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

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American Society for Microbiology
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PMC
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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 σF-σE-σG-σK. Of these, σG becomes active in the prespore upon completion of engulfment of the prespore by the mother cell. Transcription of the gene encoding σG, spoIIIG, is directed in the prespore by RNA polymerase containing σF but also requires the activity of σE in the mother cell. When first formed, σG is not active. Its activation requires expression of additional σE-directed genes, including the genes required for completion of engulfment. Here we report conditions in which σG becomes active in the prespore in the absence of σE 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 σE-directed regulation appears to be to coordinate σG activation with the completion of engulfment, not to control the level of σG activity. It seems plausible that the role of σE in σG 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 σG in the prespore is. Competition for core RNA polymerase between σF and σG is unlikely to be of major importance.

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