Structure of free Bg/II reveals an unprecedented scissor-like motion for opening an endonuclease

Restriction endonuclease BgIII completely encircles its target DNA, making contacts to both the major and minor grooves. To allow the DNA to enter and leave the binding cleft, the enzyme dimer has to rearrange. To understand how this occurs, we have solved the structure of the free enzyme at 2.3 Ang strom resolution, as a complement to our earlier work on the BgIII-DNA comp lex Unexpectedly, the enzyme opens by a dramatic 'scissor-like' motion, acc ompanied by a complete rearrangement of the or-helices at the dimer interfa ce. Moreover, within each monomer, a set of residues - a 'lever' - lowers o r raises to alternately sequester or expose the active site residues. Such an extreme difference in free versus complexed structures has not been repo rted for other restriction endonucleases, This elegant mechanism for captur ing DNA may extend to other enzymes that encircle DNA.