Cell-type specificity during development in Bacillus subtilis: the molecular and morphological requirements for sigma E activation.

Development in Bacillus subtilis involves the formation of two cell types with activation of the transcription factors sigma F in the forespore and sigma E in the mother cell. Activation of sigma E is due to the processing of the inactive precursor pro-sigma E, which requires the putative protease SpoIIGA and the presence of active sigma F. We have introduced missense mutations altering the promoter recognition properties of sigma F. These mutations abolish pro-sigma E processing, suggesting that sigma F is involved through its transcriptional activity and that the processing machinery responds to a signal generated by the product(s) of some unidentified gene(s) transcribed in the forespore. The role of the septum in transducing this signal was investigated. Induction of sigma F during exponential growth in cells producing SpoIIGA and pro-sigma E led to a high level of processing and sigma E activity. Moreover, pro-sigma E was efficiently processed in a mutant strain blocked prior to septation and synthesizing sigma F in active form at the onset of sporulation. Therefore, the sporulation septum is not required for induction of pro-sigma E processing and pro-sigma E can be processed in the same cell in which sigma F is active. These results suggest that some unknown mechanism must exist to prevent sigma E from becoming active in the forespore.