Structure of Hjc, a Holliday junction resolvase, from Sulfolobus solfataricus

The 2.15-Angstrom structure of Hjc, a Holliday junction-resolving enzyme fr om the archaeon Sulfolobus solfataricus, reveals extensive structural homol ogy with a superfamily of nucleases that includes type II restriction enzym es. Hjc is a dimer with a large DNA-binding surface consisting of numerous basic residues surrounding the metal-binding residues of the active sites. Residues critical for catalysis, identified on the basis of sequence compar isons and site-directed mutagenesis studies, are clustered to produce two a ctive sites in the dimer, about 29 Angstrom apart, consistent with the requ irement for the introduction of paired nicks in opposing strands of the fou r-way DNA junction substrate. Hjc displays similarity to the restriction en donucleases in the way its specific DNA-cutting pattern is determined but u ses a different arrangement of nuclease subunits. Further structural simila rity to a broad group of metal/phosphate-binding proteins, including conser vation of active-site location, is observed. A high degree of conservation of surface electrostatic character is observed between Hjc and T4-phage end onuclease VII despite a complete lack of structural homology. A model of th e Hjc-Holliday junction complex is proposed, based on the available functio nal and structural data.