On the Processive Mechanism of Escherichia coZi DNA Polymerase I

Comparison of the rates of polymerization catalyzed by DNA polymerases in the presence of a limited and a complete complement of the four deoxynucleoside triphosphates permits accurate measurement of the processivity of polymerization. The extent of processivity, Le. the number of nucleotides polymerized each time the DNA polymerase associates with the DNA template, may be measured in a range from 1.0 to several hundred nucleotides. As judged by this method, Escherichia coli DNA polymerase I is processive for values as great as 188, depending on conditions and primer-template structure. Higher ionic strength and lower temperatures result in lower processivity. With nicked DNA templates, at 37” and ionic strength = 0.085 JL, processivity is 15 to 20. With gapped DNA templates under the same conditions, processivity is 40 to 50. Poly[d(A-T)l templates yield processivity values as high as 188, but changes in conditions can diminish values to 3.3. In contrast, DNA polymerase /3 from KB cells is nonprocessive (enzyme-DNA dissociation after addition of each nucleotide) under all conditions examined. A procedure is described to compare the “static affinity”