Expression of spoIIIJ in the Prespore Is Sufficient for Activation of σ G and for Sporulation in Bacillus subtilis

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Abstract
During sporulation in Bacillus subtilis , the prespore-specific developmental program is initiated soon after asymmetric division of the sporangium by the compartment-specific activation of RNA polymerase sigma factor σ F . σ F directs transcription of spoIIIG , encoding the late forespore-specific regulator σ G . Following synthesis, σ G is initially kept in an inactive form, presumably because it is bound to the SpoIIAB anti-sigma factor. Activation of σ G occurs only after the complete engulfment of the prespore by the mother cell. Mutations in spoIIIJ arrest sporulation soon after conclusion of the engulfment process and prevent activation of σ G . Here we show that σ G accumulates but is mostly inactive in a spoIIIJ mutant. We also show that expression of the spoIIIGE155K allele, encoding a form of σ G that is not efficiently bound by SpoIIAB in vitro, restores σ G -directed gene expression to a spoIIIJ mutant. Expression of spoIIIJ occurs during vegetative growth. However, we show that expression of spoIIIJ in the prespore is sufficient for σ G activation and for sporulation. Mutations in the mother cell-specific spoIIIA locus are known to arrest sporulation just after completion of the engulfment process. Previous work has also shown that σ G accumulates in an inactive form in spoIIIA mutants and that the need for spoIIIA expression for σ G activation can be circumvented by the spoIIIGE155K allele. However, in contrast to the case for spoIIIJ , we show that expression of spoIIIA in the prespore does not support efficient sporulation. The results suggest that the activation of σ G at the end of the engulfment process involves the action of spoIIIA from the mother cell and of spoIIIJ from the prespore.