SdpAB are required for post-translational modification of SdpC

Bacillus subtilis is a Gram-positive spore-forming soil bacterium. Under environmental stress conditions such as starvation, B. subtilis enters the pathway of sporulation. Earlier work demonstrated that B. subtilis can delay sporulation by undergoing cannibalism. Sporulating cells secrete toxins that kill nearby siblings, thus allowing cells to feed on the released contents. One of these toxins, SdpC, is encoded by the sdpABC operon. To uncover the requirements for SdpC toxic activity during cannibalism, all proteins in the sdpABC operon were analyzed. We report that mutations of SdpC which block signal peptidase cleavage also block toxin production. In addition, production and secretion of SdpC do not require SdpA and SdpB. Our results indicate that SdpC secretion is indispensable for induction of the immunity operon sdpRI . Furthermore, SdpC secreted from a Δ sdpAB strain does not fully induce sdpRI expression and has decreased toxicity to cells that are sensitive to wild type SdpC. Lastly, differences in SdpC mobility are observed in the presence of SdpA and SdpB. Thus, we propose that SdpA and SdpB may function by post-translationally modifying SdpC into the active form of the toxin.