Genetic rearrangement of DNA induces knots with a unique topology: implications for the mechanism of synapsis and crossing-over.

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Abstract
The topological sign of the knots produced by a cycle of phage .lambda. integrative recombination was determined. To insure that these knots reflect intrinsic features of the reaction interwrapping of segments of DNA played minimal role in the topological outcome. The knotted DNA was coated with the bacteriophage T4 uvsX gene product and examined in the EM to determine the nature of each crossing point or node. All of the knots were identical; they were trefoils with 3 nodes of positive sign. One recombination site, which previous work had indicated is organized into a nucleosome-like structure, is apparently wrapped with a handedness identical to that found in nucleosomes. This wrapping may explain the dependence of recombination on supercoiling of the substrate DNA. The topological result sharply limits acceptable mechanisms for the details of strand exchange.