Ribosome biosynthesis in Tetrahymena pyriformis. Regulation in response to nutritional changes.

Abstract

Ribosome contents of growing and 12 h-starved T. pyriformis (strain B) were compared. Starved cells contained 74% of the ribosomes found in growing cells, growing cells devoted 20% of their protein synthetic activity to ribosomal protein production, and > 3% of the protein synthesized in starved cells was ribosomal protein. Ribosome metabolism was studied in starved cells which had been refed. For the first 1.5 h after refeeding, there was no change in ribosome number per cell. Between 1.5 and 2 h, there was an abrupt increase in rate of ribosome accumulation but little change in rate of cell division. By 3.5 h the number of ribosomes per cell increased to that found in growing cells. At this time the culture began to grow exponentially at a normal rate. During the first 2 h after refeeding, cells devoted 30-40% of their protein synthetic activity to ribosomal protein production. The rate of ribosomal protein synthesis per cell increased at least 80-fold during the first 1-1.5 h after refeeding, reaching the level found in exponentially growing cells. This occurred before any detectable change in ribosome number per cell. The transit time for the incorporation of these newly synthesized proteins into ribosomes was from 1-2 h during early refeeding, whereas in exponentially growing cells it was < 30 min. The relationship between ribosomal protein synthesis and ribosome accumulation was discussed.