Cooperative Function of TraJ and ArcA in Regulating the F Plasmid Operon

The F plasmid operon encodes most of the proteins required for bacterial conjugation. TraJ and ArcA are known activators of the operon promoter P, which is subject to H-NS-mediated silencing. Donor ability and promoter activity assays indicated that P is inactivated by silencers and requires both TraJ and ArcA for activation to support efficient F conjugation. The observed low-level, ArcA-independent F conjugation is caused by expression from upstream alternative promoters. Electrophoretic mobility shift assays showed that TraJ alone weakly binds to P regulatory DNA; however, TraJ binding is significantly enhanced by ArcA binding to the same DNA, indicating cooperativity of the two proteins. Analysis of binding affinities between ArcA and various DNA fragments in the P regulatory region defined a 22-bp tandem repeat sequence (from -76 to -55 of P) sufficient for optimal ArcA binding, which is immediately upstream of the predicted TraJ-binding site (from -54 to -34). Deletion analysis of the P promoter in strains deficient in TraJ, ArcA, and/or H-NS determined that sequences upstream of -103 are required by silencers including H-NS for P silencing, whereas sequences downstream of -77 are targeted by TraJ and ArcA for activation. TraJ and ArcA appear not only to counteract P silencers but also to directly activate P in a cooperative manner. Our data reveal the cooperativity of TraJ and ArcA during P activation and provide insights into the regulatory circuit controlling F-family plasmid-mediated bacterial conjugation. Conjugation is a major mechanism for dissemination of antibiotic resistance and virulence among bacterial populations. The operon in the F family of conjugative plasmids encodes most of the proteins involved in bacterial conjugation. This work reveals that activation of operon transcription requires two proteins, TraJ and ArcA, to bind cooperatively to adjacent sites immediately upstream of the major promoter P The interaction of TraJ and ArcA with the operon not only relieves P from silencers but also directly activates it. These findings provide insights into the regulatory circuit of the F-family plasmid-mediated bacterial conjugation.