A model for the condensation of the bacterial chromosome by the partitioning protein ParB

The molecular machinery responsible for faithful segregation of the chromosome in bacteria such as \textit{Caulobacter crescentus} and \textit{Bacillus} \textit{subtilis} includes the ParABS a.k.a. Spo0J/Soj partitioning system. In \textit{Caulobacter}, prior to division, hundreds of ParB proteins bind to the DNA 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 \textit{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 developed a coarse-grained Brownian dynamics model that allows for lateral and 3D protein-protein interactions among bound ParB proteins. Our model shows how such interactions can condense and organize the DNA spatially, and can control the localization and the long-range order of the DNA-bound proteins.