2 RAS GENES IN DICTYOSTELIUM-MINUTUM SHOW HIGH SEQUENCE HOMOLOGY, BUTDIFFERENT DEVELOPMENTAL REGULATION FROM DICTYOSTELIUM-DISCOIDEUM RASDAND RASG GENES

The social amoeba Dictyostelium discoideum expresses five ras genes at different stages of development. One of them, DdrasD is expressed dur ing postaggregative development and transcription is induced by extrac ellular cAMP. A homologue of DdrasD, the DdrasG gene, is expressed exc lusively during vegetative growth. We cloned two aas homologues Dmras1 and Dmras2 from the primitive species D. minutum, which show high hom ology to DdrasD and DdrasG and less homology to the other Ddras genes. In contrast to the DdrasD and DdrasG genes, both the Dmras1 and Dmras 2 genes are expressed during the entire course of development. The exp ression levels are low during growth, increase at the onset of starvat ion and do not decrease until fruiting bodies have formed. Expression of neither Dmras1 or Dmras2 is regulated by cAMP. So even though the h igh degree of homology between the ras genes of different species sugg ests conservation of function, this function is apparently not associa ted with a specific developmental stage.