Phosphorylation in vivo of Ribosomes in Tetrahymena pyriformis

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Abstract
Phosphorylation of ribosomal protein in vivo was studied in exponentially growing and starved cells of the ciliated protozoan, T. pyriformis. No phosphorylation of ribosomal proteins could be demonstrated in cells growing exponentially in complex nutrient media. When Tetrahymena cells were transferred into a non-nutrient medium, pronounced phosphorylation of a single ribosomal protein was observed. During 2-dimensional polyacrylamide gel electrophoresis the phosphorylated ribosomal protein migrated in a manner virtually identical to that of the phosphorylated ribosomal protein S6 of rat liver. The phosphorylated ribosomal protein had a MW of 38,000 as estimated by dodecylsulfate polyacrylamide gel electrophoresis. The phosphorylated ribosomal protein of starved Tetrahymena was apparently homologous with the ribosomal protein which was predominantly phosphorylated in higher eukaryotes. When phosphorylated ribosomes were dissociated by treatment with high concentration of KCl, the phosphorylated protein was found only on the small subunit. If dissociation was achieved by dialysis against a buffer low in MgCl2, the phosphorylated protein was distributed almost equally between the 2 subunits. The phosphorylated ribosomal protein was probably located at the interface between the 2 subunits.