Structural and functional properties of three mammalian nuclear DNA-dependent RNA polymerases

Three DNA-dependent RNA polymerases AI, BI and BII were purified from calf thymus. These enzymes were characterized by their sensitivity to amanitin, their subunit pattern and their template specificities. Enzyme AI is resistant to amanitin, while B enzymes are inhibited by this poison. Each of these enzymes contains two subunits of high molecular weight: A1 (M.W. 197 000) and A2 (M.W. 126 000) for enzyme AI, Bl (M.W. 214 000) and B3 (M.W. 140 000) for enzyme BI, B2 (M.W. 180 000) and B3 for enzyme BII. In addition, each enzyme contains several subunits of lower molecular weight. Purification of B enzymes from rat liver showed the presence of two enzymes similar to calf thymus RNA polymerases BI and BII and suggested the existence of an additional B enzyme in rat liver. The number of RNA polymerase form B molecules was determined in a variety of animal tissues by binding of labelled amanitin. The values ranged from 1 × 104 to 6.5 × 104 molecules per haploid genome.

Studies with derivatives of rifamycin indicated that calf thymus B enzymes were able to recognize specific initiation sites on calf thymus DNA, which were different from those recognized by enzyme AI or E. coli RNA polymerase. These results favour the hypothesis that the multiplicity of animal RNA polymerases could play an active role in the control of transcription.