(a) A3A structure with a 2Fo−Fc electron density map contoured at 1σ. The protein is presented as a green-coloured ribbon diagram and the bound DNA is in stick representation (carbons and phosphates, orange; nitrogens, blue; oxygens, red). A zinc ion at the active centre is depicted as a magenta-coloured sphere. The side chains of zinc-coordinating residues H70, C101 and C106 are shown as sticks (carbons, green; nitrogens, blue; oxygen, red; sulfurs, yellow). DNA binding at the active site of A3A is presented in (b) ribbon and (c) surface representation. (d) Conformational changes of residues R28, H29 and Y132 upon DNA binding are indicated by arrows, with side chains in stick representation (white and green-coloured carbon for the apo (PDB code 4XXO) and DNA-bound forms, respectively). Surface electrostatic potentials of (e) apo and (f) DNA-bound A3A are coloured red to blue for negative and positive charges, respectively, using a scale of −5 to +5 kT e−1.