Genes selectively expressed in proliferating Dictyostelium amoebae

Few eukaryotic genes are expressed only during cell growth and division. We found that the slime mold Dictyostelium discoideum is unusual in that it expresses many genes only during proliferation. Thirty-two percent (304/950) of the sequences in a cDNA library made from vegetative mRNA were homologous to RNAs that are present at high levels during growth but at low or undetectable levels during differentiation when no cell growth occurs. In vitro translation assays confirmed that one-third of the vegetative cell mRNAs decreased in steady-state levels during differentiation. These vegetative cell-specific transcripts identified a diverse coordinately regulated class of genes: (i) 9 of the 10 cDNAs tested hybridized to unique small transcripts ranging from 400 to 620 bases long; (ii) the sequences showed various degrees of homology to related species; (iii) transcript levels synchronously fell by a factor of greater than 20 during development and synchronously increased during germination. This class of genes may play important roles in normal cell proliferation.