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.