Figure 7. Models for C. perfringens toxin gene regulation by the Agr-like QS system and VirS/VirR TCRS. (A) Regulation of toxin production in type C and D strains. The Agr-like QS system generates an autoinducing peptide (AIP) that interacts with one or more TCRS, possibly including VirS/VirR and/or for ETX regulation in type D strain CN3718, another still unidentified TCRS (sensor = membrane sensor; RR = response regulator). This signaling results in direct or indirect increased production of CPA and PFO in both type C and D strains, CPB in type C strain CN3685 and ETX in type D strain CN3718 (but not in type B strains). (B) Regulation of sporulation and CPE production in CPE-positive type A strains. The Agr-like QS system generates AIP, which (possibly via a TCRS) increases Spo0A production and, under sporulation-inducing conditions, alternative sigma factor expression levels to promote sporulation and CPE production.