EXTRACELLULAR CAMP DEPLETION TRIGGERS STALK GENE-EXPRESSION IN DICTYOSTELIUM - DISPARITIES IN DEVELOPMENTAL TIMING AND DOSE-DEPENDENCY INDICATE THAT PRESPORE INDUCTION AND STALK REPRESSION BY CAMP ARE MEDIATEDBY SEPARATE SIGNALING PATHWAYS

During Dictyostelium development, amoebae differentiate into spores an d stalk cells. Earlier studies showed that extracellular cAMP is essen tial for induction of prespore differentiation and that cAMP represses stalk gene expression in vitro. We show that the repressive pathway i s operative in vivo, because activation of the stalk-specific promoter region of the ecmB gene is strongly enhanced by overexpression of a p hosphodiesterase that depletes extracellular cAMP. To test whether a s ingle cAMP transduction pathway controls the choice between prespore o r stalk cell differentiation, we compared the timing and dose dependen cy of the effects of cAMP on both responses. Cells acquire competence for cAMP repression of ecmB promoter activity 4 hr later than for pres pore gene induction. Half-maximal prespore induction requires 30 mu M stable cAMP analog Sp-cAMPs, while ecmB induction is half-maximally re pressed by 200 nM Sp-cAMPs, which is equivalent to about 3 to 13 nM cA MP. At concentrations exceeding 10 mu M, Sp-cAMPs stimulates ecmB expr ession from the intact promoter, but not from the stalk-specific subre gion. These data suggest that distinct signaling pathways operating at different developmental stages control induction of prespore genes on one hand and repression of stalk genes on the other. Both stalk gene repression and prespore gene induction by Sp-cAMPs are antagonized by millimolar adenosine concentrations. However, an adenosine analog that is resistant to extracellular metabolism is active at 10 mu M. Since adenosine inhibits cAMP binding to cAMP receptors, it may facilitate s talk gene expression by reducing the perceived cAMP concentration. (C) 1996 Academic Press, Inc.