Molecular-Weight Determinations and Stoichiometric Measurements of 40-S and 60-S Ribosomal Proteins of the Cellular Slime Mold Dictyostelium discoideum

The stoichiometry of ribosomal proteins of the cellular slime mold Dictyostelium discoideum was investigated. The average number of copies per mole ribosomal subunit of each of 34 proteins from 40-S subunits and 48 proteins from 60-S subunits was measured on the basis of the molecular weights and the mass ratio of proteins estimated by combining two disc electrophoretic systems with urea-polyacrylamide and dodecylsulfate-polyacrylamide gels. According to the stoichio-metric classification of Escherichia coli ribosomal proteins suggested by Weber[1], D. discoideum ribosomal proteins were divided into at least four groups; fractional (F), unit (U), fractional repeated (FR) and repeated (R) proteins with the average number of copies of 0.10–0.69, 0.70 to 1.30, 1.31–1.70 and more than 1.71 per mole subunit, respectively. The data indicated that 40-S and 60-S proteins from isolated ribosomal subunit populations contain many F-, R- and FR-proteins besides U-protein. We concluded that the isolated ribosomal subunits of D. discoideum are structurally heterogeneous.