The Two-Component System Bacillus Respiratory Response A and B (BrrA−BrrB) Is a Virulence Factor Regulator in Bacillus anthracis^†

Bacillus anthracis, a bioterrorism threat as well as an agricultural concern, has complex mechanisms for regulation of its major virulence factors. Genome searches identified the putative two-component system that we designated Bacillus anthracis respiratory response (Brr)A−BrrB. A brrA deletion strain was constructed, and real-time reverse transcriptase polymerase chain reaction and Western blot analysis were used to assess the effect of BrrA−BrrB on levels of virulence factors, the regulator atxA, and growth characteristics. When brrA was deleted, the genes for anthrax toxins (lethal factor, protective antigen, and edema factor) where expressed 4−6-log₁₀-fold less than in the parent Sterne strain. The global regulator atxA was downregulated when compared to atxA in the Sterne strain. Thus, the BrrA−BrrB two-component system positively regulates B. anthracis toxin genes as well as the atxA regulator. Aerobic growth was not affected by the ΔbrrA mutation, but colonies showed differences in morphology, the mutant did not sporulate, and the strain lost the ability to synthesize cytochrome aa₃. Gel-shift mobility assays demonstrated that BrrA bound to the promoters of genes for both protective antigen and cytochrome aa₃, demonstrating that BrrA is a transcription factor. BrrA−BrrB has sequence similarity with the virulence regulator SrrA−SrrB in Staphylococcus aureus and the aerobic/anaerobic regulator, ResD−ResE, in B. subtilis, and appears to share regulatory mechanisms with ResD−ResE.