Cooperative function of TraJ and ArcA in regulating the F plasmid tra operon.

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