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

Chromosome segregation in Pseudomonas aeruginosa is assisted by the tripartite ParAB- parS system, composed of an ATPase (ParA), a DNA-binding protein (ParB) and its target parS sequence(s). ParB forms a nucleoprotein complex around four parS s ( parS1-parS4 ) that overlaps oriC and facilitates relocation of newly synthesized ori domains inside the cells by ParA. Remarkably, ParB of P. aeruginosa also binds to numerous heptanucleotides (half- parS 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- parS complexes and a shift of ParB to half- parS s were observed during the transition from the exponential to stationary phase of growth on rich medium, suggesting the role of half- parS s in retaining ParB on the nucleoid within non-dividing P. aeruginosa cells. The ChIP-seq analysis of strains expressing ParB variants unable to dislocate from parS s showed that the ParB spreading ability is not required for ParB binding to half- parS s. Finally, a P. aeruginosa strain with mutated 25 half- parS s of the highest affinity towards ParB was constructed and analyzed. It showed altered ParB coverage of the oriC region and moderate changes in gene expression. Overall, this study characterizes a novel aspect of conserved bacterial chromosome segregation machinery.