Molecular Insights on the Assembling of ParB1 and ParB1-parS1 Partition Complex.

ParB, which is a condensate-forming DNA clamp and a CTP switch, is a key member of the bacterial chromosome segregation apparatus. Molecular features that govern the assembly of ParB for partition assembly condensate formation that leads to DNA segregation are sparsely understood. We report that ParB1 from the multipartite bacteria Vibrio cholerae is a CTPase. The AI-predicted structural model of the ParB1-parS1 nucleoprotein complex consistent with small-angle X-ray scattering revealed how parS1 loading primes ParB1 for sliding, with the formation of a lumen for housing the sliding DNA. The CTPase domains in this dimeric ParB1-parS1 structure are accessible for the formation of higher-order assemblies. Furthermore, we showed that a truncated N-terminal segment of ParB1 containing the CTPase domain undergoes concentration-dependent oligomerization. A flexible linker joining this oligomerization-prone N-terminal segment and the C-terminal dimerization domain appears to intrinsically restrain the self-association of full-length ParB1. Our results suggest that the oligomerization propensity of the N-terminal segment of ParB1 is a key factor for higher-order partition assembly formation.