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SgrAI is a type II restriction endonuclease that cuts an unusually long recognition sequence and exhibits allosteric self-activation with expansion of DNA-sequence specificity. The three-dimensional crystal structures of SgrAI bound to cleaved primary-site DNA and Mg2+ and bound to secondary-site DNA with either Mg2+ or Ca2+ are presented. All three structures show a conformation of enzyme and DNA similar to the previously determined dimeric structure of SgrAI bound to uncleaved primary-site DNA and Ca2+ [Dunten et al. (2008), Nucleic Acids Res. 36, 5405-5416], with the exception of the cleaved bond and a slight shifting of the DNA in the SgrAI/cleaved primary-site DNA/Mg2+ structure. In addition, a new metal ion binding site is located in one of the two active sites in this structure, which is consistent with proposals for the existence of a metal-ion site near the 3'-O leaving group.

Supporting information

PDB references: SgrAI, 3n7b; 3n78; 3mqy


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