The Amount of Ribosomal RNA Genes in Tetrahymena pyriformis in Different Physiological States

DNA‐RNA hybridization studies have been performed to determine the amount of rRNA genes present in purified nuclear DNA of Tetrahymena pyriformis cultured under various conditions of growth. It was found that 0.32% of the nuclear DNA isolated from exponentially growing cells hybridizes with an equimolar mixture of 17‐S and 25‐S rRNA. Nuclear DNA isolated from starved cells and from cells in the stationary growth phase hybridizes 30–40% less with rRNA than the nuclear DNA of the exponentially growing cells. This variation of the amount of rRNA genes to the total amount of nuclear DNA suggests that the replication of the rRNA genes and the rest of the nuclear DNA are under separate controls. With the possible exception of a minor homology between 17‐S and 25‐S rRNA, the saturation and competition experiments indicate that there are separate genes for the two classes of rRNA.