"We have determined the topological sign of the knots produced by a cycle of phage lambda integrative recombination. To insure that these knots reflect intrinsic features of the reaction mechanism, the substrate was constructed so that random interwrapping of segments of DNA played a minimal role in the topological outcome. The knotted DNA was coated with the bacteriophage T4 uvsX gene product and examined in the electron microscope to determine the nature of each crossing point or node. All of the knots were identical; they were trefoils with three nodes of positive sign. We interpret this result to mean that one recombination site, which previous work had indicated is organized into a nucleosome-like structure, is wrapped with a handedness identical to that found in nucleosomes. Therefore, this wrapping may explain the dependence of recombination on supercoiling of the substrate DNA. Moreover, we show that the topological result sharply limits acceptable mechanisms for the details of strand exchange."