Compartmentalized distribution of the proteins controlling the prespore‐specific transcription factor σ F of Bacillus subtilis

Background : Differential gene expression during sporulation in the prespore and mother cell of Bacillus subtilis is dependent on the correct timing and localization of the activity of specific transcription (σ) factors. The first σ factor activated is σ F , which directs gene expression specifically in the prespore compartment. Release of σ F activity is tightly controlled through a series of complex interactions involving an anti‐σ factor, SpoIIAB, an anti‐anti‐σ factor SpoIIAA and a phosphoprotein phosphatase SpoIIE. In vitro studies have shown that SpoIIAB binds to σ F , preventing transcription of the σ F regulon, and that it can also phosphorylate SpoIIAA, thereby inactivating it. However, non‐phosphorylated SpoIIAA can displace σ F from SpoIIAB. The SpoIIE phosphatase provides a means of reactivating SpoIIAA‐P. Results : We have directly determined the cellular distibutions of σ F , SpoIIAB, SpoIIAA‐P and SpoIIAA during sporulation, using recently developed immunofluorescence methods. While σ F activity is restricted to the prespore, the protein is present in both compartments. As development proceeds the σ F signal disappears. The anti‐σ factor SpoIIAB is also distributed throughout both cells and rapidly disappears from both cellular compartments soon after σ F becomes active. Disappearance of SpoIIAB seems to be closely associated with the activation of the second prespore‐specific σ factor σ F . The distribution of phosphorylated SpoIIAA closely mimics that of SpoIIAB, being non‐compartmentalized and disappearing soon after σ F activation occurs. Significantly, the active, non‐phosphorylated form of the anti‐anti‐σ factor, SpoIIAA, accumulates in the prespore just before σ F becomes active. Conclusion : These results support the hypothesis that the accumulation of SpoIIAA within the prespore is the single most important requirement for activation of σ F .