Analysis of cellular interactions involved in differential control of prestalk genes in Dictyostelium discoideum

The prestalk-specific genes ecmA and ecmB in Dictyostelium are both induced by cAMP and DIF-1. In an attempt to understand the control mechanism of the differential expression of the two genes in multicellular aggregates, we examined the requirement for additional cellular interaction using low-cell-density cultures. We show that the whole process of inducing expression of these genes depends on high cell density. However, cells which have become responsive to DIF-1 expressed both genes at a low cell density in the presence of DIF-1 when incubated in a medium previously conditioned by developing cells. 8-Br-cAMP, which is believed to penetrate the cell membrane and activate protein kinase A, induced ecmB, but not ecmA, in the absence of the conditioned medium. These results suggest that there may be a specific inducer of ecmB in the conditioned medium which acts via activation of protein kinase A. Previously, the two genes were shown to respond differently to cAMP in late development at high cell densities, where there were cellular interactions. However, cAMP given to low-density-plated cells inhibited the conditioned medium-dependent induction of both genes to the same extent, suggesting that cAMP itself does not directly show the different effects on the two genes.