Septation, dephosphorylation, and the activation of sigma(F) during sporulation in Bacillus subtilis

Cell-specific activation of transcription factor sigma(F) during sporulatio n in Bacillus subtilis requires the formation of the polar septum and the a ctivity of a serine phosphatase (SpoIIE) located in the septum. The SpoIIE phosphatase indirectly activates sigma(F) by dephosphorylating a protein (S poIIAA-P) in the pathway that controls the activity of the transcription fa ctor. By use of a SpoIIE-GFP fusion protein in time-course and time-lapse e xperiments and by direct visualization of septa in living cells, we show th at SpoIIE is present in the predivisional sporangium, where it often locali zes near both cell poles in structures known as E-rings. We also present ev idence consistent with the view that SpoIIE is present in both progeny cell s after polar division. These findings are incompatible with a model for th e control of sigma(F) activity in which the phosphatase is simply sequester ed to one cell. Instead, we conclude that the function of SpoIIE is subject to regulation, and we present evidence that this occurs in two stages. The first stage, which involves the phosphatase function of SpoIIE, depends on the cell division protein FtsZ and could correspond to the FtsZ-dependent assembly of SpoIIE into E-rings. The second stage occurs after the dephosph orylation of SpoIIAA-P and is dependent on the later-acting, cell-division protein DivIC. Evidence based on the use of modified and mutant forms of th e phosphatase protein indicates that SpoIIE blocks the capacity of unphosph orylated SpoIIAA to activate sigma(F) until formation of the polar septum i s completed.