On the role of centromere dispersion in stability of linear bacterial plasmids

The N15 prophage-plasmid is linear, and the four centromere sites that enable its active partition are scattered rather than confined to a tandem array. This unusual arrangement suggested that the two features might be linked: centromere dispersion could enable condensation of linear DNA through interaction of partition complexes and so facilitate movement of segregating plasmid molecules. The present study examines this possibility. Linear N15 derivatives varying in centromere-site (IR) position, number and spacing were constructed, and stabilization of these plasmids by N15 SopAB proteins was measured. Stabilization increased in proportion to the number of IR sites and the distance between IR sites, the result expected if condensation mediated by partition complexes (SopB-IR) improves efficiency of SopA-directed segregation. However, visualization of two IR sites on the same molecule revealed that their colocalization did not depend on SopB but resulted from spontaneous folding of the linear DNA. Segregation of these sites by SopA was limited and incomplete compared to partition of plasmid copies. These observations imply that forces responsible for intrinsic folding of linear DNA tend to counter attempts to partition centromeres in cis to each other. We suggest that the beneficial effects of IR number and spacing on partition stem not from condensation but from provision of more numerous and better arranged substrates for SopA action.