Topoisomerase IV can support oriC DNA replication in vitro

Escherichia coli has two type II topoisomerases, DNA gyrase and topoisomerase IV (Topo IV). Topo IV is required for the decatenation of the linked daughter chromosomes at the terminal stages of DNA replication, whereas gyrase, because of its ability to convert to negative supercoils the positive supercoils generated by replication fork progression in a circular chromosome, is required to support nascent chain elongation. Using an oriC DNA replication system in vitro, we show that Topo IV, which can relax positive supercoils, can also support replication fork progression. This activity is only observed at substoichiometric ratios of Topo IV to template, at higher ratios, the template becomes relaxed and initiation of DNA replication cannot occur. Topo IV was capable of supporting bidirectional DNA replication from oriC, although, unlike the case with gyrase, some templates apparently replicated unidirectionally. This suggests that either gyrase itself or a certain minimum superhelical density is required for proper initiation of DNA replication from oriC.