PSF AND CMF, AUTOCRINE FACTORS THAT REGULATE GENE-EXPRESSION DURING GROWTH AND EARLY DEVELOPMENT OF DICTYOSTELIUM

Throughout growth and development, Dictyostelium cells secrete autocri ne factors that accumulate in proportion to cell density. At sufficien t concentration, these factors cause changes in gene expression. Veget ative Dictyostelium cells continuously secrete prestarvation factor (P SF). The bacteria upon which the cells feed inhibit their response to PSF, allowing the cells to monitor their own density in relation to th at of their food supply. At high PSF/bacteria ratios, which occur duri ng late exponential growth, PSF induces the expression of several gene s whose products are needed for cell aggregation. When the food supply has been depleted, PSF production declines, and a second density-sens ing pathway is activated. Starving cells secrete conditioned medium fa ctor (CMF), a glycoprotein of Mr 80 kDa that is essential for the deve lopment of differentiated cell types. Antisense mutagenesis has shown that cells lacking CMF cannot aggregate, and preliminary data suggest that CMF regulates cAMP signal transduction. Calculations indicate tha t a mechanism of simultaneously secreting and recognizing a signal mol ecule, as used by Dictyostelium to monitor cell density, could also be used to determine the total number of cells in a tissue.