Structural basis of initial RNA polymerase II transcription

During transcription initiation by RNA polymerase (Pol) II, a transient open promoter complex (OC) is converted to an initially transcribing complex (ITC) containing short RNAs, and to a stable elongation complex (EC). We report structures of a Pol II–DNA complex mimicking part of the OC, and of complexes representing minimal ITCs with 2, 4, 5, 6, and 7 nucleotide (nt) RNAs, with and without a nonhydrolyzable nucleoside triphosphate (NTP) in the insertion site þ1. The partial OC structure reveals that Pol II positions the melted template strand opposite the active site. The ITC-mimicking structures show that two invariant lysine residues anchor the 30-proximal phosphate of short RNAs. Short DNA–RNA hybrids adopt a tilted conformation that excludes the þ1 template nt from the active site. NTP binding induces complete DNA translocation and the standard hybrid conformation. Conserved NTP contacts indicate a universal mechanism of NTP selection. The essential residue Q1078 in the closed trigger loop binds the NTP 20-OH group, explaining how the trigger loop couples catalysis to NTP selection, suppressing dNTP binding and DNA synthesis