A Membrane-Embedded Amino Acid Couples the SpoIIQ Channel Protein to Anti-Sigma Factor Transcriptional Repression during Bacillus subtilis Sporulation

ABSTRACT SpoIIQ is an essential component of a channel connecting the developing forespore to the adjacent mother cell during Bacillus subtilis sporulation. This channel is generally required for late gene expression in the forespore, including that directed by the late-acting sigma factor σ G . Here, we present evidence that SpoIIQ also participates in a previously unknown gene regulatory circuit that specifically represses expression of the gene encoding the anti-sigma factor CsfB, a potent inhibitor of σ G . The csfB gene is ordinarily transcribed in the forespore only by the early-acting sigma factor σ F . However, in a mutant lacking the highly conserved SpoIIQ transmembrane amino acid Tyr-28, csfB was also aberrantly transcribed later by σ G , the very target of CsfB inhibition. This regulation of csfB by SpoIIQ Tyr-28 is specific, given that the expression of other σ F -dependent genes was unaffected. Moreover, we identified a conserved element within the csfB promoter region that is both necessary and sufficient for SpoIIQ Tyr-28-mediated inhibition. These results indicate that SpoIIQ is a bifunctional protein that not only generally promotes σ G activity in the forespore as a channel component but also specifically maximizes σ G activity as part of a gene regulatory circuit that represses σ G -dependent expression of its own inhibitor, CsfB. Finally, we demonstrate that SpoIIQ Tyr-28 is required for the proper localization and stability of the SpoIIE phosphatase, raising the possibility that these two multifunctional proteins cooperate to fine-tune developmental gene expression in the forespore at late times. IMPORTANCE Cellular development is orchestrated by gene regulatory networks that activate or repress developmental genes at the right time and place. Late gene expression in the developing Bacillus subtilis spore is directed by the alternative sigma factor σ G . The activity of σ G requires a channel apparatus through which the adjacent mother cell provides substrates that generally support gene expression. Here we report that the channel protein SpoIIQ also specifically maximizes σ G activity as part of a previously unknown regulatory circuit that prevents σ G from activating transcription of the gene encoding its own inhibitor, the anti-sigma factor CsfB. The discovery of this regulatory circuit significantly expands our understanding of the gene regulatory network controlling late gene expression in the developing B. subtilis spore.