The structurally conserved core of the Bacterial T. aquaticus RNAP (A) and Archaeal S. shibatae RNAP (C) can easily be recognised by close inspection of their X-ray structures. Important functional features such as the active site (B, metal A shown as pink sphere), the bridge helix (highlighted in orange) and the main DNA-binding channel are well conserved. The homologous RNAP subunits are colour-coded according to the key in the figure.