question: What is the function of the AtxA pleiotropic regulator?
answer: AtxA is the gene encoding the trans-activator of anthrax toxin synthesis and is essential for virulence of B. anthracis. It is located on the resident 185-kb plasmid pXO1 and its activation is stimulated by bicarbonate. AtxA controls the expression of more than a hundred genes belonging to all genetic elements, the chromosome and both virulence plasmids, including those encoding the major virulence factors. AtxA can activate or repress gene expression. In atxA+ strains, toxin gene expression is increased 5- to 20-fold in cells grown in 5% CO2 relative to cells grown in air. Dual promoters control expression of AtxA.  Transcription of the atxA gene occurs from two independent promoters, P1 and P2, whose transcription start sites are separated by 650 bp.