Bacterial RNAP holoenzyme. A. The Thermus aquaticus RNAP holoenzyme. The β (pink), β" (pale green), α2 (yellow, orange; without their C-terminal domains) and ω (cyan) subunits are identified, and the β subunit flap (red), which is the attachment site of σ domain 4 and gp33, as well as the β" coiled-coil (green), which is the docking site of σ domain 2 and gp55, are emphasized. σ domains 1.2, 2, 3 and 4 (dark blue) are identified. B. The same, with σ removed (i.e., RNAP core, but with the coordinates of the holoenzyme) (Adapted from ).