Conserved C-terminal region of global repressor KorA of broad-host-range plasmid RK2 is required for co-operativity between KorA and a second RK2 global regulator, KorB.

KorA and KorB proteins of IncP1 plasmid RK2 are encoded in the central control region (ccr) of the plasmid and act as global regulators of plasmid genes for replication, transfer and stable inheritance. KorA represses seven promoters on RK2, by binding to a defined operator site, OA, which always occurs in promoter regions. KorB recognises another operator, OB, which is found 12 times on the RK2 genome, but not always in promoter regions. At five of the KorA-regulated promoters, an OBsequence is also present. The presence of both KorA and KorB leads to severely decreased promoter activity. By measuring repression at different levels of KorA and KorB alone and in combination, we showed that there is at least 3. 4-fold co-operativity between them at korApin vivo. Testing the ability of previously isolated KorA mutants to act in a co-operative way in the presence of KorB in vivo or in vitro showed that the C-terminal part of KorA between amino acid positions 68 and 83 is required for this co-operativity. This region is part of a segment that is highly conserved between KorA and two other RK2 proteins, TrbA and KlcB. We propose that this conserved region may provide the basis for co-operativity with KorB either indirectly, by modulating DNA structure near the KorB binding site, or directly by serving as the "recognition" patch of each protein by KorB. It may thus serve as a key domain in allowing a sensitive response of the global circuits to changes in repressor concentration and thus modulation of replication, transfer and maintenance.