Regulatory pathways governing the transition to stationary phase in <i>Bacillus subtilis</i>
Bacteria enter stationary phase when they exhaust the nutrients available to them, or when other adverse environmental changes occur. The transition to stationary phase requires dramatic changes in gene expression in which suites of genes are turned on that allow the cells to adapt to unfavorable circumstances. These changes in gene expression are governed by signal transduction pathways that sense the onset of adverse conditions and respond by activating (or inactivating) global regulatory proteins. One such global regulator is the alternate sigma factor σS, which governs the transition to stationary phase in Escherichia coli. In the spore forming bacterium Bacillus subtilis, the subject of this work, the transition to stationary phase is governed in large part by the master regulator of sporulation, Spo0A∼P, and the AbrB repressor.
The AbrB protein is a repressor of numerous genes that are switched on during the transition from exponential to stationary phase. The abrB gene is directly repressed by the master regulator for sporulation, Spo0A∼P. It has generally been assumed that derepression of genes under the negative control of AbrB is achieved by Spo0A∼P-mediated repression of abrB gene followed by degradation of the AbrB protein. Here I report that a decrease in AbrB levels is not the entire basis by which AbrB-controlled genes become derepressed. Rather, AbrB is inactivated by the product of a previously uncharacterized gene, abbA, whose transcription is turned on by Spo0A∼P. AbbA is an antirepressor that binds to AbbA and prevents it from binding to DNA. I further report that AbrB binds AbbA by interacting with the same amino acids with which it contacts DNA. Thus, it appears that AbbA occludes the DNA binding domain of AbrB, and thereby mediates derepression of genes under the negative control of AbrB.
Spo0A is activated is activated by phosphorylation via a multicomponent phosphorelay, by multiple histidine kinases. I present evidence that the activity of one of the kinases, KinD, depends on the lipoprotein Med, whose function until now has been mysterious. I show that the absence of Med impairs and that the over production of Med stimulates the transcription of genes involved in cannibalism (sdp and skf), as well as formation of biofilms, all of which are known to depend on Spo0A∼P. These effects of Med are specifically dependent on KinD. I also report that over production of Med bypasses the dominant-negative effect of a truncated KinD on sdp expression. I propose that Med directly or indirectly interacts with KinD in the cytoplasmic membrane, and that this interaction is required for KinD-dependent phosphorylation of Spo0A.