Switching between polymerase and exonuclease sites in DNA polymerase ε

The balance between exonuclease and polymerase activities promotes DNA synthesis over degradation when nucleotides are correctly added to the new strand by replicative B-family polymerases. Misincorporations shift the balance toward the exonuclease site, and the balance tips back in favor of DNA synthesis when the incorrect nucleotides have been removed. Most B-family DNA polymerases have an extended beta-hairpin loop that appears to be important for switching from the exonuclease site to the polymerase site, a process that affects fidelity of the DNA polymerase. Here, we show that DNA polymerase epsilon can switch between the polymerase site and exonuclease site in a processive manner despite the absence of an extended beta-hairpin loop. K967 and R988 are two conserved amino acids in the palm and thumb domain that interact with bases on the primer strand in the minor groove at positions n-2 and n-4/n-5, respectively. DNA polymerase epsilon depends on both K967 and R988 to stabilize the 3'-terminus of the DNA within the polymerase site and on R988 to processively switch between the exonuclease and polymerase sites. Based on a structural alignment with DNA polymerase delta, we propose that arginines corresponding to R988 might have a similar function in other B-family polymerases.