The kinetic properties of Escherichia coli DNA polymerase I were simplified to those of a 1 deoxynucleotide substrate reaction by the use of polynucleotide templates. With poly(dA)-oligo(dT) as the template-primer complex, Mg2+ decreases the Km of the substrate dTTP but has little or no effect on the Km of the substrate Mg-dTTP, suggesting that multiple pathways involving the binding of Mg2+, dTTP, and Mg-dTTP are operative in forming the active complex. The Km of free Mg2+, extrapolated to zero concentration of substrate (830 = 62 muM), agrees within a factor of 2 with the dissociation constant of magnesium from 4 +/- 1 sites on the enzyme determined previously by binding studies (Slater, J.P., Tamir, I., Loeb, L.A., and Mildvan, A.S. (1972) J. Biol. Chem. 247, 6784-6794). The maximal turnover number with poly(dA) as template is 5.7 +/- 0.7 s-1. Changing the nature of the base in the polydeoxynucleotide template alters the maximal rate of polydeoxynucleotide synthesis by an overall factor of 31 with poly(dC) is greater than poly(dT) is greater than poly(dA) is greater than poly(dG), indicating that pyrimidine templates are copied faster than purine templates. Changing the sugar structure from poly(dA) to poly(rA) causes a 3-fold increase in the rate of template copying. A study of the kinetic effects of all noncomplementary deoxynucleotides with all deoxynucleotide templates, as well as with poly(rA)-oligo(dT), yields complex patterns of activation and inhibition requiring from 1 to 2 additional binding sites for the noncomplementary nucleotides. The kinetically determined affinities of the active site of the enzyme-template-primer complex for the complementary free nucleotide (as measured by Km) generally exceed those for the noncomplementary neuclotides (as measured by KI slope) by 1 or more than 3 orders of magnitude.