CTP and parS coordinate ParB partition complex dynamics and ParA-ATPase activation for ParABS-mediated DNA partitioning

ParABS partition systems, comprising the centromere-like DNA sequence parS, the parS-binding ParB-CTPase, and the nucleoid-binding ParA-ATPase, ensure faithful segregation of bacterial chromosomes and low-copy-number plasmids. F-plasmid partition complexes containing ParB(F) and parS(F) move by generating and following a local concentration gradient of nucleoid-bound ParA(F). However, the process through which ParB(F) activates ParA(F)-ATPase has not been defined. We studied CTP- and parS(F)-modulated ParA(F)–ParB(F) complex assembly, in which DNA-bound ParA(F)-ATP dimers are activated for ATP hydrolysis by interacting with two ParB(F) N-terminal domains. CTP or parS(F) enhances the ATPase rate without significantly accelerating ParA(F)–ParB(F) complex assembly. Together, parS(F) and CTP accelerate ParA(F)–ParB(F) assembly without further significant increase in ATPase rate. Magnetic-tweezers experiments showed that CTP promotes multiple ParB(F) loading onto parS(F)-containing DNA, generating condensed partition complex-like assemblies. We propose that ParB(F) in the partition complex adopts a conformation that enhances ParB(F)–ParB(F) and ParA(F)–ParB(F) interactions promoting efficient partitioning.