Robust ParB Binding to Half- parS Sites in Pseudomonas aeruginosa -A Mechanism for Retaining ParB on the Nucleoid?

Chromosome segregation in is assisted by the tripartite ParAB- system, composed of an ATPase (ParA), a DNA-binding protein (ParB) and its target sequence(s). ParB forms a nucleoprotein complex around four s () that overlaps and facilitates relocation of newly synthesized ori domains inside the cells by ParA. Remarkably, ParB of also binds to numerous heptanucleotides (half-s) scattered in the genome. Here, using chromatin immunoprecipitation-sequencing (ChIP-seq), we analyzed patterns of ParB genome occupancy in cells growing under conditions of coupling or uncoupling between replication and cell division processes. Interestingly, a dissipation of ParB- complexes and a shift of ParB to half-s were observed during the transition from the exponential to stationary phase of growth on rich medium, suggesting the role of half-s in retaining ParB on the nucleoid within non-dividing cells. The ChIP-seq analysis of strains expressing ParB variants unable to dislocate from s showed that the ParB spreading ability is not required for ParB binding to half-s. Finally, a strain with mutated 25 half-s of the highest affinity towards ParB was constructed and analyzed. It showed altered ParB coverage of the region and moderate changes in gene expression. Overall, this study characterizes a novel aspect of conserved bacterial chromosome segregation machinery.