Localization and condensation of the parB-DNA complex in the bacterial chromosome
Chase Broedersz (Princeton University)
The molecular machinery responsible for faithful segregation of the chromosome in bacteria such as Caulobacter crescentus and Bacillus subtilis includes the ParABS a.k.a. Spo0J/Soj partitioning system. In Caulobacter, prior to division, hundreds of ParB proteins bind to the chromosome near the origin of replication, and localize to one pole of the cell. Subsequently, the ParB-DNA complex is translocated to the far pole by the binding and retraction of the ParA spindle-like apparatus. Remarkably, the localization of ParB proteins to specific regions of the chromosome appears to be controlled by only a few centromeric parS binding sites. Although lateral interactions between DNA-bound ParB are likely to be important for their localization, the long-range order of ParB domains on the chromosome appears to be inconsistent with a picture in which protein-protein interactions are limited to neighboring DNA-bound proteins. We propose a simple model to study the localization and condensation of the ParB-DNA complex, which includes both lateral and 3D protein-protein interactions among DNA-bound ParB proteins. Our model shows how such interactions can condense and organize the chromosome spatially, and can control both the localization and the long-range order of the DNA-bound proteins. Interestingly, the condensed ParB-DNA complex adopts a topology in which loops of DNA extend outside the cluster exposed for transcription; this structure may have implications for expression levels of genes in the vicinity of the parS site.