Figure 5.

Anticooperative DNA binding by vTopo. For substrates with long 3′ base paired regions, a strain component to the observed binding energy is indicated. The duplex strain in the noncovalent complex results in weakening of the interactions in the major groove. Upon formation of the covalent species the strain is at least partially released allowing the major groove interactions to become stronger. Although not shown for clarity, the α5 helix of the larger catalytic domain also interacts with the major groove along with the β5 strand of the amino terminal domain.