Fig. 5.

DNA supercoiling-induced tightening of DNA knots facilitates their efficient unknotting by type IIA DNA topoisomerases. (A and B) DNA supercoiling-induced tightening of knotted portions makes these more bent than the remainder of the molecule, which stimulates topoisomerase binding to knotted DNA regions. (C and D) Hooked juxtapositions, believed to be preferential substrates triggering type II topo passages, are encountered frequently in the knotted portions, which favors type II topo action within the knotted domain still further and directs strand-passage reactions toward efficient unknotting.