FIG. 2.
Morphological changes during B. subtilis sporulation and the approximate time and location at which different σ factors become active. (A) σA and σH RNA polymerases transcribe genes whose products cause polar septation and axial filament formation. (B) The axial filament consists of two chromosomes extending the length of the cell with their replication origin-proximal regions attached at opposite ends of the cell. The polar septum forms around the axial filament, capturing one-third of one chromosome in the forespore. The remaining two-thirds of that chromosome is translocated into the forespore. (C) Upon completion of polar septation, σF becomes active in the forespore, and this leads to activation of σE in the mother cell. (D) Products of genes under σE control drive migration of the septal membranes around the forespore in the phagocytic-like process of engulfment. (E) Completion of engulfment pinches off the forespore as a free protoplast within the mother cell. Two membranes surround the forespore and separate its contents from the mother cell cytoplasm. σG becomes active in the forespore, leading to activation of σK in the mother cell. Primarily, genes under σE and σK control cause synthesis of a loosely cross-linked peptidoglycan termed cortex, between the two membranes surrounding the forespore, and synthesis of proteins that assemble on the surface of the forespore to produce the spore coat (F). Not shown are subsequent steps, which include spore maturation and release of the spore via lysis of the mother cell. Adapted from reference 67 with permission.