Differential and dynamic localization of topoisomerases in Bacillus subtilis

Visualization of topoisomerases in live Bacillus subtilis cells showed that Topo I, Topo IV, and DNA gyrase differentially localize on the nucleoids but are absent at cytosolic spaces surrounding the nucleoids, suggesting that these topoisomerases interact with many regions of the chromosome. While both subunits of Topo IV were uniformly distributed throughout the nucleoids, Topo I and gyrase formed discrete accumulations, or foci, on the nucleoids in a large fraction of the cells, which showed highly dynamic movements. Three-dimensional time lapse microscopy showed that gyrase foci accumulate and dissipate within a 1-min time scale, revealing dynamic assembly and disassembly of subcellular topoisomerase centers. Gyrase centers frequently colocalized with the central DNA replication machinery, suggesting a major role for gyrase at the replication fork, while Topo I foci were frequently close to or colocalized with the structural maintenance of chromosomes (SMC) chromosome segregation complex. The findings suggest that different areas of supercoiling exist on the B. subtilis nucleoids, which are highly dynamic, with a high degree of positive supercoiling attracting gyrase to the replication machinery and areas of negative supercoiling at the bipolar SMC condensation centers recruiting Topo I.