Yeast RNA polymerase II at 5 angstrom resolution

Appropriate treatment of X-ray diffraction from an unoriented 18-heavy atom cluster derivative of a yeast RNA polymerase II crystal gave significant p hase information to 5 Angstrom resolution. The validity of the phases was s hown by close similarity of a 6 Angstrom electron density map to a 16 Angst rom molecular envelope of the polymerase from electron crystallography. Com parison of the 6 Angstrom X-ray map with results of electron crystallograph y of a paused transcription elongation complex suggests functional roles fo r two mobile protein domains: the tip of a flexible arm forms a downstream DNA clamp; and a hinged domain may serve as an RNA clamp, enclosing the tra nscript from about 8-18 residues upstream of the 3'-end in a tunnel.