Figure 1.

Diagram of spore formation and gene regulation network for sporulation in B. subtilis. (A) Diagram of spore formation. Shown are different stages of sporulation in B. subtilis from stage 0 to stage VII. Stage 0: vegetative cell; stage I: genome replication; stage II: asymmetric division (stage II is shown in two different substages, one with formation of asymmetric septum (IIa) and the other with the completion of asymmetric division (IIb)); stage III: forespore engulfment; stage IV: spore cortex formation; stage V: spore coat formation; stage VI: mother cell lysis; stage VII: phase-bright free spore. (B) Signal transduction and phosphorelay for activation of sporulation in B. subtilis. The three membrane-associated sporulation kinases (KinB, KinC, and KinD) and the cytoplasmic histidine kinase KinA act together in activating sporulation by sensing a diverse set of environmental and cellular signals. KinD has an extracellular CACHE domain involved in direct sensing of plant root-released l-malic acid [16]. KinE is proposed to act as a phosphatase rather than a kinase under normal sporulation conditions [17]. Spo0F (0F), Spo0B (0B), and Spo0A (0A) constitute the phosphorelay. High levels of phosphorylated Spo0A (0A~P) directly and indirectly activate hundreds of genes involved in sporulation, some of which are through mother cell or forespore-specific sigma factors that function in a cascade. The phosphatase Spo0E negatively regulates Spo0A~P and the Rap family phosphatases negatively regulate Spo0F~P. (C) Increased Spo0A~P levels lead to activation of the first sporulation sigma factor F (σ^F) in the forespore (Fs), followed by sequential activation of several other alternative sigma factors (σ^E, σ^G, and σ^K) in either the mother cell (Mc) or the forespore compartment [12].