Pools and Turnover Rates of Nuclear Ribosomal RNA in Tetrahymena pyriformis

The labeling kinetics of rRNA in exponentially growing Tetrahymena pyriformis were analyzed quantitatively in order to determine pool‐sizes and turnover rates of nuclear 17 and 25 S rRNA. The mathematical analysis of the system indicates that, during the steady state of growth, there is an inverse proportionality between pool‐size and turnover rate of nuclear rRNA. Thus, from the labeling kinetics of rRNA and the growth rate constant (4.6 × 10 −3 min −1 ), the pool‐size of macronuclear 17 S rRNA was estimated at 0.5% of that of cytoplasmic 17 S rRNA corresponding to a turnover rate constant of 0.92 min −1 . The analogues figures for 25 S rRNA were estimated at 1.6% and 0.29 min −1 . Since the RNA content of macronuclei is about 2% of the cellular RNA content, it is concluded that during growth, most nuclear RNA is found in precursors of ribosomes and that macronuclei contain few, if any, finished ribosomes. Apart from its use on rRNA, the method described here may be generally useful in determining pool‐sizes and turnover rates of other precursors, such as free ribosomal proteins.