Ribosomal protein synthesis during spore germination and vegetative growth in Dictyostelium discoideum

We have investigated the regulation of the synthesis of each ribosomal protein during spore germination and in vegetatively growing cells of Dictyostelium discoideum. Germinating spores and exponentially growing amoebae were labeled with [35S]methionine and the individual ribosomal proteins were resolved by electrophoresis on two-dimensional polyacrylamide gels. Functional mRNA levels for ribosomal proteins were analyzed by translating the slime mold RNA in a wheat germ cell-free system and separating the translation products by two-dimensional polyacrylamide gel electrophoresis. All slime mold ribosomal proteins were synthesized at every stage of spore germination and during vegetative growth; and the amount of synthesis of these proteins was highest at 1 to 2 h of germination. A comparison between the synthesis of ribosomal proteins in germinating spores with those in vegetatively growing cells suggests that although the majority of them seem to be similarly regulated, the regulation of the synthesis of some of the proteins may be different in both conditions. Although dormant spores lacked functional mRNAs for 16 ribosomal proteins, these mRNAs were detected as early as 1 h of germination. The mRNAs corresponding to all ribosomal proteins were present throughout germination, and their levels were highest at 2 h of germination. These results are consistent with the conclusion that the synthesis of the majority of ribosomal proteins during spore germination is regulated transcriptionally. However, vegetative amoebae lack three proteins (A, E, and L in our nomenclature), yet contain mRNA for these proteins that can be translated in vitro. Therefore, the synthesis of these proteins is apparently regulated translationally.