Intercellular signal transduction within the mother cell compartment during Bacillus subtilis sporulation

Intercellular signaling contributes to the spatiotemporal regulation of gene expression during sporulation in Bacillus subtilis. The mother cell transcription factor σE is initially produced as an inactive precursor protein pro-σE and activated by the processing enzyme SpoIIGA in response to the forespore-produced putative signaling molecule SpoIIR. However, the mechanism underlying the SpoIIR-mediated signal transduction remains poorly understood. In this study, we showed that the spoIIR-positive, spoIIGA-deleted strain was able to induce SpoIIGA-dependent pro-σE processing in co-cultured spoIIR-deleted, spoIIGA-positive strains. This signaling was dependent on SpoIIR expression and did not involve DNA transfer. Extracellular materials including secreted proteins and membrane vesicles were unlikely to be involved in this signaling pathway. Interestingly, cessation of co-incubation shaking enhanced the signaling, while the addition of membrane-solubilizing detergent abolished it. In addition, SpoIIR signaling did not necessitate release from the forespore membrane or extracellular translocation. A SpoIIR variant lacking the putative signal peptide-like hydrophobic domain produced solely in the mother cell compartment was still able to activate pro-σE. Overall, the study findings suggested that the forespore-produced SpoIIR is neither secreted nor externally translocated. Instead, SpoIIR appeared to be transferred into the mother cell compartment and interacts with the SpoIIGA cytoplasmic domain to trigger pro-σE processing.