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Escherichia coli possesses two type 1A topoisomerases, Topo I (topA ) and Topo III (topB ). Topo I relaxes excess negative supercoiling, andtopA mutants can grow only in the presence of compensatory mechanisms, such as gyrase mutations.topB mutants grow as well as wild-type cells.In vitro , Topo III, but not Topo I, can efficiently decatenate DNA during replication. However,in vivo , a chromosome segregation defect is seen only when both type 1A topoisomerases are absent. Here we present experimental evidence for an interplay between gyrase and type 1A topoisomerases in chromosome segregation. We found that both the growth defect and the Par− phenotypes of agyrB (Ts) mutant at nonpermissive temperatures were significantly corrected by deletingtopA , but only whentopB was present. Overproducing Topo IV, the major cellular decatenase, could not substitute fortopB . We also show that overproducing Topo III at a very high level could suppress the Par− phenotype. We previously found that the growth and chromosome segregation defects of a tripletopA rnhA gyrB (Ts) mutant in which gyrase supercoiling activity was strongly inhibited could be corrected by overproducing Topo III (V. Usongo, F. Nolent, P. Sanscartier, C. Tanguay, S. Broccoli, I. Baaklini, K. Drlica, and M. Drolet, Mol. Microbiol.69: 968-981, 2008). We show here that this overproduction could be bypassed by substituting thegyrB (Ts) allele for agyrB + one or by growing cells in a minimal medium, conditions that reduced bothtopA - andrnhA -dependent unregulated replication. Altogether, our data point to a role for Topo III in chromosome segregation when gyrase is inefficient and suggest that Topo I plays an indirect role via supercoiling regulation.

Interplay between Type 1A Topoisomerases and Gyrase in Chromosome Segregation in Escherichia coli