Cooperativity between KorB and TrbA repressors of broad-host-range plasmidRK2

The KorB and TrbA proteins of broad-host-range plasmid RK2 are key regulato rs of the plasmid genes required for conjugative transfer. trbBp is the pri mary promoter responsible for expression of mating pair formation genes, We show that despite the targets for KorB and TrbA at trbBp being about 165 b p apart, 189 bp upstream of the transcription start point and overlapping t he -10 region, respectively, these two proteins show up to 10-fold cooperat ivity for the repression of trbBp, Deletion analysis of TrbA showed that th e C-terminal domain (CTD), which has a high degree of sequence conservation with the CTD of KorA, is required for this cooperativity: with KorB, Weste rn blotting demonstrated that the apparently mutual enhancement of repressi on is not due simply to elevation of repressor level by the presence of the second protein, suggesting that the basis for cooperativity is interaction between KorB and TrbA bound at their respective operators.