Crystal Structure of Bacillus cereus HlyIIR, a Transcriptional Regulator of the Gene for Pore-forming Toxin Hemolysin II

Production of Bacillus cereus and Bacillus anthracis toxins is controlled by a number of transcriptional regulators. Here we report the crystal structure of B. cereus HlyIIR, a regulator of the gene encoding the pore-forming toxin hemolysin II. We show that HlyIIR forms a tight dimer with a fold and overall architecture similar to the TetR family of repressors. A remarkable feature of the structure is a large internal cavity with a volume of 550 Å3 suggesting that the activity of HlyIIR is modulated by binding of a ligand, which triggers the toxin production. Virtual ligand library screening shows that this pocket can accommodate compounds with molecular masses of up to 400–500 Da. Based on structural data and previous biochemical evidence, we propose a model for HlyIIR interaction with the DNA.