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The Demonstration of Separate DNA Polymerase Activities In Intact Isolated Rat Liver Nuclei by Means of Response to Bleomycin and Arabinosyl Cytosine 5-Triphosphate

Rat hepatic nuclei, isolated and studied in vitro, tend to reflect changes occurring in DNA synthesis in the intact rat (1). Nuclei from normal rat liver synthesize minimal amounts of DNA, while nuclei from a partially hepatectomized rat show a marked increase in thymidine tri-phosphate incorporation. This increase probably represents continuation of elongation of replicating DNA strands begun in vivo (2). Studies of extracts of rat liver nuclei suggest that there are at least two separate DNA polymerase activities: a tightly-bound activity which rises in quantity along with DNA replication following hepatectomy and is inhibited by cytosine arabinoside 5′-triphosphate (ara-CTP), and a loosely-bound activity which rises little or not at all after partial hepatectomy and is insensitive to ara-CTP (3). We have previously reported that Bleomycin causes dose-related and time-related increases of 3H-thymidine triphosphate incorporation into DNA of isolated normal rat liver nuclei (4). This incorporation is presumably carried out by a DNA polymerase reacting to structural DNA damage produced by the Bleomycin. The studies reported herein provide evidence that the increased 3H-thymidine triphosphate incorporation which follows addition of Bleomycin to isolated rat liver nuclei is mediated by a DNA polymerase activity that is different from the polymerase activity that increases following partial hepatectomy.