Characterization of a morphological checkpoint coupling cell-specific transcription to septation in Bacillus subtilis

Early in the process of spore formation in Bacillus subtilis, asymmetric ce ll division produces a large mother cell and a much smaller prespore. Diffe rentiation of the prespore is initiated by activation of an RNA polymerase sigma factor, sigma(F), specifically in that cell. sigma(F) is controlled b y a regulatory cascade involving an anti-sigma factor, SpoIIAB, an anti-ant i-sigma factor, SpoIIAA, and a membrane-bound phosphatase, SpoIIE, which co nverts the inactive, phosphorylated form of SpoIIAA back to the active form . SpoIIE is required for proper asymmetric division and much of the protein is sequestered into the prespore during septation. Importantly, activation of sigma(F) is dependent on formation of the asymmetric septum, We have no w characterized this morphological checkpoint in detail, using strains affe cted in cell division and/or spoIIE function. Surprisingly, we found that s ignificant dephosphorylation of SpoIIAA occurred even in the absence of sep tation. This shows that the SpoIIE phosphatase is at least partially active independent of the morphological event and also that cells can tolerate si gnificant levels of unphosphorylated SpoIIAA without activating sigma(F). W e also describe a spoIIE mutant in which the checkpoint is bypassed, probab ly by an increase in the dephosphorylation of SpoIIAA. Taken together, the results support the idea that sequestration of SpoIIE protein into the pres pore plays an important role in the control of sigma(F) activation and in c oupling this activation to septation.