Archaea-specific D-family DNA polymerase forms a heterotetramer consisting of two large polymerase subunits and two small exonuclease subunits. We analyzed the structure of the N-terminal 200 amino-acid regulatory region of the small subunit by NMR and revealed that the N-terminal approximately 70 amino-acid region is folded. The structure consists of a four-alpha-helix bundle including a short parallel beta-sheet, which is similar to the N-terminal regions of the B subunits of human DNA polymerases alpha and epsilon, establishing evolutionary relationships among these archaeal and eukaryotic polymerases. We observed monomer-dimer equilibrium of this domain, which may be related to holoenzyme architecture and/or functional regulation.
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