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Transcription of theBacillus anthracis structural genes for the anthrax toxin proteins and biosynthetic operon for capsule is positively regulated by AtxA, a transcription regulator with unique properties. Consistent with the role ofatxA in virulence factor expression, aB. anthracis atxA -null mutant is avirulent in a murine model for anthrax. In culture, multiple signals impactatxA transcript levels, and the timing and steady-state level ofatxA expression are critical for optimal toxin and capsule synthesis. Despite the apparent complex control ofatxA transcription, only onetrans -acting protein, the transition state regulator AbrB, has been demonstrated to interact directly with theatxA promoter. Here we employ 5′ and 3′ deletion analysis and site-directed mutagenesis of theatxA control region to demonstrate thatatxA transcription from the major start site P1 is dependent upon a consensus sequence for the housekeeping sigma factor SigA and an A+T-rich upstream element for RNA polymerase. We also show that an additionaltrans -acting protein(s) binds specifically toatxA promoter sequences located between −13 and +36 relative to P1 and negatively impacts transcription. Deletion of this region increases promoter activity up to 15-fold. Site-directed mutagenesis of a 9-bp palindromic sequence within the region prevents binding of thetrans -acting protein(s), increasing promoter activity 7-fold and resulting in a corresponding increase in AtxA and anthrax toxin production. Notably, anatxA promoter mutant that produced elevated levels of AtxA and toxin proteins during culture was unaffected for virulence in a murine model for anthrax.

cis -Acting Elements That Control Expression of the Master Virulence Regulatory Gene atxA in Bacillus anthracis