Eukaryotic DNA-dependent RNA polymerases (Pol I-III) share a conserved core of 12 subunits, which is closely related to archaeal RNA polymerases. Rpb8, a subunit found in Pol I, II and III, was thought to be restricted to eukaryotes. We show here that Rpb8 closely resembles an archaeal protein called G, found only in Crenarchaea, which identifies a last missing link between the core structure of archaeal and eukaryotic RNA polymerases.