The two major virulence factors of Bacillus anthracis are the tripartite toxin 
and the polyglutamate capsule, which are encoded by genes on the large plasmids, 
pXO1 and pXO2, respectively. The genes atxA, located on pXO1, and acpA, located 
on pXO2, encode positive trans-acting proteins that are involved in 
bicarbonate-mediated regulation of toxin and capsule production, respectively. A 
derivative strain cured of pXO1 produced less capsular substance than the parent 
strain harbouring both pXO1 and pXO2, and electroporation of the strain cured of 
pXO1 with a plasmid containing the cloned atxA gene resulted in an increased 
level of capsule production. An acpA-null mutant was complemented by not only 
acpA but also the atxA gene. The cap region, which is essential for 
encapsulation, contains three genes capB, capC, and capA, arranged in that 
order. The atxA gene stimulated capsule synthesis from the cloned cap region. 
Transcriptional analysis of cap by RNA slot-blot hybridization and 
primer-extension analysis revealed that atxA activated expression of cap in 
trans at the transcriptional level. These results indicate that cross-talk 
occurs, in which the pXO1-located gene, atxA, activates transcription of the cap 
region genes located on pXO2. We identified two major apparent transcriptional 
start sites, designated P1 and P2, located at positions 731 bp and 625 bp, 
respectively, upstream of the translation-initiation codon of capB. 
Transcription initiated from P1 and P2 was activated by both atxA and acpA, and 
activation appeared to be stimulated by bicarbonate. Deletion analysis of the 
upstream region of the cap promoter revealed that activation by both atxA and 
acpA required a DNA segment of 70 bp extending upstream of the P1 site. These 
results suggest that cross-talk by atxA to the genes encoding capsule synthesis 
is caused by the interaction of the atxA gene product with a regulatory sequence 
upstream of cap.