RNA polymerase II elongation complex showing positions of bridge helix (BH) and trigger loop (TL). (a) Cartoon representation of 10-subunit Pol II elongation complex (PDB: 2e2h)  with protein shown in charcoal gray, template DNA in blue, RNA product in red, and non-template DNA in green. Buried within, near the active site, are the TL (cyan) and BH (magenta). (b) Transparent view of (a) showing the position of the BH and TL relative to the nucleic acid scaffold within Pol II. (c) Rotated view of (b) showing the position of the BH and TL relative to the 3' end of the RNA product (the site of transcript elongation, shown by an arrow). This figure was created with PyMOL.
Kaplan and Kornberg Journal of Biology 2008 7:39 doi:10.1186/jbiol99