Nuclear deoxyribonucleic acid polymerase. Further observations on the structure and properties of the enzyme from human KB cells

Abstract

At low ionic strength KB cell DNA polymerase N1 forms large aggregates of a size comparable to those of DNA polymerase C. However, in contrast to polymerase C, the polymerase N1 aggregate: (a) retains the distinctive features of the polymerase N1 monomer, specifically its relative insensitivity to salt and to p-hydroxymercuribenzoate, and its pI of 9.3; and (b) is quantitatively converted to the polymerase N1 monomer form at appropriate ionic strength. It is important to recognize that since both polymerase N1 and polymerase C undergo salt-dependent association-dissociation reactions, attempts to distinguish these clearly indedependent polymerase species on the basis of size criteria can be very misleading. This is particularly true in relatively impure enzyme fractions that are generally isolated from eukaryotic tissue sources in low ionic strength buffers. We had earlier reported (Wang, T. S.-F., Sedwick, W. D., and Korn, D. (1974) J. Biol. Chem. 249,841-850; Sedwick, W. D., Wang, T. S.-F., and Korn, D. (1972) J. Biol. Chem. 247,5026-5033; Sedwick, W. D., Wang, T. S.-F., and Korn, D. (1974) Methods Enzymol. 29, 89-102) that DNA polymerase N1 could not utilize homoribopolymer templates. We have re-examined this question with a modified and more stringent method of product assay, and we show here that a greater than or equal 95% homogeneous preparation of polymerase N1 can copy the primer-template (A)n-(dT)-/16 at about one-half the rate that it copies activated DNA under optimum incubation conditions.