Chapter 6 : Endospore-Forming Bacteria: an Overview

Stages of sporulation in B. subtilis. Successive stages in spore formation of B. subtilis are shown. Vegetative cells (photo 1) divide medially. After the final medial septation at the entry into stationary phase, the two chromosomes of the cell form an axial filament structure (photo 2). Septation occurs near one pole of the cell (photo 3), after which the mother cell cytoplasmic membrane begins to engulf the forespore (photo 4). After completion of engulfment (photo 5), the forespore is surrounded by two membranes derived from the forespore and mother cell and lies fully within the cytoplasm of the mother cell. Cortex, a peptidoglycan-like substance, is synthesized between the two forespore membranes (gray-white material in photo 6). As cortex synthesis nears completion, spore coat proteins begin to assemble around the forespore (photo 7). The inside layers of coat protein are less electron dense than are the outside layers (photo 8). The fully assembled and mature spore is eventually released by lysis of the mother cell. (A version of this figure appeared in the doctoral thesis of S. Jin [ 1995 ].)

Sporulation in M. polyspora. Nomarski differential interference contrast micrographs show various stages in the life cycle of M. polyspora. (a) A germinated spore emerging from the spore coat; (b) a cell undergoing asymmetric septation at both poles; (c) a cell with three forespore compartments; (d) a mother cell containing four mature spores. (This figure was supplied by E. Angert, Harvard University.)

The Spo0A phosphorelay. Three different histidine protein kinases autophosphorylate and then transfer their phosphate groups to Spo0F. Through the intermediary of a phosphotransferase, Spo0B, the phosphate is finally transferred to an aspartate residue on Spo0A. Spo0A-phosphate is active as a DNA-binding transcription factor, having both negative and positive effects on gene expression. For additional details, see Burbulys et al., 1991 ; Hoch, 1993 ; and LeDeaux et al., 1995 .

The sporulation sigma factor cascade. At the onset of stationary phase, activation of Spo0A by phosphorylation ( Fig. 3 ) leads to expression of the spoIIA, spoIIG, and spoIIE operons. An additional Spo0A∼P-dependent gene of unknown identity is required for asymmetric septation. σF, a product of the spoIIA operon, interacts with core RNA polymerase to direct transcription of the early class of forespore-specific genes. Activation of σF requires its release from an inhibitory complex with SpoIIAB, a process that depends on SpoIIAA, after the latter is dephosphorylated by SpoIIE. One of the early forespore-specific genes is spoIIIG, which codes for σG. When activated, a step that requires the mother cell-expressed spoIIIA operon, σG directs transcription of late forespore-specific genes, including those that encode internal (Ssp) proteins of the spore, is encoded in the spoIIG operon as an inactive precursor; activation by cleavage depends on an early forespore protein, SpoIIR. Upon activation, σE-containing RNA polymerase transcribes genes for early mother cell-specific proteins. Among these proteins is the precursor of σκ. Activation by cleavage of pro-σK depends on a late forespore protein (SpoIVB), as well as on other mother cell proteins. When activated, σκ recognizes promoters for late mother cell genes, including spore coat protein genes.