Tracking topoisomerase activity at the single-molecule level.

The recent development of new techniques to manipulate single DNA molecules has opened new opportunities for the study of the enzymes that control DNA topology: the type I and II topoisomerases. These single-molecule assays provide a unique way to study the uncoiling of single supercoiled DNA molecules and the unlinking of two intertwined DNAs. They allow for a detailed characterization of the activity of topoisomerases, including the processivity, the chiral discrimination, and the dependence of their enzymatic rate on ATP concentration, degree of supercoiling, and the tension in the molecule. These results shed new light on the mechanism of these enzymes and their function in vivo.