Spatial control over near-critical-point operation ensures fidelity of ParABS-mediated bacterial genome segregation

In bacteria, most low copy number plasmid and chromosomally encoded partition systems belong to the tripartite ParABS partition machinery. ParABS system presents a prototypic model to understand the mechanisms of bacterial genome segregation, which despite its importance in genetic inheritance, are not well understood. Combining theory and experiment, we provided evidences that the ParABS system, partitioning via the ParA gradient based Brownian ratcheting, operates near a critical point in vivo. This near critical point operation adapts the segregation distance of replicated plasmids to the half length of the elongating nucleoid, ensuring both cell halves to inherit one copy of the plasmids. Further, we demonstrated that the plasmid localizes the cytoplasmic ParA to buffer the partition fidelity against the large cell to cell fluctuations in ParA level. We suggest that the spatial control over the near-critical-point operation ensures both sensitive adaption and robust execution of partitioning, a basic principle that could shed light on bacterial genome partition.