Two regulatory genes, acpA and atxA, have been reported to control expression of 
the Bacillus anthracis capsule biosynthesis operon capBCAD. The atxA gene is 
located on the virulence plasmid pXO1, while pXO2 carries acpA and the cap 
genes. acpA has been viewed as the major regulator of the cap operon because it 
is essential for capsule gene expression in a pXO1(-) pXO2(+) strain. atxA is 
essential for toxin gene transcription but has also been implicated in control 
of the cap genes. The molecular functions of the regulatory proteins are 
unknown. We examined cap gene expression in a genetically complete pXO1(+) 
pXO2(+) strain. Our results indicate that another pXO2 gene, acpB (previously 
called pXO2-53; accession no. NC002146.1:49418-50866), has a role in cap 
expression. The predicted amino acid sequence of AcpB is 62% similar to that of 
AcpA and 50% similar to that of AtxA. Assessment of cap gene transcription 
revealed that cap expression was not affected in a pXO1(+) pXO2(+) acpB-null 
mutant and was slightly reduced in an isogenic acpA mutant. However, cap gene 
expression was abolished in an acpA acpB double mutant. Microscopic examination 
of capsule synthesis by the mutants corroborated these findings. acpA and acpB 
expression is controlled by atxA; capsule synthesis and transcription of acpA 
and acpB were markedly reduced in an atxA mutant. The data suggest that, in a 
strain containing both virulence plasmids, atxA is the major regulator of 
capsule synthesis and controls capBCAD expression indirectly, via positive 
regulation of acpA and acpB.