RECOGNITION AND CLEAVAGE OF DNA BY TYPE-II RESTRICTION ENDONUCLEASES

Restriction endonucleases are enzymes which recognize short DNA sequen ces and cleave the DNA in both strands. Depending on the enzymological properties different types are distinguished. Type II restriction end onucleases are homodimers which recognize short palindromic sequences 4-8 bp in length and, in the presence of Mg2+, cleave the DNA within o r next to the recognition site. They are capable of non-specific bindi ng to DNA and make use of linear diffusion to locate their target site , Binding and recognition of the specific site involves contacts to th e bases of the recognition sequence and the phosphodiester backbone ov er approximately 10-12 bp. In general, recognition is highly redundant which explains the extreme specificity of these enzymes. Specific bin ding is accompanied by conformational changes over both the protein an d the DNA. This mutual induced fit leads to the activation of the cata lytic centers. The precise mechanism of cleavage has not yet been esta blished for any restriction endonuclease. Currently two models are dis cussed: the substrate-assisted catalysis mechanism and the two-metal-i on mechanism. Structural similarities identified between EcoRI, EcoRV, BamHI, PvuII and Cfr10I suggest that many type II restriction endonuc leases are not only functionally but also evolutionarily related.