The structure of ribosome-like particles from rat-liver mitochondria

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

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.

Characterization of mitochondrial and cytoplasmic ribosomes from Paramecium aurelia

The ribosomes extracted from the mitochondria of the ciliate, Paramecium aurelia, have been shown to sediment at 80S in sucrose gradients. The cytoplasmic ribosomes also sediment at 80S but can be distinguished from their mitochondrial counterparts by a number of criteria. Lowering of the Mg++ concentration, addition of EDTA, or high KCl concentrations results in the dissociation of the cytoplasmic ribosomes into 60S and 40S subunits, whereas the mitochondrial ribosomes dissociate into a single sedimentation class at 55S. Furthermore, the relative sensitivity of the two types of ribosome to dissociating conditions can be distinguished. Electron microscopy of negatively stained 80S particles from both sources has also shown that the two types can be differentiated. The cytoplasmic particles show dimensions of 270 X 220 A whereas the mitochondrial particles are larger (330 X 240 A). In addition, there are several distinctive morphological features. The incorporation of [14C]leucine into nascent polypeptides associated with both mitochondrial and cytoplasmic ribosomes has been shown: the incorporation into cytoplasmic 80S particles is resistant to erythromycin and chloramphenicol but sensitive to cycloheximide, whereas incorporation into the mitochondrial particles is sensitive to erythromycin and chloramphenicol but resistant to cycloheximide.

On the synthesis of viral ribonucleic acids and ribonucleoproteins in the submitochondrial system completely free of interfering cytoplasmic contaminations

The synthesis of virus-specific macromolecules was studied in the reconstituted system containing inner membrane-matrix fraction from rat liver mitochondria and infectious RNA of Venezuelian equine encephalomyelitis (VEE) virus. In a series of preliminary experiments it was shown that isolated submitochondrial fraction was completely free of interfering cytoplasmic contaminations and particularly, of cytoplasmic 80S ribosomes. VEE RNA when added to submitochondrial system caused significant stimulation of RNA and protein synthesis. These processes were resistant to actinomycin D which inhibited profoundly the synthesis of proper mitochondrial macromolecules. The stimulating effect of VEE RNA in experiments with submitochondrial system was about three times higher than that with intact mitochondria. The stimulation of 14C-amino acid incorporation increased as a function of incubation time; a certain lag-period being observed. The newly formed virus-specific RNA's and ribonucleoproteins were identified with the aid of sedimentation analysis. In particular, radioactive RNA's with sedimentation coefficients 40S and 26-18S were isolated from the incubated system. These RNA's are similar respectively to VEE genome RNA and double-stranded VEE replicative RNA. In double labelling experiments with 3H-uridine and 14C-amino acids it was shown that VEE RNA induced synthesis of ribonucleoproteins containing newly formed RNA and protein. These RNP possessed sedimentation coefficients 60-80S, 140S and 300S in sucrose gradient and buoyant densities 1.32 and 1.50 g/cm3 in cesium chloride gradients. These properties of ribonucleoproteins synthesized de novo in submitochondrial system are close to those of RNP intermediates of VEE virus reproduction in the infected cells. We concluded that viral RNA could program virus-specific synthesis in the submitochondrial system under conditions that eliminated the contribution of cytoplasmic ribosomes.

Electron microscope study of mitochondrial 60S and cytoplasmic 80S ribosomes from Locusta migratoria

Purified mitochondrial ribosomes (60S) have been isolated from locust flight muscle. Purification could be achieved after lysis of mitochondria in 0.055 M MgCl(2). Mitochondrial 60S and cytoplasmic 80S ribosomes were investigated by electron microscopy in tissue sections, in sections of pellets of isolated ribosomes, and by negative staining of ribosomal suspensions. In negatively stained preparations, mitochondrial ribosomes show dimensions of approximately 270 x 210 x 215 A; cytoplasmic ribosomes measure approximately 295 x 245 x 255 A. From these values a volume ratio of mitochondrial to cytoplasmic ribosomes of 1: 1.5 was estimated. Despite their different sedimentation constants, mitochondrial ribosomes after negative staining show a morphology similar to that of cytoplasmic ribosomes. Both types of particles show bipartite profiles which are interpreted as "frontal views" and "lateral views." In contrast to measurements on negatively stained particles, the diameter of mitochondrial ribosomes in tissue sections is approximately 130 A, while the diameter of cytoplasmic ribosomes is approximately 180-200 A. These data suggest a volume ratio of mitochondrial to cytoplasmic ribosomes of 1:3. Subunits of mitochondrial ribosomes (40S and 25S) were obtained by incubation under dissociating conditions before fixation in glutaraldehyde. After negative staining, mitochondrial large (40S) subunits show rounded profiles with a shallow groove on a flattened side of the profile. Mitochondrial small subunits (25S) display elongated, triangular profiles.