The dynamics of partition protein B (ParB) spreading on the DNA and bridging-induced DNA condensation.

Abstract

The ParABS system plays an essential role in prokaryotic chromosome segregation. After loading at the parS site on the genome, ParB proteins rapidly redistribute to distances of ∼15 kb away from the loading site and form a partition complex. It has remained puzzling how this large-distance spreading can occur along DNA that is loaded with hundreds of proteins as well as its mechanism in DNA condensation. Using in vitro single-molecule fluorescence imaging, we here show that ParB proteins can load onto the DNA distantly of parS loading site by a novel ParB-ParB recruitment. We observe the proteins loading in-cis and in-trans whereby, at low tensions within the DNA, newly recruited ParB can bypass roadblocks as it gets loaded to spatially proximal but genomically distant DNA regions. Moreover, we investigate the detailed mechanism of ParB-induced DNA-condensation, where previously spread ParB proteins can fix distant genomic regions in close proximity via transient loop formation. We investigate this process in the light of polymer-polymer phase-separation (PPPS) and detect which molecular domains are involved in this bridging-induced condensation. ParB-ParB recruitment and ParB-DNA condensation explains how ParB can form a bacterial segregation apparatus involving substantial genomic distance during chromosome segregation which is vital for the bacterial cell cycle.