Characterization of a Dictyostelium factor that acts synergistically with DIF to induce terminal stalk cell...

The differentiation of Dictyostelium prestalk cells is induced by the chlorinated hexaphenone DIF-1 and their maturation into stalk cells at culmination occurs by activation of the cAMP-dependent protein kinase (PKA). Medium harvested from developing Dictyostelium cells will act synergistically with DIF-1 to induce prestalk cell differentiation in a low-density monolayer assay (Yamada and Okamoto, 1994). Using HPLC, we have partially purified from such conditioned medium an activity we term STIF (stalk-inducing factor). It is hydrophilic and of low molecular weight. There are multiple classes of prestalk cells, which are defined by their patterns of expression of the ecmA and ecmB genes and that can be further subcategorized because they utilize different elements within the promoters of the two genes. We show that, in a monolayer assay, STIF acts synergistically with DIF-1 to induce ecmB gene expression via promoter elements that are normally activated strongly only in cells that have entered the stalk tube and which are therefore committed to differentiate into stalk cells. The combination of STIF and DIF-1 also induces morphological maturation of prestalk cells into stalk cells but does not efficiently induce expression of ecmA: a gene that is selectively expressed in cells within the anterior, prestalk region of the slug. Inactivation of PKA, by cell type-specific expression of a dominant inhibitor, represses the action of STIF. These data suggest that STIF is an extracellular signal that acts to induce the terminal differentiation of stalk cells.