Forespore‐specific disappearance of the sigma‐factor antagonist SpollAB: implications for its role in determination of cell fate in Bacillus subtilis

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Abstract
Endospore formation in Bacillus subtilis is a morphologically complex process in which the bacterium divides into two compartments (forespore and mother cell) that follow different developmental paths. Compartment‐specific transcription in the forespore is initiated by RNA polymerase containing σ;F, and results in the forespore‐specific production of σ;G, which directs most of the subsequent forespore‐specific transcription. The activity of σ;F is thought to be restricted to the forespore by the sigma factor antagonist SpollAB. We used antibodies against SpollAB to monitor its accumulation during sporulation. We found that SpollAB accumulates early after the initiation of sporulation, and that it was present in the mother‐cell compartment 2h after σ;F became active in the forespore. SpollAB disappeared preferentially from the forespore during development, and its disappearance from the forespore compartment correlated with the activation of σ;G in that compartment, raising the possibility that SpollAB may be involved regulating σ;G activity. We tested whether SpollAB could antagonize σ;G activity by replacing the σ;F‐dependent promoter that drives expression of spolllG, the structural gene for σ;G, with a σ;H‐dependent promoter. This resulted in a lytic phenotype that was supressed by the simultaneous expression of a plasmid‐borne copy of spollAB. This suggests that SpollAB can suppress this effect of σ;G expression. Moreover, these cells formed spores efficiently. Since σ;G synthesis in these cells was not restricted to the forespore by the σ;F‐dependent transcription of its structural gene that normally occurs in wild‐type cells, the forespore‐specific activity of σ;G required for Sporulation appears to have resulted from expression of SpollAB.