RNA polymerase structures. The three-dimensional structures of RNA polymerases from a prokaryote (Thermus aquaticus) and a eukaryote (Saccharomyces cerevisiae). The two largest subunits for each structure are shown in dark red and dark blue. Notice that both structures contain a central metal ion (green) in their active sites, near a large cleft on the right. The similarity of these structures reveals that these enzymes have the same evolutionary origin and have many mechanistic features in common.

[Drawn from 1I6V.pdb and 1I6H.pdb.]