A PorX/PorY and σ Feedforward Regulatory Loop Controls Gene Expression Essential for Porphyromonas gingivalis Virulence.

The PorX/PorY two-component system in the periodontal pathogen Porphyromonas gingivalis controls the expression of the genes, encoding a type IX secretion system, and the gene, encoding sigma factor σ. Previous results implied that PorX/PorY and σ formed a regulatory cascade because the PorX/PorY-activated σ enhanced the genes, including , via binding to their promoters. We recently showed that PorX also binds to the promoters, thus suggesting that an alternative mechanism is required for the PorX/PorY- and σ-governed expression. Here, our assays show the PorX response regulator binds to the promoter at a sequence shared with the promoter and enhances its transcription, mediated by a reconstituted P. gingivalis RNA polymerase holoenzyme. Merely producing σ in fails to reverse the transcription in a mutant, which further argues against the action of the proposed regulatory cascade. An transcription assay using a reconstituted RNA polymerase-σ holoenzyme verifies the direct role of PorX in transcription, since transcription is enhanced by a pure PorX protein. Accordingly, we propose that the PorX/PorY system coordinates with σ to construct a coherent regulatory mechanism, known as the feedforward loop. Specifically, PorX will not only bind to the promoter to stimulate the expression of σ, but also bind to the promoter to facilitate the RNA polymerase-σ-dependent transcription. Importantly, mutations at the and genes attenuate bacterial virulence in a mouse model, demonstrating that this regulatory mechanism is essential for P. gingivalis pathogenesis. The anaerobic bacterium Porphyromonas gingivalis is not only the major etiologic agent for chronic periodontitis, but also prevalent in some common noncommunicable diseases such as cardiovascular disease, Alzheimer's disease, and rheumatoid arthritis. We present genetic, biochemical, and biological results to demonstrate that the PorX/PorY two-component system and sigma factor σ build a specific regulatory network to coordinately control transcription of the genes encoding the type IX secretion system, and perhaps also other virulence factors. Results in this study verify that the response regulator PorX stimulates the expression of the genes encoding both σ and the type IX secretion system by binding to their promoters. This study also provides evidence that σ, like the PorX/PorY system, contributes to P. gingivalis virulence in a mouse model.