Separation of mercury substituted RNA synthesized in isolated rat liver nuclei

The population of RNA molecules synthesized in isolated rat liver nucleiin vitro in the presence of ³H]CTP and Hg-UTP was succesfully fractionated into at least two subfractions containing various proportions of mercury label. Fractionation was achieved either by step-wise chromatography of Hg-RNA on thiopropyl-Sepharose columns or by density gradient centrifugation in metrizamide. The fraction of RNA heavily labeled with Hg-UTP was composed mainly of 4-18S RNA and contained virtually all radioactivity derived from -³²P]ATP or -³²P]GTP. The slightly mercurated RNA fraction consisted mainly of longer RNA molecules (12->28S) and was not labeled with -³²P]ATP or -³²P]GTP. Labeling with -³²P nucleoside triphosphates was sensitive both to rifamycin AF/013 and heparin whereas labeling with ³H]CTP was fully resistant to the inhibitors and showed sensitivity to low doses of -amanitin. We assume that the observed subpopulation of heavily mercurated RNAs consists of RNA molecules initiated in vitro.