A conserved mechanism drives partition complex assembly on bacterial chromosomes and plasmids.

Chromosome and plasmid segregation in bacteria are mostly driven by ParAB systems. These DNA partitioning machineries rely on large nucleoprotein complexes assembled on centromere sites (). However, the mechanism of how a few -bound ParB proteins nucleate the formation of highly concentrated ParB clusters remains unclear despite several proposed physico-mathematical models. We discriminated between these different models by varying some key parameters using the F plasmid partition system. We found that "Nucleation & caging" is the only coherent model recapitulating data. We also showed that the stochastic self-assembly of partition complexes (i) is a robust mechanism, (ii) does not directly involve ParA ATPase, (iii) results in a dynamic structure of discrete size independent of ParB concentration, and (iv) is not perturbed by active transcription but is by protein complexes. We refined the "Nucleation & caging" model and successfully applied it to the chromosomally encoded Par system of , indicating that this stochastic self-assembly mechanism is widely conserved from plasmids to chromosomes.