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

Abstract

ABSTRACT 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.