Molecular structures of DNA-dependent RNA polymerases (II) from calf thymus and rat liver.

DNA-dependent RNA polymerase II has been purified to high specific activity and apparent homogeneity from both calf thymus and rat liver. Two form II enzymes are present in rat-liver preparations, one with the molecular structure [(190,000)(1)(150,000)(1)(35,000)(1)(25,000)(1)], the other with a molecular structure of [(170,000)(1)(150,000)(1)(35,000)(1)(25,000)(1)] (molecular weights are within +/-5% but the absolute values are approximate). Inclusion of a proteolytic inhibitor during the isolation procedure decreases the proportion of the molecule containing the 170,000 subunit. Calf-thymus RNA polymerase preparations typically exhibit four components on polyacrylamide gels that contain sodium dodecyl sulfate, with an apparent molecular structure of [(190,000)(1)(150,000)(1)(35,000)(1)(25,000)(1)]. In addition, some calf-thymus polymerase II preparations contain small quantities of the [(170,000)(1)(150,000)(1)(35,000)(1)(25,000)(1)] species; the quantity of this species may also be increased from less than 5% in the normal preparation to at least 40% in an "aged" preparation. Thus, the 170,000 subunit may be derived from the 190,000 subunit in both tissues. Until unequivocal evidence is obtained on this point, however, the possibility that the large subunits are unique species should not be eliminated. The general structural similarity of the eukaryotic RNA polymerase II with that of the prokaryotic polymerase suggests that the modes of action and regulation may be analogous.