sequence that does not resemble a TATA box. Only TAF(I)96 ( the mammalian TAF(I)68, yeast Rrn7p homolog) overlaps significantly with the DNA interaction cleft of pol I based on modeling to the pol II crystal structure. None of the pol I-specific subunits that are localized on the lips of the cleft (A49 and A34.5) or the pol I-specific stalk (A43 and A14) cross-link to DNA. Pol I does not extend significantly upstream of the promoter-proximal border of the factor complex ( - 11 to - 14), and similarly in the promoter proximal elongation complex, the enzyme does not contact DNA upstream of its normal exit from the cleft.