9. Eucaryotic RNA Polymerases

Abstract

In this chapter, the properties of the recently discovered mammalian cytoplasmic RNA polymerase activity is described with those of the animal nuclear enzymes because it is not yet known whether it is a true cytoplasmic enzyme, a nuclear enzyme that is leached out during homogenization, or a precursor of the nuclear enzymes. Purified B RNA polymerases (calf thymus, rat liver, or KB cells), calf thymus AI RNA polymerase, and rat liver AI and AII RNA polymerases sedimented through glycerol gradients at about 14-15 S faster than the E. coli core enzyme [MW 380,000-400,000]. B enzymes sediment slightly faster than A enzymes. These observations suggest a molecular weight of about 500,000. In contrast to E. coli RNA polymerase, there was no drastic modification of the sedimentation rate of animal enzymes as the ionic strength was varied. Molecular weights of 550,000 ± l0 %, 600,000 ± l0%, and 570,000 ± 10% were found for calf thymus AI, BI, and BII enzymes, respectively, by electrophoresis in polyacrylamide gels of increasing porosity. The molecular weight of the “cytoplasmic” RNA polymerase C from rat liver is probably similar because it was not resolved from the B enzymes by ultracentrifugation through sucrose gradients. Both AI and B enzymes are acidic protein, which migrates toward the anode at pH 8. After electrofocusing KB cell B enzymic activity peaked at around pH 4.74.