Recently determined crystal structures of type II restriction endonucleases have produced a plethora of information on the basis for target site sequence selectivity. The positioning and role of metal ions in DNA recognition sites might reflect important properties of protein-DNA interaction. Although acidic and basic groups in the active sites can be identified, and in some cases divalent-metal binding sites delineated, a convincing picture clarifying the way in which the attacking hydroxide ion is generated, and the leaving group stabilized, has not been elucidated for any of the enzymes. We have examined the interatomic distances between metal ions and proposed key catalytic residues in the binding sites of seventeen type II restriction endonucleases whose crystal structures are documented in literature. The summary and critical evaluation of structural assignments and predictions made earlier have been useful to group these enzymes. All the enzymes used for this study have been categorized on the basis of the number of metal ions identified in their crystal structures. Among 17 experimentally characterized (not putative) type II REases, whose apparently full-length sequences are available in REBASE, we predict 8 (47%) to follow the single metal ion mechanism, 5 to follow the two metal ion mechanism, 2, the three metal ion mechanism, 1, the four metal ion mechanism and 1 the six metal ion mechanism. (C) 2010 Elsevier Inc. All rights reserved.