title: 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.
mesh_terms: D000818:Animals; D007700:Kinetics; D008715:Methionine / Q000378:metabolism; D008745:Methylation; D008970:Molecular Weight; D009698:Nucleoproteins / Q000096:biosynthesis*; D012335:RNA, Ribosomal / Q000096:biosynthesis*; D012261:Ribonucleoproteins / Q000096:biosynthesis*; D012269:Ribosomal Proteins / Q000096:biosynthesis*; D013769:Tetrahymena pyriformis / Q000378:metabolism*; D014529:Uridine / Q000378:metabolism
chemical_list: D009698:Nucleoproteins; D012335:RNA, Ribosomal; D012261:Ribonucleoproteins; D012269:Ribosomal Proteins; D008715:Methionine; D014529:Uridine