Mechanisms that coordinate chromosome segregation with cell division are poorly characterized in bacteria. By contrast, plasmid segregation is well-studied and is often mediated by the tripartite plasmid-partitioning (Par) system, which comprises two proteins (the ATPase ParA and the DNA-binding protein ParB) and a cis-acting DNA binding site (parS). ParB binds to the parS site(s) and ParA interacts with the ParB–parS complex to facilitate the segregation of plasmids into daughter cells. Chromosomal par genes are also widespread in bacteria but their functions are not as clear-cut. Two new papers from the Waldor laboratory now reveal that the chromosomal Par systems of
Vibrio cholerae
segregate its chromosomes independently.
The two circular chromosomes of V. cholerae (chrI and chrII) each have their own par loci (parABS1 and parABS2). Reporting in Genes and Development, Fogel and Waldor show that parABS1 positions the origin regions of replicated chrI at the poles of pre-divisional cells to mediate chrI segregation. The locations of fluorescently-labelled ParA1 and ParB1 were monitored using microscopy and automated image analysis. The distribution of the Par proteins in thousands of cells at different stages of the cell cycle was determined. ParB1 (bound to the chromosome at three origin-proximal parS1 sites) localized to the cell poles, but only in the presence of functional ParA1. ParA1 had a dynamic distribution and seemed to form a cytoskeletal structure that retracted to the cell pole just prior to division. The authors invoked a model in which ParA1 binds to the ParB–parS complex and 'pulls' the bound chromosome across the cell to the pole, in a process similar to mitosis. The chrI par genes were not essential, and mutants lacking these genes were able to segregate chrI to daughter cells, so other systems must also participate in chrI segregation. This Par-directed mitotic pulling mechanism for chromosome segregation might be conserved in other bacteria.
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