A Threshold Mechanism Governing Activation of the Developmental Regulatory Protein σF in Bacillus subtilis*

The RNA polymerase sigma factor σF is a developmental regulatory protein that is activated in a cell-specific manner following the formation of the polar septum during the process of spore formation in the bacterium Bacillus subtilis. Activation of σF depends on the membrane-bound phosphatase SpoIIE, which localizes to the septum, and on the formation of the polar septum itself. SpoIIE is responsible for dephosphorylating and thereby activating the phosphoprotein SpoIIAA, which, in turn, triggers the release of σF from the anti-σF factor SpoIIAB. Paradoxically, however, the presence of unphosphorylated SpoIIAA is insufficient to cause σF activation as SpoIIAA reaches substantial levels in mutants blocked in polar septation. We now describe mutants of SpoIIE, SpoIIAA, and SpoIIAB that break the dependence of σF activation on polar division. Analysis of these mutants indicates that unphosphorylated SpoIIAA must reach a threshold concentration in order to trigger the release of σF from SpoIIAB. Evidence is presented that this threshold is created by the action of SpoIIAB, which can form an alternative, long lived complex with SpoIIAA. We propose that formation of the SpoIIAA-SpoIIAB complex serves as a sink that traps SpoIIAA in an inactive state and that only when unphosphorylated SpoIIAA is in excess to the sink does activation of σF take place.