title: Different rates of synthesis and turnover of ribosomal RNA in rat brain and liver.
abstract: The kinetics of in vivo labeling of cellular free UMP and nucleolar, nucleoplasmic, and cytoplasmic rRNA with [14C]orotate in rat brain and liver were investigated. Evaluation of the experimental data shows: (a) The rate of nucleolar precursors of ribosomal RNA (pre-rRNA) synthesis and the deduced rate of ribosome formation in brain is about fivefold lower than in liver and corresponds to 220-260 ribosomes/min/nucleus. (b) The lower rate of in vivo pre-rRNA synthesis is correlated with a lower activity of RNA polymerase I in isolated brain nuclei. (c) The half-lives of nucleolar rRNA in brain and liver are 210 and 60 min, respectively, thus showing a slower rate of processing of pre-rRNA in brain nucleoli. (d) The nucleo-cytoplasmic transport of ribosomes in brain is also markedly slower than in liver and reflects the lower rates of synthesis and processing of pre-rRNA. (e) Cytoplasmic ribosomes in brain and liver turn over with half-lives of about 6 and 4 days, respectively. It is concluded that the markedly lower rate of ribosome biogenesis in brain is specified mainly at the level of transcription of rRNA genes.
mesh_terms: D000818:Animals; D001921:Brain / Q000378:metabolism*; D002466:Cell Nucleolus / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008297:Male; D009694:Nucleic Acid Precursors / Q000096:biosynthesis*; D009963:Orotic Acid / Q000378:metabolism; D012318:RNA Polymerase I / Q000378:metabolism; D012322:RNA Precursors; D012335:RNA, Ribosomal / Q000096:biosynthesis*; D051381:Rats; D011919:Rats, Inbred Strains; D012270:Ribosomes / Q000378:metabolism; D014542:Uridine Monophosphate / Q000378:metabolism
chemical_list: D009694:Nucleic Acid Precursors; D012322:RNA Precursors; D012335:RNA, Ribosomal; D009963:Orotic Acid; D014542:Uridine Monophosphate; D012318:RNA Polymerase I