Loss of compartmentalization of σ^E activity need not prevent formation of spores by Bacillus subtilis

Compartmentalization of the activities of RNA polymerase sigma factors is a hallmark of formation of spores by Bacillus subtilis. It is initiated soon after the asymmetrically located sporulation division takes place with the activation of σ^F in the smaller cell, the prespore. σ^F then directs a signal via the membrane protease SpoIIGA to activate σ^E in the larger mother cell by processing of pro-σ^E. Here, we show that σ^E can be activated in the prespore with little effect on sporulation efficiency, implying that complete compartmentalization of σ^E activity is not essential for spore formation. σ^E activity in the prespore can be obtained by inducing transcription in the prespore of spoIIGA or of sigE*, which encodes a constitutively active form of σ^E, but not of spoIIGB, which encodes pro-σ^E. We infer that σ^E compartmentalization is partially attributed to a competition between the compartments for the activation signaling protein SpoIIR. Normally, SpoIIGA is predominantly located in the mother cell and as a consequence confines σ^E activation to it. In addition, we find that CsfB, previously shown to inhibit σ^G, is independently inhibiting σ^E activity in the prespore. CsfB thus appears to serve a gatekeeper function in blocking the action of two sigma factors in the prespore: it prevents σ^G from becoming active before completion of engulfment and helps prevent σ^E from becoming active at all.