FIG. 11.

The active site of the RecB nuclease domain. The active site of the RecB nuclease domain is marked out by the coordination of a Ca²⁺ ion (yellow) with conserved nuclease motifs. The three motifs characteristic of a diverse collection of nuclease enzymes are numbered (20). Motif 1 (orange) contains a glutamate residue (E1020) that may be involved in the coordination of a second divalent cation. Motifs 2 (blue) and 3 (red) contain aspartate residues (D1067 and D1080) that coordinate the bound Ca²⁺ ion. A motif specific to the “RecB-like nuclease” family is shown in green and contains highly conserved tyrosine and glutamine residues (Q1110 and Y1114). A histidine residue (magenta) that is completely conserved, but does not form part of a defined nuclease motif, also coordinates the bound Ca²⁺ ion (H956).