Architecture of RNA polymerase II and implications for the transcription mechanism

A backbone model of a 10-subunit yeast RNA polymerase II has been derived f rom x-ray diffraction data extending to 3 angstroms resolution. All 10 subu nits exhibit a high degree of identity with the corresponding human protein s, and 9 of the 10 subunits are conserved among the three eukaryotic RNA po lymerases I, II, and III. Notable features of the model include a pair of j aws, formed by subunits Rpb1, Rpb5, and Rpb9, that appear to grip DNA downs tream of the active center. A clamp on the DNA nearer the active center, fo rmed by Rpb1, Rpb2, and Rpb6, may be Locked in the closed position by RNA, accounting for the great stability of transcribing complexes. A pore in the protein complex beneath the active center may allow entry of substrates fo r polymerization and exit of the transcript during proofreading and passage through pause sites in the DNA.