title: Mitochondrial and cytoplasmic ribosomes from mammalian tissues. Further characterization of ribosomal subunits and validity of buoyant-density methods for determination of the chemical composition and partial specific volume of ribonucleoprotein particles.
abstract: 1. At 0-4 degrees C mitochondrial ribosomes (55S) dissociate into 39S and 29S subunits after exposure to 300mm-K(+) in the presence of 3.0mm-Mg(2+). When these subunits are placed in a medium containing a lower concentration of K(+) ions (25mm), approx. 75% of the subparticles recombine giving 55S monomers. 2. After negative staining the large subunits (20.3nm width) usually show a roundish profile, whereas the small subunits (12nm width) show an elongated, often bipartite, profile. The dimensions of the 55S ribosomes are 25.5nmx20.0nmx21.0nm, indicating a volume ratio of mitochondrial to cytosol ribosomes of 1:1.5. 3. The 39S and 29S subunits obtained in high-salt media at 0-4 degrees C have a buoyant density of 1.45g/cm(3); from the rRNA content calculated from buoyant density and from the rRNA molecular weights it is confirmed that the two subparticles have weights of 2.0x10(6) daltons and 1.20x10(6) daltons; the weights of the two subunits of cytosol ribosomes are 2.67x10(6) and 1.30x10(6) daltons. 4. The validity of the isodensity-equilibrium-centrifugation methods used to calculate the chemical composition of ribosomes was reinvestigated; it is confirmed that (a) reaction of ribosomal subunits with 6.0% (v/v) formaldehyde at 0 degrees C is sufficient to fix the particles, so that they remain essentially stable after exposure to dodecyl sulphate or centrifugation in CsCl, and (b) the partial specific volume of ribosomal subunits is a simple additive function of the partial specific volumes of RNA and protein. The RNA content is linearly related to buoyant density by the equation RNA (% by wt.)=349.5-(471.2x1/rho(CsCl)), where 1/rho(CsCl)=[unk](RNP) (partial specific volume of ribonucleoprotein). 5. The nucleotide compositions of the two subunit rRNA species of mitochondrial ribosomes from rodents (42% and 43% G+C) are distinctly different from those of cytoplasmic ribosomes.
mesh_terms: D000818:Animals; D002499:Centrifugation, Density Gradient; D002586:Cesium; D055598:Chemical Phenomena; D002621:Chemistry; D003593:Cytoplasm / Q000032:analysis*; D005557:Formaldehyde; D006454:Hemoglobins / Q000096:biosynthesis; D051379:Mice; D008854:Microscopy, Electron; D008930:Mitochondria, Liver / Q000032:analysis* / Q000648:ultrastructure; D009698:Nucleoproteins / Q000032:analysis*; D009711:Nucleotides / Q000032:analysis; D012335:RNA, Ribosomal / Q000032:analysis; D011817:Rabbits; D051381:Rats; D012261:Ribonucleoproteins / Q000032:analysis*; D012270:Ribosomes / Q000032:analysis* / Q000648:ultrastructure; D012492:Salts
chemical_list: D006454:Hemoglobins; D009698:Nucleoproteins; D009711:Nucleotides; D012335:RNA, Ribosomal; D012261:Ribonucleoproteins; D012492:Salts; D005557:Formaldehyde; D002586:Cesium