comparative structural studies of the free enzyme vs. the "bgl"ii-dna complex showed that the enzyme opens by a dramatic scissor-like motion, accompanied by a complete rearrangement of the α-helices at the dimer interface. these structural studies also revealed that within each monomer a set of residues lowers or raises to alternatively sequester or expose the active site residues. these dramatic differences in structure in the free vs. bound enzyme have yet to be observed in any other restriction endonuclease and may possibly represent a novel mechanism for capturing dna that may extend to other proteins that encircle dna.