Fig. 2.

Detailed view of the I-DmoI active site. (A and B) Anomalous difference maps illustrate the presence of only 1 atom of calcium in the DNA bound structure (A) and 2 manganese ions in the structure of the I-DmoI product complex (B). The protein is shown in yellow and the DNA in green. (C) Schematic diagram of the hypothetical enzymatic mechanism proposed for I-DmoI. Hydrolysis of the phosphodiester bonds would follow a 2-metal ion mechanism. The first metal ion (site1, colored in yellow in 1) is bound in 1 active site and the water nucleophile (colored in red) is positioned in the central site and can attack the coding strand (in 2). The regeneration of the central water together with a second metal ion in the second site (site2, colored in cyan in 3) would enable the second attack. The D21, G20 and E117, A116 are contributed by the LAGLIDADG motifs of the enzyme.