Specific Nucleolar and Nucleoplasmic RNA Polymerases

The DNA-dependent RNA polymerase activity present in rat liver nuclei has been solubilized and purified from whole nuclei and from subnuclear fractions. As reported earlier (Roeder, R. G., and W. J. Rutter, Nature, 224, 234 (1969)), two major chromatographically distinct enzymatic species (I and II) are present in whole nuclei. Subfractionation of whole nuclei into nucleolar and nucleoplasmic fractions had little effect on the total recovery of activity. Purified nucleoli contain predominantly polymerase I, whereas the nucleoplasmic fraction is greatly enriched for polymerase II. A third minor peak of activity has also been resolved in the nucleoplasmic preparations. We conclude that the RNA polymerases are specifically localized within the nucleus and may, therefore, play specific roles in the regulation of genetic transcription.