Expression of genes for Bacillus anthracis toxin and capsule virulence factors 
are dependent upon the AtxA transcription factor. The mechanism by which AtxA 
regulates the transcription of its target genes is unknown. Here we report that 
bioinformatic analyses suggested the presence in AtxA of two PTS 
(phosphenolpyruvate : sugar phosphotransferase system) regulation domains (PRD) 
generally regulated by phosphorylation/dephosphorylation at conserved histidine 
residues. By means of amino acid substitutions that mimic the phosphorylated (H 
to D) or the unphosphorylated (H to A) state of the protein, we showed that 
phosphorylation of H199 of PRD1 is likely to be necessary for AtxA activation 
while phosphorylation of H379 in PRD2 is inhibitory to toxin gene transcription. 
In vivo labelling experiments with radioactive phosphate allowed us to propose 
that H199 and H379 are AtxA residues subject to regulated phosphorylation. In 
support to these notions, we also show that deletion of ptsHI, encoding the HPr 
intermediate and the EI enzymes of PTS, or growth in the presence of glucose 
affect positively and negatively, respectively, the activity of AtxA. Our 
results link virulence factor production in B. anthracis to carbohydrate 
metabolism and, for the first time, provide a mechanistic explanation for AtxA 
transcriptional activity.