Efficient Regulation of σF, the First Sporulation-specific Sigma Factor in B. subtilis

Differential gene expression is established in the prespore and mother-cell compartments of Bacillus subtilis through the successive activation of a series of cell-type-specific sigma factors. Crucial to the success of this process is the control of the first prespore-specific sigma factor, σF. σF is regulated by the proteins SpoIIAB, SpoIIAA and SpoIIE. SpoIIAB forms an inhibitory complex with σF, which can be dissociated by interaction with SpoIIAA. During this interaction SpoIIAA is phosphorylated. SpoIIE is a membrane-bound phosphatase that dephosphorylates SpoIIAA, thereby re-activating it. It is not understood how σF is activated specifically in the prespore but not in the mother cell. Here, we use a recently developed fluorescence spectroscopy technique to follow in real time the formation of σF.SpoIIAB complexes and their dissociation by SpoIIAA. We show that complete activation of σF is induced by a tenfold increase in SpoIIE activity. This result demonstrates that relatively small changes in SpoIIE activity, which could arise from asymmetric septation, can achieve the all-or-nothing response in σF activity required by the cell. For long-term σF activation, we find that sustained SpoIIE activity is required to counteract the activity of SpoIIAB. Even though the continual phosphorylation and dephosphorylation of SpoIIAA by these two enzymes will expend some ATP, the formation of SpoIIAA.SpoIIAB.ADP complexes greatly diminishes the rate of the phosphorylation reaction, and thus minimizes the wastage of energy. These features provide a very efficient system for regulating σF.