Crystal structure of the fission yeast mitochondrial Holliday junction resolvase Ydc2

Resolution of Holliday junctions into separate DNA duplexes requires enzyma tic cleavage of an equivalent strand from each contributing duplex at or cl ose to the point of strand exchange. Diverse Holliday junction-resolving en zymes have been identified in bacteria, bacteriophages, archaea and pox vir uses, but the only eukaryotic examples identified so far are those from fun gal mitochondria. We have now determined the crystal structure of Ydc2 (als o known as SpCce1), a Holliday junction resolvase from the fission yeast Sc hizosaccharomyces pombe that is involved in the maintenance of mitochondria l DNA. This first structure of a eukaryotic Holliday junction resolvase con firms a distant evolutionary relationship to the bacterial RuvC family, but reveals structural features which are unique to the eukaryotic enzymes. De tailed analysis of the dimeric structure suggests mechanisms for junction i somerization and communication between the two active sites, and together w ith site-directed mutagenesis identifies residues involved in catalysis.