Characterization of a small PlcR-regulated gene co-expressed with cereolysin O

Background: In the human pathogen Bacillus cereus, the expression of most extracellular virulence factors is controlled by the transcriptional activator PIcR. Among these virulence factors, cereolysin O (Clo) is an haemolysin belonging to the cholesterol-dependant cytolysins, a protein family extensively studied in Gram-positive bacteria. Results: In the genomes of bacteria belonging to the B. cereus group, including Bacillus anthracis and Bacillus thuringiensis, a small gene encoding a 26-amino acid peptide was present in multicopy. One copy was always found upstream from the gene encoding Clo. In B. cereus ATCC 14579, the small gene and the clo gene are co- transcribed. Transcriptional fusions showed that the three paralogues identified in this strain were expressed in a PIcR-dependent manner. We propose to name these peptides Spp for small PIcR-regulated peptides. We show that a synthetic peptide corresponding to the deduced product of the spp genes displayed antibacterial activity. Conclusion: The co-expression of spp, a small PIcR-regulated multicopy gene with clo suggests a yet unidentified relationship between Spp and the cholesterol-dependent cytolysin in bacteria belonging to the B. cereus group.