Just-in-Time Control of Spo0A Synthesis in Bacillus subtilis by Multiple Regulatory Mechanisms
The response regulator Spo0A governs multiple developmental processes inBacillus subtilis, including most conspicuously sporulation. Spo0A is activated by phosphorylation via a multicomponent phosphorelay. Previous work has shown that the Spo0A protein is not rate limiting for sporulation. Rather, Spo0A is present at high levels in growing cells, rapidly rising to yet higher levels under sporulation-inducing conditions, suggesting that synthesis of the response regulator is subject to a just-in-time control mechanism. Transcription ofspo0Ais governed by a promoter switching mechanism, involving a vegetative, σA-recognized promoter, Pv, and a sporulation σH-recognized promoter, Ps, that is under phosphorylated Spo0A (Spo0A∼P) control. Thespo0Aregulatory region also contains four (including one identified in the present work) conserved elements that conform to the consensus binding site for Spo0A∼P binding sites. These are herein designated O1, O2, O3, and O4in reverse order of their proximity to the coding sequence. Here we report that O1is responsible for repressing Pvduring the transition to stationary phase, that O2is responsible for repressing Psduring growth, that O3is responsible for activating Psat the start of sporulation, and that O4is dispensable for promoter switching. We also report that Spo0A synthesis is subject to a posttranscriptional control mechanism such that translation of mRNAs originating from Pvis impeded due to RNA secondary structure whereas mRNAs originating from Psare fully competent for protein synthesis. We propose that the opposing actions of O2and O3and the enhanced translatability of mRNAs originating from Pscreate a highly sensitive, self-reinforcing switch that is responsible for producing a burst of Spo0A synthesis at the start of sporulation.