Cyclic changes in polyribosomes of synchronized Tetrahymena pyriformis

Preparative particle separation in density gradients

The purpose of this review is to summarize the present status of preparative zonal centrifugation and to indicate where the remaining problems, in the reviewer's opinion, may lie. The field is sufficiently new for interest in practical separartions to be uppermost. However, a number of theoretical problems relating to gradient capacity, zonal rotor configuration and particle—particle interactions remain to be fully explored.

Phosphorylation in vivo of Ribosomes in Tetrahymena pyriformis

Phosphorylation of ribosomal proteins in vivo was studied in exponentially growing and starved cells of the ciliated protozoan, Tetrahymena pyriformis. No phosphorylation of ribosomal proteins could be demonstrated in cells growing exponentially in complex nutrient media. However, when Tetrahymena cells were transferred into a non-nutrient medium, pronounced phosphorylation of a single ribosomal protein was observed. During two-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 has a molecular weight of 38000 as estimated by dodecylsulfate polyacrylamide gel electrophoresis. Thus, the phosphorylated ribosomal protein found in starved Tetrahymena is apparently homologous with the ribosomal protein which is 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 two subunits. This indicates that the phosphorylated ribosomal protein is located at the interface between the two subunits.