REGULATION OF RIBOSOME SYNTHESIS IN TETRAHYMENA-PYRIFORMIS .1. COORDINATION OF SYNTHESIS OF RIBOSOMAL-PROTEINS AND RIBOSOMAL-RNA DURING NUTRITIONAL SHIFT-DOWN

When exponentially growing cells of T. pyriformis are transferred to a non-nutrient medium the loss of whole cell RNA, 90% of which is ribosomal RNA, exhibits biphasic kinetics; whole cell protein is lost at a constant rate. The ratio RNA/protein declines during the 1st 5 h of starvation and then remains constant during the subsequent period of starvation. The synthesis of the majority of the ribosomal proteins is coordinately regulated during a nutritional shift-down. Exponentially growing cells devote 17% of their capacity for protein synthesis to the production of ribosomal proteins. Upon starvation this proportion is rapidly reduced 3.5-fold. In long-time-starved cells the absolute rate of ribosomal protein synthesis is only .apprx. 4.5% of that of exponentially growing cells. The synthesis of ribosomal RNA and ribosomal proteins appears tightly coupled during the transition from growth to starvation. In log-time-starved cells the syntheses of ribosomal RNA and ribosomal proteins are stoichiometrically balanced with no significant degradation of de novo synthesized ribosomal proteins.