Blocking Chromosome Translocation during Sporulation of Bacillus subtilis Can Result in Prespore-Specific Activation of σG That Is Independent of σE and of Engulfment

摘要

Formation of spores by Bacillus subtilis is characterized by cell compartment-specific gene expression directed by four RNA polymerase σ factors, which are activated in the order σ^F-σ^E-σ^G-σ^K. Of these, σ^G becomes active in the prespore upon completion of engulfment of the prespore by the mother cell. Transcription of the gene encoding σ^G, spoIIIG, is directed in the prespore by RNA polymerase containing σ^F but also requires the activity of σ^E in the mother cell. When first formed, σ^G is not active. Its activation requires expression of additional σ^E-directed genes, including the genes required for completion of engulfment. Here we report conditions in which σ^G becomes active in the prespore in the absence of σ^E activity and of completion of engulfment. The conditions are (i) having an spoIIIE mutation, so that only the origin-proximal 30% of the chromosome is translocated into the prespore, and (ii) placing spoIIIG in an origin-proximal location on the chromosome. The main function of the σ^E-directed regulation appears to be to coordinate σ^G activation with the completion of engulfment, not to control the level of σ^G activity. It seems plausible that the role of σ^E in σ^G activation is to reverse some inhibitory signal (or signals) in the engulfed prespore, a signal that is not present in the spoIIIE mutant background. It is not clear what the direct activator of σ^G in the prespore is. Competition for core RNA polymerase between σ^F and σ^G is unlikely to be of major importance.