Two distinct signaling pathways mediate DIF induction of prestalk gene expression in Dictyostelium

During Dictyostelium development, the differentiation inducing factor (DIF) triggers expression of the prestalk gene ecmB and induces stalk cell differentiation, a form of programmed cell death. The effects of DIF are mediated by a sustained increase in cytosolic Ca²⁺ levels. The Ca²⁺ ATPase inhibitor BHQ causes a similar rise in Ca²⁺ levels and also induces prestalk gene expression. We show here that Ca²⁺ is a specific intermediate for prestalk gene induction, since BHQ represses transcription of the cAMP- inducible aggregative gene PDE, the postaggregative gene CP2, and the prespore gene D19. The prestalk gene ecmA is also induced by DIF, but induction appears to occur in two steps, which occur within 1 h and after 2 h, respectively. The slow step shows the same kinetics as ecmB induction and similar to ecmB induction, this step is BHQ inducible and requires an initial round of protein synthesis. The fast step does not require protein synthesis and cannot be induced by BHQ. This indicates that in addition to the slow Ca²⁺-mediated pathway, there is probably a second fast Ca²⁺-independent signal transduction pathway for DIF.