Biophysical Characterization of ParBS Condensates suggests a physical mechanism for segregation
TL;DR
Results suggest that ParB-parS condensates dynamically rearrange while maintaining sufficient cohesion to sustain segregation forces without interfering with genomic transactions.
Abstract
The ParABS system orchestrates chromosome segregation in many bacterial species. Centromere-like parS sites nucleate binding of ParB, which subsequently spreads to adjacent non-specific DNA in the presence of CTP. Recruitment of additional ParB molecules and DNA looping via ParB-ParB interactions bring distal chromosome regions into proximity, while interaction with the ParA-ATPase motor drives segregation. In some bacterial species, ParB-parS complexes undergo phase separation into condensates, but their physico-chemical and mechanical properties remain poorly characterized. Turbidity measurements in the presence of CTP, DNA, and physiological mono and divalent salts showed that Mg2+ facilitates condensate formation, whereas K+ concentrations above ∼20 mM disfavor it. Microrheology measurements showed that ParB-parS DNA condensates are viscoelastic, with a room-temperature viscosity of ∼5 Pa·s and a network relaxation time of 0.1 s, allowing rapid responses to deformation. Fluorescence combined with force spectroscopy showed that mechanical disruption of ParB-DNA condensates requires 5–7.5 pN of DNA tension, lower than the force required to stall RNA polymerase but higher than that required for chromosome and plasmid relocation during segregation. These results suggest that ParB-parS condensates dynamically rearrange while maintaining sufficient cohesion to sustain segregation forces without interfering with genomic transactions.