FORMATION OF SUBRIBOSOMAL PARTICLES IN 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. Copyright © 1969, Wiley Blackwell. All rights reserved