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.