Physical Views on ParABS-Mediated DNA Segregation

Phase-separated ParB enforces diverse DNA compaction modes and stabilizes the parS-centered partition complex

The tripartite ParABS system mediates chromosome segregation in the majority of bacterial species. Typically, DNA-bound ParB proteins around the parS sites condense the chromosomal DNA into a higher-order multimeric nucleoprotein complex for the ParA-driven partition. Despite extensive studies, the molecular mechanism underlying the dynamic assembly of the partition complex remains unclear. Herein, we demonstrate that Bacillus subtilis ParB (Spo0J), through the multimerization of its N-terminal domain, forms phase-separated condensates along a single DNA molecule, leading to the concurrent organization of DNA into a compact structure. Specifically, in addition to the co-condensation of ParB dimers with DNA, the engagement of well-established ParB condensates with DNA allows for the compression of adjacent DNA and the looping of distant DNA. Notably, the presence of CTP promotes the formation of condensates by a low amount of ParB at parS sites, triggering two-step DNA condensation. Remarkably, parS-centered ParB-DNA co-condensate constitutes a robust nucleoprotein architecture capable of withstanding disruptive forces of tens of piconewton. Overall, our findings unveil diverse modes of DNA compaction enabled by phase-separated ParB and offer new insights into the dynamic assembly and maintenance of the bacterial partition complex.

Organization and replicon interactions within the highly segmented genome of Borrelia burgdorferi

Borrelia burgdorferi, a causative agent of Lyme disease, contains the most segmented bacterial genome known to date, with one linear chromosome and over twenty plasmids. How this unusually complex genome is organized, and whether and how the different replicons interact are unclear. We recently demonstrated that B. burgdorferi is polyploid and that the copies of the chromosome and plasmids are regularly spaced in each cell, which is critical for faithful segregation of the genome to daughter cells. Regular spacing of the chromosome is controlled by two separate partitioning systems that involve the protein pairs ParA/ParZ and ParB/Smc. Here, using chromosome conformation capture (Hi-C), we characterized the organization of the B. burgdorferi genome and the interactions between the replicons. We uncovered that although the linear chromosome lacks contacts between the two replication arms, the two telomeres are in frequent contact. Moreover, several plasmids specifically interact with the chromosome oriC region, and a subset of plasmids interact with each other more than with others. We found that Smc and the Smc-like MksB protein mediate long-range interactions on the chromosome, but they minimally affect plasmid-chromosome or plasmid-plasmid interactions. Finally, we found that disruption of the two partition systems leads to chromosome restructuring, correlating with the mis-positioning of chromosome oriC. Altogether, this study revealed the conformation of a complex genome and analyzed the contribution of the partition systems and SMC family proteins to this organization. This work expands the understanding of the organization and maintenance of multipartite bacterial genomes.

Organization and replicon interactions within the highly segmented genome of Borrelia burgdorferi

Borrelia burgdorferi , a causative agent of Lyme disease, contains the most segmented bacterial genome known to date, with one linear chromosome and over twenty plasmids. How this unusually complex genome is organized, and whether and how the different replicons interact are unclear. We recently demonstrated that B. burgdorferi is polyploid and that the copies of the chromosome and plasmids are regularly spaced in each cell, which is critical for faithful segregation of the genome to daughter cells. Regular spacing of the chromosome is controlled by two separate partitioning systems that involve the protein pairs ParA/ParZ and ParB/SMC. Here, using chromosome conformation capture (Hi-C), we characterized the organization of the B. burgdorferi genome and the interactions between the replicons. We uncovered that although the linear chromosome lacks contacts between the two replication arms, the two telomeres are in frequent contact. Moreover, several plasmids specifically interact with the chromosome oriC region, and a subset of plasmids interact with each other more than with others. We found that SMC and the SMC-like MksB protein mediate long-range interactions on the chromosome, but they minimally affect plasmid-chromosome or plasmid-plasmid interactions. Finally, we found that disruption of the two partition systems leads to chromosome restructuring, correlating with the mis-positioning of chromosome oriC . Altogether, this study revealed the conformation of a complex genome and analyzed the contribution of the partition systems and SMC family proteins to this organization. This work expands the understanding of the organization and maintenance of multipartite bacterial genomes.

Polyploidy, regular patterning of genome copies, and unusual control of DNA partitioning in the Lyme disease spirochete

Borrelia burgdorferi, the tick-transmitted spirochete agent of Lyme disease, has a highly segmented genome with a linear chromosome and various linear or circular plasmids. Here, by imaging several chromosomal loci and 16 distinct plasmids, we show that B. burgdorferi is polyploid during growth in culture and that the number of genome copies decreases during stationary phase. B. burgdorferi is also polyploid inside fed ticks and chromosome copies are regularly spaced along the spirochete's length in both growing cultures and ticks. This patterning involves the conserved DNA partitioning protein ParA whose localization is controlled by a potentially phage-derived protein, ParZ, instead of its usual partner ParB. ParZ binds its own coding region and acts as a centromere-binding protein. While ParA works with ParZ, ParB controls the localization of the condensin, SMC. Together, the ParA/ParZ and ParB/SMC pairs ensure faithful chromosome inheritance. Our findings underscore the plasticity of cellular functions, even those as fundamental as chromosome segregation.