DEDDy 3'-5' exonuclease domain of family-B DNA polymerases
The 3'-5' exonuclease domain of family-B DNA polymerases. This domain has a fundamental role in reducing polymerase errors and is involved in proofreading activity. Family-B DNA polymerases contain an N-terminal DEDDy DnaQ-like exonuclease domain in the same polypeptide chain as the polymerase domain, similar to family-A DNA polymerases. This 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 that serve as ligands for the two metal ions required for 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. Members include Escherichia coli DNA polymerase II, some eubacterial phage DNA polymerases, nuclear replicative DNA polymerases (alpha, delta, epsilon and zeta), and eukaryotic viral and plasmid-borne enzymes. Nuclear DNA polymerases alpha and zeta lack the four conserved acidic metal-binding residues. Family-B DNA polymerases are predominantly involved in DNA replication and DNA repair.