Fully virulent Bacillus anthracis bacilli are encapsulated and toxinogenic. 
These bacteria carry two plasmids, pXO1, and pXO2, encoding toxins and capsule 
synthetic-enzymes (capB, C, A, dep), respectively. The PXO1 plasmid strongly 
enhances capsule formation. This influence was studied by analysing the 
expression of a capB-lacZ fusion in various backgrounds. The beta-galactosidase 
activities were similar in a delta atxA strain and a pXO1 cured strain. 
Moreover, the capB-lacZ expression level could be restored, in a pXO1 cured 
strain, by addition of atxA in trans. Thus, we conclude that the pX01 influence 
on capsule synthesis is mediated by AtxA, the pXO1-encoded trans-activator of 
the toxin gene expression.