Multiple weak interactions between BvgA~P and ptx promoter DNA strongly activate transcription of pertussis toxin genes in Bordetella pertussis

Pertussis toxin is the preeminent virulence factor and major protective antigen produced by Bordetella pertussis, the human respiratory pathogen and etiologic agent of whooping cough. Genes for its synthesis and export are encoded by the 12 kb ptx-ptl operon, which is under the control of the pertussis promoter, Pptx. Expression of this operon, like that of all other known protein virulence factors, is regulated by the BvgAS two-component global regulatory system. Although Pptx has been studied for years, characterization of its promoter architecture vis-à-vis BvgA-binding has lagged behind that of other promoters, mainly due to its lower affinity for BvgA~P. Here we take advantage of a mutant BvgA protein (Δ127–129), which enhances ptx transcription in B. pertussis and also demonstrates enhanced binding affinity to Pptx. By using this mutant protein labeled with FeBABE, binding of six head-to-head dimers of BvgA~P was observed, with a spacing of 22 bp, revealing a binding geometry similar to that of other BvgA-activated promoters carrying at least one strong binding site. All of these six BvgA-binding sites lack sequence features associated with strong binding. A genetic analysis indicated the degree to which each contributes to Pptx activity. Thus the weak/medium binding affinity of Pptx revealed in this study explains its lower responsiveness to phosphorylated BvgA, relative to other promoters containing a high affinity binding site, such as that of the fha operon.