Differential influence of the two Bacillus anthracis plasmids on regulation of virulence gene expression.

Fully virulent Bacillus anthracis bacilli are encapsulated and toxinogenic. These bacteria contain two plasmids, pXO1 and pXO2, carrying genes coding for toxins (pag, lef, and cya) and for capsule synthetic enzymes (capB, capC, capA, and dep), respectively. A transcriptional fusion between the capB regulatory region and the lacZ reporter gene was constructed to study the regulation of capsule synthesis. A single copy of this fusion was inserted into the cap region of pXO2. The influence of environmental factors on the capB-lacZ fusion expression was initially analyzed in a pXO1-negative background: bicarbonate but not temperature induced the transcription from the capB promoter. A strain carrying the recombinant pXO2 and (delta)pag pXO1 was constructed for transregulatory studies. The pXO1 plasmid strongly enhanced capsule formation without modifying the bicarbonate-dependent induction level. A (delta)cap pXO2 was transduced into a strain containing pXO1 harboring a pag-lacZ transcriptional fusion (19). pXO2 showed no influence on the toxin gene transcription.