We have reported that a domain containing Arg682 in the Klenow fragment of Escherichia coli DNA polymerase I (pol I) is important for the template-dependent dNTP-binding function [Pandey, V.N., Kaushik, N. A., Pradhan, D. S. & Modak, M. J. (1990) J. Biol. Chem. 265, 3679-3884]. In order to further define the role of Arg682 in the catalytic process, we have performed site-directed mutagenesis of this residue. For this purpose the Klenow-coding region of the DNA-pol-I gene was selectively amplified from the genomic DNA of E. coli and was cloned in an expression vector, pET-3a. This clone under appropriate conditions overproduces the Klenow fragment in E. coli. Using this clone (pET-3a-K) as the template, two mutant polymerase clones were constructed in which arginine has been replaced with either alanine, [R682A] pol I, or lysine [R682K] pol I. Both mutant enzymes showed significantly lower specific activity as compared to the wild-type enzyme. The kinetic analyses of the mutant enzymes indicated a 3-4-fold increase in the Km for the substrate dNTP, a 20-25-fold decrease in the Vmax and an overall decrease in the processive nature of DNA synthesis in both the mutant enzymes. The reverse mutation of Ala682 to the wild-type form Arg682 fully restored the processive nature and the polymerase activity of the enzyme. These observations suggest that the positively charged guanidino group in the side chain of Arg682 is catalytically important but not absolutely essential for synthesis of DNA. Furthermore it appears to maintain high processivity of the DNA synthesis catalyzed by the enzyme.