A hemicatenane conjoins two DNA duplexes through a single-strand interlock. It has been proposed that hemicatenanes are important intermediates for replication, repair, and recombination. However, the biochemical analysis of hemicatenanes is hampered by the relative inaccessibility of such structures. We report here that a DNA topoisomerase III (Top3) from a hyperthermophilic archaeum can carry out synthesis and dissolution of hemicatenanes. We also show that a complex of human Top3α, Bloom helicase (Blm), and RecQ-mediated genome instability protein 1 and 2 has a biochemical function of reversing this hemicatenation. Our results demonstrate that type IA topoisomerases can regulate the formation of hemicatenane structures
