Polysomal organization in growing and resting cultures and its relation to protein synthesis in Tetrahymena.

Cytosol from Tetrahymena cells growing at different rates was isolated and separated by centrifugation into polysomal and non-polysomal fractions. The RNAs of either fraction were separated chromatographically into poly(A)+ RNA and poly(A)-RNA. It was found that in resting cultures the total RNA per cell is only about half of that of rapidly growing cultures. All fractions of RNA were reduced proportionally. Thus, the percentage of polysomally bound total RNA (70% of cytosol RNA) and polysomally bound poly(A)+ RNA (72% of cytosol poly(A)+ RNA) is the same in growing and resting cultures. Differences, however, were found in the polysomal structure. Polysomes from resting cultures contained significantly fewer ribosomes. The amounts of RNA bound to polysomes were related to the rate of protein synthesis under different growth conditions. The decrease in cellular RNA corresponded well with the reduction in amino acid incorporation in resting cells. The rate of protein accumulation in resting cells, on the other hand, was considerably less, suggesting that polypeptides in resting cultures are less stable.