Autogenous regulation of the Bacillus anthracis pag operon

Protective antigen (PA) is an important component of the edema and lethal toxins produced by Bacillus anthracis. PA is essential for binding the toxins to the target cell receptor and for facilitating translocation of the enzymatic toxin components, edema factor and lethal factor, across the target cell membrane. The structural gene for PA, pagA (previously known as pag), is located on the 182-kb virulence plasmid pXO1 at a locus distinct from the edema factor and lethal factor genes. Here we show that a 300-bp gene located downstream of pagA is cotranscribed with pagA and represses expression of the operon. We have designated this gene pagR (for protective antigen repressor). Two pagA mRNA transcripts were detected in cells producing PA: a short, 2.7-kb transcript corresponding to the pagA gene, and a longer, 4.2-kb transcript representing a bicistronic message derived from pagA and pagR. The 3' end of the short transcript mapped adjacent to an inverted repeat sequence, suggesting that the sequence can act as a transcription terminator. Attenuation of termination at this site results in transcription of pagR. A pagR mutant exhibited increased steady-state levels of pagA mRNA, indicating that pagR negatively controls expression of the operon. Autogenous control of the operon may involve atxA, a trans-acting positive regulator of pagA. The steady-state level of atxA mRNA was also increased in the pagR mutant. The mutant phenotype was complemented by addition of pagR in trans on a multicopy plasmid.