Characterization of Preribosomal Ribonucleoprotein Particles from Tetrahymena pyriformis

Abstract

Ribonucleoprotein particles present in extracts of nuclei prepared from Tetrahymena pyriformis labelled for 1, 2.5, 5 and 10 min with [3H]uridine during exponential growth were analysed by sedimentation through linear 10–30% sucrose gradients. After 1 min of labelling, the early ribosomal RNA precursor (36‐S) is found to be associated with slowly sedimenting particles which form a broad peak centred at approximately 50 S. Other kinds of particles sedimenting at 80 S, 66 S, 60 S and 44 S are observed when labelling is carried out for longer periods (2.5, 5 and 10 min). The 80‐S particle contains 29‐S and 18‐S RNA species together with traces of 36‐S RNA; the 60‐S and 44‐S particles contain 26‐S and 17‐S RNAs respectively. Similar results were obtained when [Me‐3H]methionine was used for labelling in place of [3H]uridine. Methylation of the RNA present in slowly sedimenting nuclear components (30–70‐S) is rapid, reaching a plateau at 5 min while that of the faster sedimenting (70–90‐S) components is still increasing after 10 min. Only three types of ribonucleoprotein particles (80‐S, 66‐S, and 44‐S) were observed when the cells were labelled after prolonged starvation. A scheme of ribosome biogenesis based on these results is presented. Copyright © 1979, Wiley Blackwell. All rights reserved