In bacteria, most low-copy-number plasmid and chromosomally encoded partition systems belong to the tripartite ParABS partition machinery. Despite the importance in genetic inheritance, the mechanisms of ParABS-mediated genome partition are not well understood. Combining theory and experiment, we provided evidence that the ParABS system—DNA partitioning in vivo via the ParA-gradient-based Brownian ratcheting—operates near a transition point in parameter space (i.e., a critical point), across which the system displays qualitatively different motile behaviors. 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. The spatial control over the near-critical-point operation not only ensures both sensitive adaptation and robust execution of partitioning but also sheds light on the fundamental question in cell biology: how do cells faithfully measure cellular-scale distance by only using molecular-scale interactions?

在细菌中，大多数低拷贝数质粒和染色体编码的分配系统属于三方ParAB S分配机制。尽管ParAB S介导的基因组分区在遗传中很重要，但其机制尚不清楚。结合理论和实验，我们提供了证据，证明 ParAB S系统（通过基于 ParA 梯度的布朗棘轮在体内进行 DNA 分配）在参数空间的过渡点（即临界点）附近运行，系统在该过渡点上显示定性不同的运动行为。这种近临界点操作使复制质粒的分离距离适应延伸核的一半长度，确保两个半细胞继承质粒的一个副本。此外，我们证明质粒定位细胞质 ParA 以缓冲分区保真度，以防止 ParA 水平的细胞间大波动。对近临界点操作的空间控制不仅确保了分区的灵敏适应和稳健执行，而且还揭示了细胞生物学中的基本问题：细胞如何仅通过分子尺度相互作用忠实地测量细胞尺度距离？