Abstract
Chromosome and plasmid segregation in bacteria are mostly driven by ParABS systems. These DNA partitioning machineries rely on large nucleoprotein complexes assembled on centromere sites (parS). However, the mechanism of how a few parS‐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 in vivo using the F plasmid partition system. We found that “Nucleation & caging” is the only coherent model recapitulating in vivo 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 Vibrio cholerae, indicating that this stochastic self‐assembly mechanism is widely conserved from plasmids to chromosomes.
Highlights
The segregation of DNA is an essential process for the faithful inheritance of genetic material
To discriminate between the different partition complex assembly models, we used two larger DNA molecules: the native 100-Kbp F plasmid (F1-10B; Appendix Table S1) and the 4.6Mbp E. coli chromosome with parSF inserted at the xylE locus, in strains either expressing (DLT1472) or not (DLT1215) ParBF from an IPTG-inducible promoter
The number of foci from parSF inserted on the chromosome is half of what is observed with the F plasmid, as expected from the twofold difference in copy number (Collins & Pritchard, 1973)
Summary
The segregation of DNA is an essential process for the faithful inheritance of genetic material. Minimalistic active partition systems, termed Par, ensure this key cell cycle step in bacteria (Baxter & Funnell, 2014) and archaea (Schumacher et al, 2015). Each replicon encodes its own ParABS system and their proper intracellular positioning depends on the interactions of the three ParABS components: ParA, a Walker A ATPase; ParB, a dimer DNA binding protein; and parS, a centromere-like DNA sequence that ParB binds . The ParA-driven mechanism that ensures the proper location and the directed segregation of replicons relies on the positioning of ParBS partition complexes within the nucleoid volume (Le Gall et al, 2016) and on a reaction diffusion-based mechanism (Hwang et al, 2013; Lim et al, 2014; Hu et al, 2017; Walter et al, 2017)
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