Timing of the ribosomal gene replication in Tetrahymena pyriformis

Regulation of ribosome synthesis in Tetrahymena pyriformis. 2. Coordination of synthesis of ribosomal proteins and ribosomal RNA during nutritional shift-up

The addition of nutrients to long-time-starved cells of Tetrahymena pyriformis leads to a 50-60-fold increase in the rate of synthesis of ribosomal proteins (r-proteins). This is achieved by a 6-fold increase in the relative rate of r-protein synthesis and a 8-10-fold increase in the rate of total protein synthesis. Synthesis of r-proteins constitutes one third of total cellular protein synthesis 2-4 h after refeeding and the absolute rate of r-protein synthesis is approximately three-times greater than in exponentially growing cells. The synthesis of the individual r-proteins is coordinately regulated during a nutritional shift-up, and de novo synthesized r-proteins are stable. Addition of actinomycin D prevents the increase in the rate of r-protein synthesis. The rates of synthesis of rRNA and r-protein increase in concert, implying coordinate regulation. Furthermore, a comparison of the observed accumulation of r-proteins with the predicted accumulation based on the accumulation of rRNA suggests that rRNA and r-protein are synthesized in a stoichiometrically balanced way during the entire refeeding period.

Ribosomal proteins in growing and starved Tetrahymena pyriformis. Starvation-induced phosphorylation of ribosomal proteins

The complements of ribosomal proteins in growing and starved cells of Tetrahymena pyriformis strain GL were examined by two-dimensional gel electrophoresis. In growing cells, the 40-S ribosomal subunit contained 30 proteins, 4 of which migrated toward the anode at pH 8.6, while the 60-S ribosomal subunit contained 46 proteins, 9 of which migrated toward the anode at pH 8.6. When exponentially growing cells were transferred into a non-nutrient medium pronounced phosphorylation of a single 40-S ribosomal subunit protein, S6, was induced. The phosphorylation was very specific; more than 99.5% of the [32P]phosphate incorporated into ribosomal proteins was associated with S6. Phosphate was incorporated into S6 as O-phosphoserine and O-phosphothreonine. Two-dimensional gel electrophoresis indicated that the complement of proteins associated with the ribosomes isolated from starved cells differed from that of growing cells. Careful examination, however, suggested that except for the phosphorylation of certain ribosomal proteins in starved cells, the observed differences did not reflect starvation-induced changes in vivo, but most probably different levels of artifactual modifications (limited proteolysis) during the preparation of the ribosomes.

Tetrahymena ribosomal RNA gene chromatin is digested by micrococcal nuclease at sites which have the same regular spacing on the DNA as corresponding sites in the bulk nuclear chromatin

Synchronised cells of Tetrahymena pyriformis GL were labelled with 3H thymidine at a stage in the cell cycle when only the mitochondrial and extrachromosomal nucleolar ribosomal DNAs were replicating. In this way it was possible to prepare nuclei labelled selectively in the DNA of the ribosomal RNA genes. Since the ribosomal RNA cistrons of these cells are also very active in serving as a template for transcription, experiments were performed to test whether these genes are organised upon a nucleoprotein subunit structure of the kind that has been found in the total chromatin of a wide range of eukaryotic cell types. Tetrahymena macronuclei were prepared labelled uniformly in their DNA with 32P and labelled only in their nucleolar ribosomal DNA with 3H. Both the ribosomal genes and the bulk chromatin were then degraded in situ using micrococcal nuclease. The DNA fragments resulting from mild digestion were analysed on gels to reveal an identical DNA degradation pattern within both the ribosomal and bulk chromatins. It is concluded that the nucleoprotein structure of nucleolar rRNA cistrons posesses a periodic repeat along the DNA which is identical to that found in the substructure of unfractionated chromatin.