DNA Polymerases of Rat Liver

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Abstract
Three distinct DNA‐dependent DNA polymerase activitives have been partially purified from normal rat liver. Soluble activities are separable into two distinct fractions (P1 and P2) by phosphocellulose chromatography. A low‐molecular‐weight DNA polymerase was isolated from purified nuclei. The enzymes were characterized according to chromatographic and sedimentation behavior, enzymological properties, and response to various inhibitors. The results indicate that fraction P1 corresponds to the high‐molecular‐weight enzyme and suggest that polymerase P2 may be derived from partial dissociation of the high‐molecular‐weight enzyme. The molecular weight of polymerase P1 was estimated to be about 250000 by Sephadex column chromatography. Both fraction P2 and nuclear DNA polymerase appeared to be low‐molecular‐weight enzymes. However, the molecular size of these activities was apparently different. The estimated molecular weights of nuclear and P2 enzyme are about 40000 and 25000, respectively.As with the nuclear enzyme, polymerase P2 (but not P1) appeared to be free of detectable exonuclease activity. All of these polymerases showed a marked preference for initiated polydeoxyribonucleotide templates. The rat liver polymerases differed in their ability to use poly[d(A‐T)] primer‐template, as is shown by the ratios of their activity with this synthetic polymer to that with activated DNA: 0.5, 2.75, and 1.34 for P1, P2, and nuclear polymerase, respectively. Denatured DNA was a poor template for both enzymes P1 and P2, but it was inert as template for the nuclear enzyme.Although each of these polymerases required all four deoxynucleoside triphosphates for maximal activity, they catalyzed a high rate of synthesis in the absence of one or more deoxynucleoside triphosphates. Such a ‘limited’ synthesis was much more extensive for polymerase P2 and nuclear enzyme than for P1. Polymerase P1 was the most sensitive of the three to sulphydryl reagents, ethidium bromide, heparin, and single‐stranded DNA. The responses of P2 and nuclear enzymes to various inhibitors were very similar. However, these two enzymes respond differently to heat and high ionic strength.