sentences:  Activation of the 4 catalytic serine residues within the Hin tetramer make a 2-bp double stranded DNA break and forms a covalent reaction intermediate.The DNA cleavage event also requires the divalent metal cation magnesium.A large conformational change reveals a large hydrophobic interface that allows for subunit rotation which may be driven by superhelical torsion within the protein-DNA complex.