Potent catenation of supercoiled and gapped DNA circles by topoisomerase I in the presence of a hydrophilic polymer.

An exceptionally potent DNA catenation activity, identified in an extract from Escherichia coli, has been purified and partially characterized. Catenation results from the sequential action of the following two polypeptides: beta, 34 kDa and identical to exonuclease III; and alpha, 101 kDa and identical to DNA topoisomerase I (omega protein). An additional requirement is that a small proportion of the circles be nicked in order to provide the substrate for exonuclease III to generate gaps, estimated to be about 100 nucleotides long. Following exonuclease III digestion, one molecule of topoisomerase I can interlock per minute at 30 degrees C about 20 supercoiled and gapped DNA circles into a massively catenated network. The reaction requires Mg2+ and a hydrophilic polymer (polyvinyl alcohol or polyethylene glycol) at about 7%, but neither ATP nor spermidine. The hydrophilic polymer appears to drive catenation by condensing the DNA; decatenation by topoisomerase I proceeds upon removal of the polymer.