The Pseudomonas aeruginosa cyclic AMP (cAMP)-Vfr system (CVS) is a global regulator of virulence gene expression. Regulatory targets include type IV pili, secreted proteases, and the type III secretion system (T3SS). The mechanism by which CVS regulates T3SS gene expression remains undefined. Single-cell expression studies previously found that only a portion of the cells within a population express the T3SS under inducing conditions, a property known as bistability. We now report that bistability is altered in a vfr mutant, wherein a substantially smaller fraction of the cells express the T3SS relative to the parental strain. Since bistability usually involves positive-feedback loops, we tested the hypothesis that virulence factor regulator (Vfr) regulates the expression of exsA. ExsA is the central regulator of T3SS gene expression and autoregulates its own expression. Although exsA is the last gene of the exsCEBA polycistronic mRNA, we demonstrate that Vfr directly activates exsA transcription from a second promoter (PexsA) located immediately upstream of exsA. PexsA promoter activity is entirely Vfr dependent. Direct binding of Vfr to a PexsA promoter probe was demonstrated by electrophoretic mobility shift assays, and DNase I footprinting revealed an area of protection that coincides with a putative Vfr consensus-binding site. Mutagenesis of that site disrupted Vfr binding and PexsA promoter activity. We conclude that Vfr contributes to T3SS gene expression through activation of the PexsA promoter, which is internal to the previously characterized exsCEBA operon. IMPORTANCE: Vfr is a cAMP-dependent DNA-binding protein that functions as a global regulator of virulence gene expression in Pseudomonas aeruginosa. Regulation by Vfr allows for the coordinate production of related virulence functions, such as type IV pili and type III secretion, required for adherence to and intoxication of host cells, respectively. Although the molecular mechanism of Vfr regulation has been defined for many target genes, a direct link between Vfr and T3SS gene expression had not been established. In the present study, we report that Vfr directly controls exsA transcription, the master regulator of T3SS gene expression, from a newly identified promoter located immediately upstream of exsA.
Bibliographical noteFunding Information:
This study was supported by the National Institutes of Health (R01-AI055042 to T.L.Y. and R01-AI069116 to M.C.W.). A.E.M. was supported by NIH training grant T32 AI07511. K.H.S. was supported by NIH training grants T32 GM082729 and T32 AI07511. The data presented here were obtained at the Flow Cytometry Facility, which is a Carver College of Medicine/Holden Comprehensive Cancer Center core research facility at the University of Iowa. The facility is funded through user fees and the financial support of the Carver College of Medicine, Holden Comprehensive Cancer Center, and Iowa City Veteran's Administration Medical Center. This work, including the efforts of Timothy L. Yahr, was funded by HHS | National Institutes of Health (NIH) (R01-AI055042). This work, including the efforts of Matthew C. Wolfgang, was funded by HHS | National Institutes of Health (NIH) (R01-AI069116).