Summary
In bacteria, faithful DNA segregation of chromosomes and plasmids is mainly mediated by ParABS systems. These systems, consisting of the ParA ATPase, the DNA binding ParB CTPase, and the centromere site parS, orchestrate the separation of newly replicated DNA copies and their positioning along the cell’s longitudinal axis. Accurate segregation relies on the assembly of a high-molecular-weight complex made of hundreds of ParB dimers assembled from parS nucleation sites. This complex assembles in a multi-step process and exhibits dynamic liquid-droplet properties. Despite various proposed models, including ‘Nucleation & caging’ and ‘Clamping & sliding’, the complete mechanism for partition complex assembly remains elusive. This study, focusing on the plasmid F, investigates the impact of DNA supercoiling on ParB DNA binding profiles in vivo. We found that variation in DNA supercoiling does not significantly affect any step of the assembly of the partition complex. Physical modeling, based on ChIP-seq data from linear plasmids F, challenges the sufficiency of the ‘Clamping & sliding’ model alone. It demonstrates that while this model fits ParB DNA binding profiles up to ∼2-kbp from parS, additional mechanisms like ‘Nucleation & caging’ are needed for distant ParB binding. The study also highlights the dominant role of ParB-ParB interactions in DNA compaction within ParB condensates.
Competing Interest Statement
The authors have declared no competing interest.