Each of the three eukaryotic RNA polymerases has more subunits or 12 and so these are big complex enzymes. The genes encoding some of the subunits of each  eukaryotic  enzyme  show  DNA  sequence  similarities  to  genes  encoding subunits of the core enzyme α2  ββ of E. coli RNA polymerase. Furthermore,  four to seven other subunits of every eukaryotic  RNA polymerase  are unique  in which they show  no similarity  either  with  bacterial  RNA  polymerase subunits or with the subunits of other eukaryotic RNA polymerases.