inactive DEDDy 3'-5' exonuclease domain of eukaryotic DNA polymerase alpha, a family-B DNA polymerase
The 3'-5' exonuclease domain of eukaryotic DNA polymerase alpha. DNA polymerase alpha is a family-B DNA polymerase with a catalytic subunit that contains a DnaQ-like 3'-5' exonuclease domain. It is one of the three DNA-dependent type B DNA polymerases (delta and epsilon are the other two) that have been identified as essential for nuclear DNA replication in eukaryotes. DNA polymerase alpha is almost exclusively required for the initiation of DNA replication and the priming of Okazaki fragments during elongation. It associates with DNA primase and is the only enzyme able to start DNA synthesis de novo. The catalytic subunit contains both polymerase and 3'-5' exonuclease domains, but only exhibits polymerase activity. The 3'-5' exonuclease domain contains three sequence motifs termed ExoI, ExoII and ExoIII, without the four conserved acidic residues that are crucial for metal binding and catalysis. This explains why in most organisms, that no specific repair role, other than check point control, has been assigned to this enzyme. The exonuclease domain may have a structural role.