A mutation in GerE that affects cotC promoter activation in Bacillus subtilis

One perturbation of the mother cell gene regulatory network suppresses the effects of another during sporulation of Bacillus subtilis

In the mother cell of sporulating Bacillus subtilis, a regulatory network functions to control gene expression. Four transcription factors act sequentially in the order sigma(E), SpoIIID, sigma(K), GerE. sigma(E) and sigma(K) 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 sigma(K) RNA polymerase inhibits early sporulation gene expression, resulting in reduced accumulation of sigma(E) and SpoIIID late during sporulation. Second, GerE represses sigK transcription, reducing sigma(K) accumulation about twofold. Third, SpoIIID represses cotC, which encodes a spore coat protein, delaying its transcription by sigma(K) 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 the sigK promoter region, are reported. Accumulation of pro-sigma(K) and sigma(K) was increased, but no spore defects were detected. Expression of sigma(K)-dependent reporter fusions was altered, increasing the expression of gerE-lacZ and cotC-lacZ and decreasing the expression of cotD-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 elevated sigK expression late during sporulation. This restored the expression of sigma(K)-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 express cotC about 2 h earlier than normal, did not cause a detectable spore defect.

Maintaining the transcription factor SpoIIID level late during sporulation causes spore defects in Bacillus subtilis

During sporulation of Bacillus subtilis, four regulatory proteins act in the order sigma(E), SpoIIID, sigma(K), and GerE to temporally control gene expression in the mother cell. sigma(E) and sigma(K) work sequentially with core RNA polymerase to transcribe different sets of genes. SpoIIID and GerE are small, sequence-specific DNA-binding proteins that activate or repress transcription of many genes. Previous studies showed that transcriptionally active sigma(K) RNA polymerase inhibits early mother cell gene expression, reducing accumulation of SpoIIID late in sporulation. Here, the effects of perturbing the mother cell gene regulatory network by maintaining the SpoIIID level late during sporulation are reported. Persistent expression was obtained by fusing spoIIID to the sigma(K)-controlled gerE promoter on a multicopy plasmid. Fewer heat- and lysozyme-resistant spores were produced by the strain with persistent spoIIID expression, but the number of spores resistant to organic solvents was unchanged, as was their germination ability. Transmission electron microscopy showed structural defects in the spore coat. Reporter fusions to sigma(K)-dependent promoters showed lower expression of gerE and cotC and higher expression of cotD. Altered expression of cot genes, which encode spore coat proteins, may account for the spore structural defects. These results suggest that one role of negative feedback by sigma(K) RNA polymerase on early mother cell gene expression is to lower the level of SpoIIID late during sporulation in order to allow normal expression of genes in the sigma(K) regulon.