A new species of DNA polymerase has been purified more than 10 000-fold from the cytoplasm of erythroid hyperplastic bone marrow. This DNA polymerase, in contrast to previously described eukaryotic DNA polymerases, is associated with a very active 3′ to 5′ exonuclease activity. Similar to the 3′ to 5′ exonuclease activity associated with prokaryotic DNA polymerases, this enzyme catalyzes the removal of 3′-terminal nucleotides from DNA, as well as a template-dependent conversion of deoxyribonucleoside triphosphates to monophosphates. The exonuclease activity is not separable from the DNA polymerase activity by chromatography on DEAE-Sephadex or hydroxylapatite, and upon sucrose density gradient centrifugation the two activities cosediment at 7 S or at 11 S depending on the ionic strength. Both exonuclease and polymerase activities have identical rates of heat inactivation and both are equally sensitive to hemin and Rifamycin AF/ 013, inhibitors of DNA synthesis that act by binding to DNA polymerase and causing its dissociation from its template/ primer. These results are consistent with the coexistence of two enzyme activities in a single protein.
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