Engulfment‐regulated proteolysis of SpoIIQ: evidence that dual checkpoints control σ K activity

Summary During Bacillus subtilis sporulation, the engulfment checkpoint is thought to directly regulate late forespore transcription but to indirectly regulate late mother cell transcription, via the σ G ‐produced protease SpoIVB. We here demonstrate that SpoIIQ is subject to σ G ‐independent, but engulfment‐dependent, proteolysis that depends on SpoIVB. Thus, SpoIVB produced before engulfment supports some SpoIVB‐dependent events, suggesting that its activity or access to substrates must be regulated by engulfment. Furthermore, a mutation ( bofA ) that allows σ K to be active without σ G does not allow σ K activity in engulfment mutants, although the pro‐σ K processing enzyme (SpoIVFB) is localized to the septum in engulfment mutants, suggesting that engulfment comprises a second checkpoint for σ K Finally, we find that SpoIIQ and another protein required for σ G activity (SpoIIIAH), which directly interact and assemble helical structures around the forespore, recruit the σ K ‐processing enzyme SpoIVFB to the forespore and these structures. We suggest that these foci serve a synapse‐like role, allowing engulfment to simultaneously control both σ G and σ K , and integrating multiple checkpoints and signalling pathways.