ADENYLYL-CYCLASE-G, AN OSMOSENSOR CONTROLLING GERMINATION OF DICTYOSTELIUM SPORES

Dictyostelium cells express a G-protein-coupled adenylyl cyclase, ACA, during aggregation and an atypical adenylyl cyclase, ACG, in mature s pores, The ACG gene was disrupted by homologous recombination. acg(-) cells developed into normal fruiting bodies with viable spores, but sp ore germination was no longer inhibited by high osmolarity, a fairly u niversal constraint for spore and seed germination, ACG activity, meas ured in aca(-)/ACG cells, was strongly stimulated by high osmolarity w ith optimal stimulation occurring at 200 milliosmolar. RdeC mutants, w hich display unrestrained protein kinase A (PKA) activity and a cell l ine, which overexpresses PKA under a prespore specific promoter, germi nate very poorly, both at high and low osmolarity, These data indicate that ACG is an osmosensor controlling spore germination through activ ation of protein kinase A.