TOPOISOMERASE-III, BUT NOT TOPOISOMERASE-I, CAN SUPPORT NASCENT CHAINELONGATION DURING THETA-TYPE DNA-REPLICATION

Topoisomerase III, but not topoisomerase I, could, in the absence of D NA gyrase, support bidirectional DNA replication in an oriC plasmid DN A 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 effici ent as gyrase-stimulated DNA replication. In addition, topoisomerase I II was also able, in the absence of DNA gyrase, to decatenate the repl icating daughter DNA molecules to form monomer product. Thus, of the f our 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.