Segrosome structure revealed by a complex of ParR with centromere DNA

A crystal structure of the plasmid partition protein ParR bound to centromeric DNA is described. ParR binds the centromeric DNA repeats as a dimer-of-dimers, which assemble in a super-helical array to form a large segrosome with a solenoid-shaped structure. The stable inheritance of genetic material depends on accurate DNA partition. Plasmids serve as tractable model systems to study DNA segregation because they require only a DNA centromere, a centromere-binding protein and a force-generating ATPase. The centromeres of partition ( par ) systems typically consist of a tandem arrangement of direct repeats 1 , 2 , 3 , 4 , 5 , 6 , 7 . The best-characterized par system contains a centromere-binding protein called ParR and an ATPase called ParM. In the first step of segregation, multiple ParR proteins interact with the centromere repeats to form a large nucleoprotein complex of unknown structure called the segrosome, which binds ParM filaments 4 , 8 , 9 , 10 . pSK41 ParR binds a centromere consisting of multiple 20-base-pair (bp) tandem repeats to mediate both transcription autoregulation and segregation. Here we report the structure of the pSK41 segrosome revealed in the crystal structure of a ParR–DNA complex. In the crystals, the 20-mer tandem repeats stack pseudo-continuously to generate the full-length centromere with the ribbon–helix–helix (RHH) fold of ParR binding successive DNA repeats as dimer-of-dimers. Remarkably, the dimer-of-dimers assemble in a continuous protein super-helical array, wrapping the DNA about its positive convex surface to form a large segrosome with an open, solenoid-shaped structure, suggesting a mechanism for ParM capture and subsequent plasmid segregation.