Klenow and Klentaq are the large fragment domains of the Pol I DNA polymerases from Escherichia coli and Thermus aquaticus, respectively. Herein, we show that both polymerases can significantly stimulate complementary intermolecular end-joining ligations by E.coli DNA ligase when the polymerases are present at concentrations lower than that of the DNA substrates. In contrast, high polymerase concentrations relative to the DNA substrates inhibit the intermolecular ligation activity of DNA ligase. Neither polymerase was able to stimulate the DNA ligase from T4 bacteriophage. Additionally, nick-closure by E. coli DNA ligase (but not T4 ligase) is slightly stimulated by both polymerases, but only at about 10% of the magnitude seen for end joining enhancement. The data represent one of the first observations of direct polymerase-ligase interactions in prokaryotes, and suggest that the polymerases stabilize the associated DNA ends during intermolecular ligation, and that such a complex can be taken advantage of by some, but not all, DNA ligases. Published by Elsevier Inc.