Biochemical characterization of the Hjc Holliday junction resolvase of Pyrococcus furiosus

The Hjc protein of Pyrococcus furiosus is an endonuclease that resolves Holliday junctions, the intermediates in homologous recombination. The amino acid sequence of Hjc is conserved in Archaea, however, it is not similar to any of the well-characterized Holliday junction resolvases. In order to investigate the similarity and diversity of the enzymatic properties of Hjc as a Holliday junction resolvase, highly purified Hjc produced in recombinant Escherichia coli was used for detailed biochemical characterizations. Hjc has specific binding activity to the Holliday-structured DNA, with an apparent dissociation constant (K(d)) of 60 nM. The dimeric form of Hjc binds to the substrate DNA. The optimal reaction conditions were determined using a synthetic Holliday junction as substrate. Hjc required a divalent cation for cleavage activity and Mg²⁺ at 5-10 mM was optimal. Mn²⁺ could substitute for Mg²⁺, but it was much less efficient than Mg²⁺ as the cofactor. The cleavage reaction was stimulated by alkaline pH and KCI at ~200 mM. In addition to the high specific activity, Hjc was found to be extremely heat stable. In contrast to the case of Sulfolobus, the Holliday junction resolving activity detected in P.furiosus cell extract thus far is only derived from Hjc.