Flexibility in repression and cooperativity by KorB of broad host range IncP-1 plasmid RK2.

KorB, encoded by plasmid RK2, belongs to the ParB family of active partitioning proteins. It binds to 12 operators on the RK2 genome and was previously known to repress promoters immediately adjacent to operators O(B)1, O(B)10 and O(B)12 (proximal) or up to 154 bp away (distal) from O(B)2, O(B)9 and O(B)11. To achieve strong repression, KorB requires a cooperative interaction with one of two other plasmid-encoded repressors, KorA or TrbA.

The partitioning activity of the RK2 central control region requires only incC, korB and KorB-binding site O(B)3 but other KorB-binding sites form destabilizing complexes in the absence of O(B)3.

The sector of the genome of broad-host-range IncP plasmid RK2 from kb coordinate 54.0 to 60.0 confers an active partitioning phenotype, increasing the segregational stability of low-copy-number unstable plasmids.

Divergence and conservation of the partitioning and global regulation functions in the central control region of the IncP plasmids RK2 and R751.

The central control region (Ctl) of IncP plasmids is associated with two phenotypes: the coordinate expression of replication and transfer genes; and the ability to increase the segregational stability of a low-copy number test plasmid. This region of the IncP beta plasmid R751 shows significant sequence divergence from the IncP alpha plasmid RK2 sequence, and two genes, korF and korG, present in the IncP alpha region are missing in the IncP beta Ctl. In other respects the organization of the Ctl is basically the same.