DEDDy 3'-5' exonuclease domain of eukaryotic DNA polymerase delta, a family-B DNA polymerase
The 3'-5' exonuclease domain of eukaryotic DNA polymerase delta. DNA polymerase delta is a family-B DNA polymerase with a catalytic subunit that contains a DEDDy-type DnaQ-like 3'-5' exonuclease domain. It is one of the three DNA-dependent type B DNA polymerases (alpha and epsilon are the other two) that have been identified as essential for nuclear DNA replication in eukaryotes. DNA polymerase delta is the enzyme responsible for both elongation and maturation of Okazaki fragments on the lagging strand. It is also implicated in mismatch repair (MMR) and base excision repair (BER). The catalytic subunit displays both polymerase and 3'-5' exonuclease activities. The exonuclease domain contains three sequence motifs termed ExoI, ExoII and ExoIII, with a specific YX(3)D pattern at ExoIII. These motifs are clustered around the active site and contain four conserved acidic residues necessary for metal binding and catalysis. The exonuclease domain of family B polymerase also contains a beta hairpin structure that plays an important role in active site switching in the event of nucleotide misincorporation.