ParABS-type DNA segregation (chromosomes and type I plasmids)

Reusable module for the bacterial ParABS DNA-segregation machine, which moves newly replicated copies of a replicon into the two daughter-cell halves. A centromere-like DNA site (parS on chromosomes; sopC, parS or O_B on plasmids) is bound by a ParB-type protein that, in the characterized systems, loads as a CTP-dependent sliding clamp and spreads to form a partition complex. A deviant Walker-type ParA ATPase binds the nucleoid non-specifically as an ATP-bound dimer, and the partition complex stimulates its ATP hydrolysis, releasing it from DNA; the resulting ParA gradient positions and separates the partition complexes. The same two roles are filled by chromosome-encoded ParA/ParB (Soj/Spo0J) at origin-proximal parS sites and by type I plasmid partition proteins such as SopA/SopB of F, ParA/ParB of P1 and IncC/KorB of RK2.

Boundary. The module covers the two protein roles and their coupling. The parS/sopC/O_B site is part of the mechanism but is a DNA element, so it is described in role descriptions rather than modeled as a participant. SMC-ScpAB and MukBEF condensins, the nucleoid-occlusion protein Noc, and type II (actin-like ParM/ParR/parC) and type III (tubulin-like TubZ/TubR) plasmid partition systems are outside the boundary. So are the other routes to plasmid maintenance: high-copy plasmids that lack a canonical partition system and are thought to segregate by random distribution (a model PMID:25107339 questions), multimer resolution, and post-segregational killing. Two further activities of type I systems are real but peripheral here: transcriptional autoregulation of the par operon by ParA and ParB, and partition-mediated plasmid incompatibility. Mechanism. Older work on F SopA described ATP-dependent filaments that pull plasmids apart as they depolymerize; reviews from that period (PMID:25107339) present this alongside a diffusion-ratchet model in which partition complexes follow a gradient of nucleoid-bound ParA-ATP. Reconstitutions since then (PMID:34286695) describe F partition complexes moving along a ParA gradient, so the module's description follows the gradient model. The ParB activation of ParA ATP hydrolysis, modeled as the connection between the two parts, is common to both. Ontology gap. GO has no process term covering both chromosome segregation (GO:0007059, under cell cycle process) and plasmid partitioning (GO:0030541, under plasmid maintenance); their only shared ancestor is cellular process. The module therefore carries both terms as concepts and attaches the right one to each replicon-specific variant. A parent such as "DNA segregation by a partition system" would let one term describe the shared mechanism. Family structure does not follow replicon type. In PANTHER, F-plasmid SopA sits in the same subfamily as chromosomal ParA/Soj (PTHR13696:SF52), and RK2 KorB in the same subfamily as chromosomal ParB/Spo0J (PTHR33375:SF1), whereas SopB and P1 ParB form a separate family (PTHR38973). The NCBIFAM model TIGR00180 and the CDD architecture that names Pseudomonas ParB likewise mix chromosomal and plasmid members. The replicon is therefore modeled as a variant axis, and replicon-specific process terms should not be transferred by family membership alone. CTPase activity is asserted for plasmid ParB only where shown (F SopB, RK2 KorB). Whether P1 ParB binds and hydrolyses CTP is not established by the sources cited here.