Increased phosphorylation of a ribosomal protein during aggregation of the slime mold Dictyostelium discoideum

A single ribosomal protein (M r 32 000) becomes phosphorylated during differentiation of Dictyostelium discoideum amoebae. This protein is tentatively identified as the 40 S ribosomal protein S6. Phosphorylation of S6 is monitored by incorporation of 32Pi and by two‐dimensional polyacrylamide gel electrophoresis. S6 is minimally phosphorylated in growing cells. Upon starvation, S6 is progressively phosphorylated, the degree of phosphorylation being maximal during the aggregation phase of the developmental cycle.