One Perturbation of the Mother Cell Gene Regulatory Network Suppresses the Effects of Another during Sporulation of Bacillus subtilis

In the mother cell of sporulatingBacillus subtilis , a regulatory network functions to control gene expression. Four transcription factors act sequentially in the order σE , SpoIIID, σK , GerE. σE and σK direct RNA polymerase to transcribe different regulons. SpoIIID and GerE are DNA-binding proteins that activate or repress transcription of many genes. Several negative regulatory loops add complexity to the network. First, transcriptionally active σK RNA polymerase inhibits early sporulation gene expression, resulting in reduced accumulation of σE and SpoIIID late during sporulation. Second, GerE repressessigK transcription, reducing σK accumulation about twofold. Third, SpoIIID repressescotC , which encodes a spore coat protein, delaying its transcription by σK RNA polymerase. Partially circumventing the first feedback loop, by engineering cells to maintain the SpoIIID level late during sporulation, causes spore defects. Here, the effects of circumventing the second feedback loop, by mutating the GerE binding sites in thesigK promoter region, are reported. Accumulation of pro-σK and σK was increased, but no spore defects were detected. Expression of σK -dependent reporter fusions was altered, increasing the expression ofgerE-lacZ andcotC-lacZ and decreasing the expression ofcotD-lacZ . Because these effects on gene expression were opposite those observed when the SpoIIID level was maintained late during sporulation, cells were engineered to both maintain the SpoIIID level and have elevatedsigK expression late during sporulation. This restored the expression of σK -dependent reporters to wild-type levels, and no spore defects were observed. Hence, circumventing the second feedback loop suppressed the effects of perturbing the first feedback loop. By feeding information back into the network, these two loops appear to optimize target gene expression and increase network robustness. Circumventing the third regulatory loop, by engineering cells to expresscotC about 2 h earlier than normal, did not cause a detectable spore defect.