Precise and rapid DNA segregation is required for proper inheritance of genetic material. In most bacteria and archaea, this process is assured by a broadly conserved mitotic-like apparatus in which a NTPase (ParA) displaces the partition complex. Competing observations and models imply starkly different 3D localization patterns of the components of the partition machinery during segregation. Here we use super-resolution microscopies to localize in 3D each component of the segregation apparatus with respect to the bacterial chromosome. We show that Par proteins locate within the nucleoid volume and reveal that proper volumetric localization and segregation of partition complexes requires ATPase and DNA-binding activities of ParA. Finally, we find that the localization patterns of the different components of the partition system highly correlate with dense chromosomal regions. We propose a new mechanism in which the nucleoid provides a scaffold to guide the proper segregation of partition complexes.

Liquid-liquid phase separated (LLPS) states are key to compartmentalise components in the absence of membranes, however it is unclear whether LLPS condensates are actively and specifically organized in the sub-cellular space and by which mechanisms. Here, we address this question by focusing on the ParAB S DNA segregation system, composed of a centromeric-like sequence (parS), a DNA-binding protein (ParB) and a motor (ParA). We show that parS-ParB associate to form nanometer-sized, round condensates. ParB molecules diffuse rapidly within the nucleoid volume, but display confined motions when trapped inside ParB condensates. Single ParB molecules are able to rapidly diffuse between different condensates, and nucleation is strongly favoured by parS. Notably, the ParA motor is required to prevent the fusion of ParB condensates. These results describe a novel active mechanism that splits, segregates and localises non-canonical LLPS condensates in the sub-cellular space.

Liquid-liquid phase separated (LLPS) states are key to compartmentalise components in the absence of membranes, however it is unclear whether LLPS condensates are actively and specifically organized in the sub-cellular space and by which mechanisms. Here, we address this question by focusing on the ParAB S DNA segregation system, composed of a centromeric-like sequence (parS), a DNA-binding protein (ParB) and a motor (ParA). We show that parS-ParB associate to form nanometer-sized, round condensates. ParB molecules diffuse rapidly within the nucleoid volume, but display confined motions when trapped inside ParB condensates. Single ParB molecules are able to rapidly diffuse between different condensates, and nucleation is strongly favoured by parS. Notably, the ParA motor is required to prevent the fusion of ParB condensates. These results describe a novel active mechanism that splits, segregates and localises non-canonical LLPS condensates in the sub-cellular space.

In this chapter, we will focus on ParABS: an apparently simple, three-component system, required for the segregation of bacterial chromosomes and plasmids. We will specifically describe how biophysical measurements combined with physical modeling advanced our understanding of the mechanism of ParABS-mediated complex assembly, segregation and positioning.

Liquid-liquid phase separated (LLPS) states are key to compartmentalise components in the absence of membranes, however it is unclear whether LLPS condensates are actively and specifically organized in the sub-cellular space and by which mechanisms. Here, we address this question by focusing on the ParAB S DNA segregation system, composed of a centromeric-like sequence ( parS ), a DNA-binding protein (ParB) and a motor (ParA). We show that parS -ParB associate to form nanometer-sized, spherical droplets. ParB molecules diffuse rapidly within the nucleoid volume, but display confined motions when trapped inside ParB droplets. Single ParB molecules are able to rapidly diffuse between different droplets, and droplet nucleation is strongly favoured by parS . Notably, the ParA motor is required to prevent the fusion of ParB droplets. These results describe a novel active mechanism that splits, segregates and localises LLPS condensates in the sub-cellular space. 80 85 90 95 100 et al., 2005) , localizes to a large extent (~90% of ParB molecules) to regions containing parS (Sanchez et al., 2015) to form a tight nucleoprotein complex (partition complex). Formation of the partition complex requires weak ParB-ParB dimer interactions mediated by its disordered, low-complexity region. Here, we used super-resolution microscopy and single-particle analysis to investigate the physical mechanisms involved in the formation of partition complexes. We show that partition complexes are small (<50nm), spherical objects.Single, isolated ParB molecules diffuse rapidly within the nucleoid volume, but display confined motions when trapped inside partition complexes. These results suggest a partition of ParB into two phases: a gas-like phase, and a dense, liquid-like condensate phase that shows nanometer-sized, spherical droplets. Next, we show that the nucleation of ParB condensates is strongly favoured by the presence of the centromeric sequence parS .Separation and proper sub-cellular localization of ParB condensates require the ParA motor.Critically, different ParB condensates collapse into a single droplet upon the degradation of ParA. These results describe a novel active mechanism that splits, segregates and localises LLPS condensates in the sub-cellular space. Results ParB assembles into spherical nano-condensatesPrevious studies have revealed that the partition complex is made of ~300 dimers of ParB (Adachi et al., 2006;Bouet et al., 2005) and ~10kb of parS -proximal DNA (Rodionov et al., 1999) . This complex is held together by specific, high-affinity interactions between ParB and parS , and by low-affinity interactions between ParB dimers that are essential for the power-law distribution of ParB around parS sites ( Figure 1A) (Debaugny et al., 2018; Sanchez 455 460 465 470 475 480 485 https://docs.google.com/document/d/1xM7w7WU8bbag5sVCGg1xatPdYe5i1xia3PzXthmzPW8/edit# 24/45 530 535 540 545 550 555 560 https://docs.google.com/document/d/1xM7w7WU8bbag5sVCGg1xatPdYe5i1xia3PzXthmzPW8/edit# 25/45 565 570 575 580 585 590 595 https:/...