2 DISTINCT MODES OF STRAND UNLINKING DURING THETA-TYPE DNA-REPLICATION

DNA gyrase and topoisomerase IV (Topo IV) are type II bacterial DNA to poisomerases that show a high degree of similarity to each other yet a ppear to have different functions during DNA replication. We show here that the torsional stress generated during theta-type DNA replication in vitro can take the form of either positive supercoils ahead of the replication fork or catenane-like right-handed windings (precatenanes ) of the two partially replicated duplexes behind the fork. Gyrase pre fers to act on the former, whereas Topo IV prefers the latter. Removal of either form of positive winding can support nascent chain elongati on, but only precatenane removal can support the final stages of DNA r eplication: processing of the late intermediate and daughter chromosom e decatenation.