THE PHOSPHODIESTERASE SECRETED BY PRESTALK CELLS IS NECESSARY FOR DICTYOSTELIUM MORPHOGENESIS

Dictyostelium discoideum secretes a cyclic nucleotide phosphodiesteras e to control cAMP levels during development. Three promoters control e xpression of the gene-one during vegetative growth, one during aggrega tion, and one which constrains phosphodiesterase synthesis to prestalk cells. In this report we show that the expression of phosphodiesteras e (PDE) in prestalk cells is necessary for morphogenesis. A gene that codes for a specific glycoprotein inhibitor of the phosphodiesterase ( K-d = 0.1 nM) was fused to the prestalk-specific promoter of the PDE g ene. Transformants carrying multiple copies of this construct secreted inhibitor in 100-fold excess after the aggregation process had occurr ed. The first effect seen was an elongated tip, followed by a block in slug formation and an inability to culminate. Stalk and spores cells are produced but morphogenesis is uncoupled from cellular differentiat ion. Overproduction of inhibitor during earlier stages delayed aggrega tion, but did not affect fruiting body formation. A phosphodiesterase mutant was transformed with a plasmid that expresses PDE only during a ggregation and not in prestalk cells. The defect in aggregation was re scued, but the defect in later development was not. The combined resul ts indicate that PDE expression in prestalk cells is critical to morph ogenesis. To ask whether the inhibitor gene under its normal regulatio n had a role in aggregation or later morphogenesis, it was destroyed b y homologous recombination. The loss of the gene did not prevent devel opment under the conditions used. (C) 1995 Academic Press, Inc.