Crystal structure of type IIE restriction endonuclease EcoRII dimer. The locations of the two effector clefts and the two catalytic half-sites are indicated. The catalytic half-sites are blocked by the N-domains and inaccessible by a substrate DNA. The two key residues Y41 and E96 in the effector clefts are shown in ball-and stick and labeled. The alpha-helix H6 is also labeled. The structure reveals an autoinhibition mechanism by a novel effector-binding fold. The autoinhibition mechanism employed by EcoRII is the first ever observed among over 3500 restriction endonucleases. See the publication: J. MOL. Biol. 335, 307-319 (2004).