Topoisomerase III, but not topoisomerase I, can support nascent chain elongation during theta-type DNA replication.

Topoisomerase III, but not topoisomerase I, could, in the absence of DNA gyrase, support bidirectional DNA replication in an oriC plasmid DNA replication system reconstituted with purified proteins. The initial rate of DNA synthesis and the efficiency of nascent chain elongation showed that topoisomerase III-stimulated DNA replication was as efficient as gyrase-stimulated DNA replication. In addition, topoisomerase III was also able, in the absence of DNA gyrase, to decatenate the replicating daughter DNA molecules to form monomer product. Thus, of the four topoisomerases in Escherichia coli, three, topoisomerases III and IV and DNA gyrase, can support nascent chain elongation, whereas only two, topoisomerases III and IV, can topologically resolve the daughter molecules.