Two distinct classes of prestalk-enriched mRNA sequences inDictyostelium discoideum

We have isolated cDNA clones derived from three mRNA sequences which are inducible by DIF, the putative stalk-specific morphogen of Dictyostelium. The three mRNA sequences are selectively expressed in cells on the stalk cell pathway of differentiation and we have compared them with previously characterized prestalk-enriched mRNA sequences. We find these latter sequences are expressed without a dependence on DIF, are much less highly enriched in prestalk over prespore cells and are expressed earlier during development than the DIF-inducible mRNA sequences. We propose two distinct mechanisms whereby a mRNA may become enriched in prestalk cells. An apparently small number of genes, represented by those we have isolated, is inducible by DIF and accumulates only in prestalk cells. We suggest that a second class of prestalk-enriched mRNA sequences are induced by cAMP to accumulate in all cells during aggregation and then become enriched in prestalk cells by selective loss from prespore cells.