STRUCTURE OF RESTRICTION-ENDONUCLEASE BAMHI AND ITS RELATIONSHIP TO ECORI

TYPE II restriction endonucleases are characterized by the remarkable specificity with which they cleave specific DNA sequences. Surprisingl y, their protein sequences are in most cases unrelated, and no recurri ng structural motif has yet been identified1,2. We have determined the structure of restriction endonuclease BamHI at 1.95 angstrom resoluti on. BamHI shows striking resemblance to the structure of endonuclease EcoRI (refs 3, 4), despite the lack of sequence similarity between the m. We also observe some curious differences between the two structures , and propose an evolutionary scheme that may explain them. The active site of BamHI is structurally similar to the active sites of EcoRI an d EcoRV (ref. 5), but the mechanism by which BamHI activates a water m olecule for nucleophilic attack may be different.