Formation of Subribosomal Particles in the Macronuclei of Tetrahymena pyriformis

The formation of subribosomal ribonucleoprotein particles isolated from macronuclei has been studied in exponentially growing Tetrahymena pyriformis GL (amicronucleate). About 50% of the nuclear RNA can be liberated as ribonucleoprotein particles by the use of deoxycholate. The nuclear ribonucleoprotein particles isolated contain predominantly 25 S RNA. A fraction of these ribonucleoprotein particles sediment in a sucrose gradient with sedimentation values of about 60 S. Extraction of the nuclei by phenol-sodium dodecyl sulfate yields two rapidly labeled nuclear RNA species which sediment about 45 S and 32 S and which can be methylated. This suggests that high molecular weight precursors of ribosomal RNA are present in Tetrahymena. The kinetics of labeling of nuclear versus cytoplasmic 17 S and 25 S RNA indicate that 17 S RNA has a shorter half-life in the macronuclei than 25 S RNA. This is presumably the reason that the major RNA constituent of the nuclei is 25 S RNA. The overall rate of migration of rRNA label from the nucleus to the cytoplasm proceeds about 10 times faster in Tetrahymena than in HeLa cells and indicates a very fast formation of 40 S and 60 S subribosomal particles. This fast rate is presumably reflected in the fact that the macronuclei only contain about 2% of the cellular RNA.