Transcriptional analysis of the Bacillus anthracis capsule regulators

The poly-d-glutamic acid capsule of Bacillus anthracis is essential for virulence. Control of capsule synthesis occurs at the level of transcription and involves positive regulation of the capsule biosynthetic operon capBCAD by a CO2/bicarbonate signal and three plasmid-borne regulators: atxA, acpA, and acpB. Although the molecular mechanism for control of cap transcription is unknown, atxA affects cap expression via positive control of acpA and acpB, two genes with partial functional similarity. Transcriptional analyses of a genetically complete strain indicate that capB expression is several hundred-fold higher during growth in 5% CO2 compared to growth in air. atxA was expressed appreciably during growth in air and induced only 2.5-fold by CO2. In contrast, expression of acpA and acpB was induced up to 23-fold and 59-fold, respectively, by CO2. The 5'-end mapping of gene transcripts revealed atxA-regulated and atxA-independent apparent transcription start sites for capB, acpA, and acpB. Transcripts mapping to all atxA-regulated start sites were increased during growth in elevated CO2. The acpA gene has one atxA-regulated and one atxA-independent start site. acpB lies downstream of capBCAD. A single atxA-independent start site maps immediately upstream of acpB. atxA-mediated control of acpB appears to occur via transcriptional read-through from atxA-dependent start sites 5' of capB. One atxA-independent and two atxA-regulated start sites map upstream of capB. Transcription from the atxA-regulated start sites of capBCAD was reduced significantly in an acpA acpB double mutant but unaffected in mutants with deletion of only acpA or acpB, in agreement with the current model for epistatic relationships between the regulators.