Yeast RNA Polymerase II at 5 Å Resolution

Appropriate treatment of X-ray diffraction from an unoriented 18-heavy atom cluster derivative of a yeast RNA polymerase II crystal gave significant phase information to 5 Å resolution. The validity of the phases was shown by close similarity of a 6 Å electron density map to a 16 Å molecular envelope of the polymerase from electron crystallography. Comparison of the 6 Å X-ray map with results of electron crystallography of a paused transcription elongation complex suggests functional roles for 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 transcript from about 8–18 residues upstream of the 3′-end in a tunnel.