PlcR is a pleiotropic regulator of extracellular virulence factor gene expression in Bacillus thuringiensis

Members of the Bacillus cereus group (B. anthracis, B. cereus, B. mycoides and B. thuringiensis) are well-known pathogens of mammals (B. anthracis and B. cereus) and insects (B. thuringiensis). The specific diseases they cause depend on their capacity to produce specific virulence factors, such as the lethal toxin of B. anthracis and the Cry toxins of B. thuringiensis. However, these Bacillus spp. also produce a variety of proteins, such as phospholipases C, which are known to act as virulence factors in various pathogenic bacteria. Few genes encoding these virulence factors have been characterized in pathogenic Bacillus spp. and little is known about the regulation of their expression. We had previously reported that in B. thuringiensis expression of the phosphatidylinositol-specific phospholipase C gene is regulated by the transcriptional activator PlcR. Here we report the identification of several extracellular virulence factor genes by the virtue of their PlcR-regulated expression. These PlcR-regulated genes encode degradative enzymes, cell-surface proteins and enterotoxins. The PlcR-regulated genes are widely dispersed on the chromosome and therefore do not constitute a pathogenic island. Analysis of the promoter region of the PlcR-regulated genes revealed the presence of a highly conserved palindromic region (TATGNAN4TNCATA), which is presumably the specific recognition target for PlcR activation. We found that the plcR gene is also present in and probably restricted to all the members of the B. cereus group. However, although the polypeptide encoded by the B. cereus PlcR gene is functionally equivalent to the B. thuringiensis regulator, the polypeptide encoded by the B. anthracis gene is truncated and not active as a transcriptional activator. PlcR is the first example described of a pleiotropic regulator involved in the control of extracellular virulence factor expression in pathogenic Bacillus spp. These results have implications for the taxonomic relationships among members of the B. cereus group, the virulence properties of these bacteria and the safety of B. thuringiensis-based biopesticides.