Two-stage regulation of an anti-sigma factor determines developmental fate during bacterial endospore formation

During endospore formation in Bacillus subtilis an asymmetric division produces two cells, forespore and mother cell, which follow different developmental paths. Commitment to the forespore-specific developmental path is controlled in part by the activation of the forespore-specific RNA polymerase sigma factor, sigma(F). Activity of sigma(F) is inhibited in the mother cell by the anti-sigma factor SpollAB. In the forespore, of directs transcription of the structural gene for sigma(G). However, sigma(G) does not become active until after engulfment of the forespore is complete. This sigma(G) activity is dependent upon the products of the spolllA operon. We showed that sigma(G) is present but mostly inactive in a spolllA mutant. We also demonstrated that the antisigma factor SpollAB can bind to sigma(G) in vitro. Moreover, a mutant form of sigma(G) that binds SpollAB inefficiently in vitro was shown to function independently of SpolllA during sporulation. These and previously reported results support a model in which SpollAB functions as an inhibitor of sigma(G) activity during sporulation. Therefore, we propose that the anti-sigma factor SpollAB antagonizes both sigma(F) and sigma(G) activities, and that this antagonism is relieved in the forespore in two stages. In the first stage, which follows septation, a SpollAA-dependent mechanism partially relieves SpollAB inhibition of of activity in the forespore. In the second stage, which follows forespore engulfment, a SpolllA-dependent process inactivates SpollAB in the forespore, resulting in the activation of sigma(G).