QUANTITATIVE BIOLOGY SEMINAR<br>Keeping It Together: Organizing the Bacterial Chromosome for Division</br>
The chromosome has been shown to be highly organized in space in many bacteria, although the origins of this organization remain unclear. The organization of the bacterial chromosome is further complicated by the requirement for replication and segregation. Partitioning proteins of the ParABS system mediate chromosomal and plasmid segregation in a variety of bacteria. At its heart, this segregation machinery includes a large ParB-DNA complex consisting of roughly 1000 ParB proteins. The nature of interactions between DNA-bound ParB proteins, and how these determine the structural properties of the partitioning module remain unclear. In particular, does ParB spread along the DNA to form a filamentous protein-DNA complex with a 1D character, or rather assemble to form a 3D complex on the DNA? Furthermore, it remains unclear how the presence of only one or even a few parS sites can lead to robust formation and localization of such a large protein-DNA complex.
We developed a simple model for interacting proteins on DNA, and found that a combination of 1D spreading bonds and a 3D bridging bond between ParB proteins constitutes the minimal model for condensation of a 3D ParB-DNA complex. These combined interactions provide an effective surface tension that prevents fragmentation of the ParB-DNA complex. Thus, ParB spreads to form multiple 1D domains on the DNA, connected in 3D by bridging interactions to assemble into a 3D ParB-DNA condensate. Importantly, this model accounts for recent experiments on ParB-induced gene-silencing and the effect of a DNA ``roadblock'' on ParB localization. Furthermore, our model provides a simple mechanism to explain how a single parS site is both necessary and sufficient for the formation and localization of the ParB-DNA complex.