Fig. 8. Model for induction of sporulation by cytokinin and SDF-2. Cytokinins activate the histidine kinase DhkB and the adenylyl cyclase AcrA in prespore cells, while SDF-2 binds to the histidine kinase DhkA, such that the cAMP phosphodiesterase RegA is no longer activated. As a result, cAMP accumulates and activates PKA, which triggers encapsulation. PKA activity also leads to the release of the SDF-2 precursor AcbA which is processed by the prestalk specific protease TagC. Glutamate blocks the effect of PKA on AcbA. GABA competes with glutamate for the receptor GrlE. The kinases PI3K and PKBR 1 are essential components of the GABA signal transduction pathway leading to release of AcbA [adapted, with permission, from Anjard and Loomis (Anjard and Loomis, 2006)].