The low molecular weight autoinhibitor of sexual development in Dictyostelium discoideum inhibits cell fusion and zygote differentiation

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Abstract
A previous study has shown that, during the sexual cycle of Dictyostelium discoideum, zygote giant cell formation is regulated by an autoinhibitor. Experiments reported here show that the autoinhibitor inhibits two events of zygote development: cell fusion and subsequent giant cell differentiation. The autoinhibitor is heat stable and has a molecular weight around 500. Medium containing the autoinhibitor can be diluted 500-fold without loss of activity. Preliminary experiments show that, although levels of ammonia double during the 8-h period of autoinhibitor production, added ammonia does not mimic the inhibiting effect. cAMP at 1 mM inhibits both binucleate formation and differentiation, but the concentration of cyclic AMP in 28-h cultures is only 13.4 pmol, a level which does not affect zygote development. Thus, it is established that neither of these critical regulators of other developmental processes in D. discoideum is the autoinhibitor.