Chirality of DNA trefoils: implications in intramolecular synapsis of distant DNA segments.

We show that supercoiling of a DNA trefoil, the simplest knotted ring, perturbs differently the spatial writhe of its two chiral forms. As a consequence, the negative-noded and positive-noded DNA trefoils can be resolved by gel electrophoresis. Analysis of the chirality of trefoils produced by cyclization of two linear DNAs demonstrates that the two chiral trefoils are produced in equal amounts, suggesting that these DNAs do not prefer intrinsic writhe of one chirality or the other. In contrast, knotting of nicked DNA rings by a molar excess of Saccharomyces cerevisiae DNA topoisomerase II produces more negative-noded than positive-noded trefoils, indicating an asymmetry in the interaction between the enzyme and DNA crossovers of different signs. These results suggest that asymmetry in DNA crossovers and intrinsic or ligand-induced writhe in a DNA might be detectable from an analysis of trefoil chirality.