RNA Polymerase. This large enzyme comprises many subunits, including β (red) and β′ (yellow), which form a “claw” that holds the DNA to be transcribed. Notice that the active site includes a Mg2+ ion (green) at the center of the structure. The curved tubes making up the protein in the image represent the backbone of the polypeptide chain.
[Drawn from 1L9Z, pdb.]