IncC of broad-host-range plasmid RK2 modulates KorB transcriptional repressor activity In vivo and operator binding in vitro.

The korAB operon of broad-host-range plasmid RK2 encodes five genes, two of which, incC and korB, belong to the parA and parB families, respectively, of genome partitioning functions. Both korB and a third gene, korA, are responsible for coordinate regulation of operons encoding replication, transfer, and stable inheritance functions. Overexpression of incC alone caused rapid displacement of RK2. Using two different reporter systems, we show that incC modulates the action of KorB. Using promoter fusions to the reporter gene xylE, we show that incC potentiates the repression of transcription by korB. This modulation of korB activity was only observed with incC1, which encodes the full-length IncC (364 amino acids [aa]), whereas no effect was observed with incC2, which encodes a polypeptide of 259 aa that lacks the N-terminal 105 aa. Using bacterial extracts with IncC1 and IncC2 or IncC1 purified through the use of a His6 tail and Ni-agarose chromatography, we showed that IncC1 potentiates the binding of KorB to DNA at representative KorB operators. The ability of IncC to stabilize KorB-DNA complexes suggests that these two proteins work together in the global regulation of many operons on the IncP-1 genomes, as well in plasmid partitioning.