Type II topoisomerase substrate geometry revealed through combined experiment and computation.

Type II topoisomerases (topo IIs) are essential enzymes that regulate DNA topology through a strand-passage mechanism in which a duplex DNA (transfer-segment) is passed through a transiently cleaved second duplex DNA (gate-segment). Biochemical and structural approaches have revealed critical details of the binding and cleavage of the gate-segment DNA. However, capture of the transfer-segment DNA has proven more difficult to resolve due to the transient nature of the interaction. Nonetheless, selection of the transfer segment with a specific conformation or orientation relative to the gate segment is predicted to govern aspects of topo II activity, including chiral discrimination and the ability to reduce topological complexity below equilibrium. To determine the conformation and orientation of the transfer-segment relative to the gate segment, we combined experimental single-molecule measurements of topo II unlinking a single DNA crossing with Brownian dynamics simulations of the DNA crossing. By correlating the unlinking rate with the geometric features of the DNA crossing, we obtain the complete three-dimensional preferred crossing geometry for strand passage. Strikingly, the preferred crossing geometries for