How Does Topoisomerase IV Distinguish Left from Right?
Keir C. Neuman, Ecole Normale Superieure 4:30pm, Physics Lecture Hall Topoisomerase IV (Topo IV) is an essential ATP-dependent bacterial type II topoisomerase that transports one segment of DNA through a transient double-strand break in a second segment of DNA. In vivo, Topo IV resolves inappropriate topological states resulting from DNA replication by removing positive supercoils and unlinking catenated daughter strands. In vitro, Topo IV relaxes positive supercoils ~20- fold faster than negative supercoils. Hypotheses explaining this asymmetry include a chirality sensing mechanism whereby Topo IV productively binds juxtaposed DNA segments with a preferred crossing angle. Single-molecule magnetic manipulation experiments permitted us to directly test this angular symmetry breaking hypothesis by measuring unlinking rates of single left- and right-handed DNA crossings. In conjunction with Monte Carlo simulations of juxtaposed DNA, these measurements yielded the preferred crossing angle
