Illegitimate recombination mediated by calf thymus DNA topoisomerase II in vitro (nonhomologous recombination/deletion/dupllcation/bacteriophage A/subtsnit exchange model)

We have found that purified calf thymus DNA topoisomerase II mediates recombination between two phage A DNA molecules in an in vitro system. The enzyme mainly produced a linear monomer recombinant DNA that can be packaged in vitro. Novobiocin and anti-calf thymus DNA topoisomerase II antibody inhibit this ATP-dependent recombination. The recombinant molecules contain duplica- tions or deletions, and most crossovers take place between nonhomologous sequences of A DNA, as judged by the se- quences of recombination junctions. Therefore, the recombi- nation mediated by the calf thymus DNA topoisomerase II is an illegitimate recombination that is similar to recombination mediated by Escherichia coli DNA gyrase or phage T4 DNA topoisomerase. The subunit exchange model, which has been suggested for the DNA gyrase-mediated recombination, is now generalized as follows: DNA topoisomerase II molecules bind to DNAs, associate with each other, and lead to the exchange of DNA strands through the exchange of topoisomerase II subunits. Illegitimate recombination might be carried out by a general mechanism in organisms ranging from prokaryotes to higher eukaryotes.