The architecture of RNA polymerases. (a) The bacterial RNA polymerase core is composed of several subunits that give the enzyme the overall shape of a crab claw. The pincers are formed from the large β and β′ subunits. At the start of transcription, the sigma factor (σ) associates with the core polymerase to form the holoenzyme (not shown). The crystal structures of (b) a bacterial RNA polymerase (Thermus aquaticus) and (c) a eukaryotic RNA polymerase II (Saccharomyces cerevisiae) are shown.