Dividing cells must transfer their genetic information to the daughter cells. In many bacterial species, the segregation of the genetic material is carried out by a tripartite partitioning system, ParABS. This complex consists of a 16 bp centromere-like DNA sequence, ParS, to which ParB, a CTPase, binds and recruits ParA, an ATP-fueled, walker protein. From the ParS nucleation site, subsequent spreading of ParB along adjacent DNA regions brings distal parts of the chromosome into proximity and forms condensates. While much is known about the molecular composition and binding dynamics of the ParABS system, how the physical properties of these assemblies contribute to their function remains less explored. Given that chromosome segregation is an inherently mechanical process, we investigated whether ParBS condensates exert sufficient force to separate the duplicated DNA. ParBS condensate formation assessed using turbidity assays indicated the importance of electrostatic interactions and was particularly enhanced by divalent magnesium ions. Microrheology data indicated that ParBS condensates were viscoelastic. Furthermore, in the presence of a macromolecular crowder, ParB bound to and co-condensed with λ DNA tethered between two optically trapped beads. From stretch and relaxation cycles, we confirmed that these condensates were liquid-like and generated a measurable force of ∼ 5 pN. These data indicate that ParBS-DNA condensates generate tension sufficient to translocate DNA, supporting their critical role in organizing and partitioning bacterial chromosomes.
Gupta et al. (Sun,) studied this question.