DNA replication and recombination generate intertwined DNA intermediates that must be decatenated for chromosome segregation to occur. We showed recently that topoisomerase IV (topo IV) is the only important decatenase of DNA replication intermediates in bacteria. Earlier results, however, indicated that DNA gyrase has the primary role in unlinking the catenated products of site-specific recombination. To address this discordance, we constructed a set of isogenic strains that enabled us to inhibit selectively with the quinolone @entity4258 topo IV, gyrase, both enzymes, or neither enzyme in vivo. We obtained identical results for the decatenation of the products of two different site-specific recombination enzymes, @entity5999 integrase and transposon Tn3 resolvase. @entity4258 blocked decatenation in wild-type strains, but had no effect in strains with drug-resistance mutations in both gyrase and topo IV. When topo IV alone was inhibited, decatenation was almost completely blocked. If gyrase alone were inhibited, most of the @entity43632 were unlinked. We showed that topo IV is the primary decatenase in vivo and that this function is dependent on the level of DNA supercoiling. We conclude that the role of gyrase in decatenation is to introduce negative supercoils into DNA, which makes better substrates for topo IV. We also discovered that topo IV has an unexpectedly strong DNA relaxation activity that, together with gyrase and topo I, is able to set the supercoiling levels in @entity526 .

Topoisomerase IV, not gyrase, decatenates products of site-specific recombination in XXXX .

@entity43632 :: ('MESH:D043863', 'Chemical') :: ['catenanes'] @entity5999 :: ('10710', 'Species') :: ['phage lambda'] @entity4258 :: ('MESH:D009643', 'Chemical') :: ['Norfloxacin', 'norfloxacin'] @entity526 :: ('562', 'Species') :: ['Escherichia coli']
@entity526 :: (562,Species) :: ['Escherichia coli']
