although restriction endonucleases show little sequence similarity, crystal structures reveal that they all share a highly similar α/β core consisting of a six-stranded β-sheet flanked by five α-helices, two of which mediate dimerization. this core carries the active site (catalytic center) and the residues that contact dna in the major groove. "bgl"ii is unique in that its α/β core is augmented by a β-sandwich subdomain that has several projections that extend outward to grip the dna, allowing "bgl"ii to completely encircle the dna molecule. this atypical feature of "bgl"ii suggests a unique hinge motion for dna binding and release.