Characterization of cDNA clones specific for sequences developmentally regulated during Dictyostelium discoideum spore germination.

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Abstract
Spore germination in the slime mold Dictyostelium discoideum was used as a model to study the developmental regulation of protein and mRNA synthesis. Changes in the synthesis of these macromolecules occur during the transition from dormant spore to amoebae. The study of the mechanisms which regulate the quantity and quality of protein synthesis can best be accomplished with cloned genes. cDNA clones which hybridized primarily with mRNAs from only spores or germinating spores and not with growing amoebae were collected. Three such clones, denoted pLK109, pLK229, and pRK270, were isolated and had inserts of approximately 500, 1,200, and 690 base pairs, respectively. Southern blot hybridization experiments suggested that each of the genes is present in multiple copies in the D. discoideum genome. RNA blot hybridizations were performed to determine the sizes of the respective mRNAs and their developmental regulation. The mRNA that hybridized to pLK109 DNA was present predominantly in spores and at 1 h after germination but was absent in growing amoebae. Its concentration dramatically dropped at 3 h. The mRNA present in spores is apparently larger (approximately 0.5 kilobase) than in the later stages of germination (0.4 kilobase), indicating processing of the RNA during germination. The mRNA that hybridized to pLK229 DNA was approximately 1.0 kilobase and was present in very low amounts during growth. Its concentration rose until 1 h after spore germination and decreased thereafter. pRK270-specific RNA was approximately 2.7 kilobases and was found predominantly at 1 h after germination. It was present in lower concentrations at 2 and 3 h after germination and was absent in spores and amoebae. In vitro translation of mRNA selected from 1-h polyadenylated RNA which was hybridized to pLK109 or pLK229 DNA gave proteins of molecular weights consistent with the sizes of the mRNAs as determined by the RNA blot analysis.