The ParB protein of Streptomyces coelicolor A3(2) recognizes a cluster of parS sequences within the origin-proximal region of the linear chromosome

The mycelial prokaryote Streptomyces coelicolor A3(2) possesses a large linear chromosome (8.67 Mb) with a centrally located origin of replication (oriC). Recently, chromosome partitioning genes (parA and parB) and putative ParB binding sites (parS sequences) were identified in its genome. The S. coelicolor chromosome contains more parS sequences than any other bacterial chromosome characterized so far. Twenty of the 24 parS sequences are densely packed within a relatively short distance (approximately 200 kb) around oriC. A series of in vitro and in vivo experiments showed that S. coelicolor ParB protein interacts specifically with the parS sequences, albeit with a rather low affinity. Our results suggested that the binding of ParB is not only determined by the parS sequence, but also by the location of target DNA close to oriC. The unusually high number and close proximity to each other of the parS sites, together with in vivo and in vitro evidence that multiple ParB molecules may assemble along the DNA from an initial ParB-parS complex, suggest that a large DNA segment around the replication origin may form a massive nucleoprotein complex as part of the replication-partitioning cycle.