Loss of Compartmentalization of σE 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 sigma(F) in the smaller cell, the prespore. sigma(F) then directs a signal via the membrane protease SpoIIGA to activate sigma(E) in the larger mother cell by processing of pro-sigma(E). Here, we show that sigma(E) can be activated in the prespore with little effect on sporulation efficiency, implying that complete compartmentalization of sigma(E) activity is not essential for spore formation. sigma(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 sigma(E), but not of spoIIGB, which encodes pro-sigma(E). We infer that sigma(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 sigma(E) activation to it. In addition, we find that CsfB, previously shown to inhibit sigma(G), is independently inhibiting sigma(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 sigma(G) from becoming active before completion of engulfment and helps prevent sigma(E) from becoming active at all.