Family D DNA polymerase (PolD) has recently been found in the Euryarchaeota subdomain of Archaea. Its genes are adjacent to several other genes related to DNA replication, repair and recombination in the genome, suggesting that this enzyme may be the major DNA replicase in Euryarchaeota. We successfully cloned, expressed, and purified the family D DNA polymerase from Pyrococcus horikoshii (PolDPho). By site-directed mutagenesis, we identified amino acid residues Asp-1122 and Asp-1124 of a large subunit as the essential residues responsible for DNA-polymerizing activity. We analysed the domain structure using proteins truncated at the N- and C-termini of both small and large subunits (DP1Pho and DP2Pho), and identified putative regions responsible for subunit interaction, oligomerization and regulation of the 3´-5´ exonuclease activity in PolDPho. It was also found that the internal region of the putative zinc finger motif (cysteine cluster II) at the C-terminal of DP2Pho is involved in the 3´-5´ exonuclease activity. Using gel filtration analysis, we determined the molecular masses of the recombinant PolDPho and the N-terminal putative dimerization domain of the large subunit, and proposed that PolD from P. horikoshii probably forms a heterotetrameric structure in solution. Based on these results, a model regarding the subunit interaction and regulation of activity of PolDPho is proposed.

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