The two eukaryotic subunits Rpb3 and Rpb11 dimerise to from aplatform onto which the other subunits of the RNA polymeraseassemble (D/L in archaea). The prokaryotic equivalent of theRpb3/Rpb11 platform is the alpha-alpha dimer. The dimerisationdomain of the alpha subunit/Rpb3 is interrupted by an insertdomain (Pfam:PF01000). Some of the alpha subunits also containiron-sulphur binding domains (Pfam:PF00037).