Mitochondrial nucleic acids and their relation to the biogenesis of mitochondria

Morphological and histochemical evidence of mitoribosomes in Manduca sexta

The mitochondria found in the neurons of the frontal ganglion of Manduca sexta contained numerous mitoribosomes. The mitochondria of the glial and perineural cells did not contain mitoribosomes. The mitoribosomes were digested in RNase whereas phospholipase C digested the cellular membranes but had no effect on the mitoribosomes.

Copper content and exchange in mammalian hearts

Determination of the copper (Cu) content of rat, rabbit, and mouse hearts proved to be a sensitive microassay for mitochondrial content and membrane area of mitochondrial cristae per unit myocardial cell volume. The direction of variations in Cu content correlated well with variations in membrane area of mitochondrial cristae determined by morphometry of electron micrographs during normal postnatal growth, ventricular hypertrophy after aortic constriction, and thyroxin-stimulated myocardial cell growth. Right and left ventricular and interventricular septal Cu contents per unit dry weight were the same, did not vary during postnatal growth, and significantly exceeded atrial Cu content. Among different mammalian species, ventricular Cu content per unit dry weight tended to vary inversely with adult animal size and thus with adult resting heart rate. The existence of mechanisms for transport of 67 Cu across the myocardial cell plasma membrane and for incorporation of 67 Cu into mitochondrial cristae from an intracellular precursor pool was demonstrated by injecting 67 Cu-labeled blood serum into rats.

Nuclear gene dosage effects on mitochondrial mass and DNA

In order to assess the effect of nuclear gene dosage on the regulation of mitochondria we have studied serial sections of a set of isogenic haploid and diploid cells of Saccharomyces cerevisiae, growing exponentially in the absence of catabolite repression, and determined the amount of mitochondrial DNA per cell. Mitochondria accounted for 14% of the cytoplasmic and 12% of the total cellular volume in all cells examined regardless of their ploidy or their apparent stage in the cell cycle. The mean number of mitochondria per cell was 22 in the diploid and 10 in the haploids. The volume distribution appeared unimodal and identical in haploids and diploids. The mitochondrial DNA accounted for 12.6 +/- 1.2% and 13.5 +/- 1.3% of the total cellular DNA in the diploid and haploid populations, respectively. These values correspond to 3.6 x 10(-15) g, 2.2 x 10(9) daltons, or 44 genomes (50 x 10(6) daltons each) per haploid and twice that per diploid cell. On this basis, the average mitochondrion in these cells contains four mitochondrial genomes in both the haploid and the diploid.

Transfer RNA methylating activity of yeast mitochondria

Mitochondria isolated from Saccharomyces cerevisiae and purified in Urografin or sucrose gradient contain tRNA methylating activity with specificities different from those of the cytoplasm. The main reaction product, using E.coli tRNA as methyl group acceptor, is N(2),-N(2)-dimethylguanine. The corresponding mitochondrial methylase is coded by nuclear DNA. A DNA methylating activity is also associated with yeast mitochondria.

Evidence for discontinuous replication of circular mitochondrial DNA molecules from Novikoff rat ascites hepatoma cells

Double-forked circular molecules of mitochondrial DNA (mtDNA) from rat tissues, indicated by their form and size to be replicative intermediates, are of two structurally distinct classes. Molecules of the first class are totally double stranded. Molecules of the second class are defined by one daughter segment being totally or partially single stranded. Length histograms of daughter segments measuring between 2% and 44% of the total 5-microm molecular contour were constructed from samples of both classes of replicating molecules derived from mtDNA or Novikoff rat ascites hepatoma cells. For single strand-containing molecules, the lengths fell into eight distinct, reproducible groups with mean values separated by 4.1-7.6% of the circular contour length. For totally double stranded molecules, the lengths fell into seven groups, corresponding to seven of the groups found for single strand-containing molecules. These results suggest that along at least 44% of the contour of mtDNA molecules there exist discrete points at which DNA synthesis tends to be arrested. This may indicate that there are pauses in normal mtDNA synthesis. However, as the DNA used in these experiments was isolated from mitochondrial fractions, the findings may indicate that continuation of synthesis beyond specific points on the nucleotide strands requires a factor which is not available after cell disruption.

Synthesis and degradation of mitochondrial components in hypertrophied rat heart

The accumulation of inner mitochondrial components of rat heart was studied 1 and 3 days after constriction of the ascending aorta of rats. By 1 day after aortic constriction, the activities of three mitochondrial respiratory enzymes/mg of cardiac homogenate protein were increased; after 3 days, specific activities had levelled off or decreased. Selective accumulation of inner mitochondrial membrane components 24h after aortic constriction was further indicated by increased left ventricular cytochrome c concentration (nmol/mg of protein). By 3 days after surgery, cytochrome c concentration was significantly diminished. Low-temperature spectroscopy of isolated mitochondria showed that the ratios of cytochromes c, b and a+a(3) remained unchanged after aortic constriction, suggesting that cytochrome c was a good indicator of the response of the other mitochondrial inner-membrane cytochromes as well. The effect of cardiac hypertrophy on the turnover of cytochrome c was also examined. Cytochrome c was labelled in its haem group with delta-amino[2,3-(3)H(2)]laevulinate 3 days before aortic constriction. By 1 day after surgery the total ventricular radioactivity in cytochrome c of aortic banded animals was significantly higher than in sham-operated controls, indicating a decreased degradation rate in the former during the first postoperative day. delta-Aminolaevulinate was shown to be a particularly suitable precursor for such turnover studies, since it results in rapid pulse-labelling of cytochrome c (peak activity in 90min), is rapidly removed from the precursor pool (t((1/2))=30min) and is not reutilized.

Mitoribosomes from Candida utilis. Morphological, physical, and chemical characterization of the monomer form and of its subunits

Highly purified mitochondrial ribosomes (mitoribosomes) have been obtained from the yeast Candida utilis. Sedimentation analysis in sucrose gradients made in 5 mM MgCl(2), 1 mM Tris, pH 7.4 and 50 mM KCl clearly distinguishes mitoribosomes (72S) from cytoplasmic ribosomes (cytoribosomes) (78S). Mitoribosomes are completely dissociated into 50S and 36S subunits at 10(-4)M MgCl(2) whereas complete dissociation of cytoribosomes into 61S and 37S subunits occurs only at 10(-6)M MgCl(2) Electron microscopy of negatively stained mitoribosomes (72S peak) shows bipartite profiles, about 265 x 210 x 200 A Characteristic views are interpreted as frontal, dorsal, and lateral projections of the particles, the latter is observed in two enantiomorphic forms Mitoribosome 50S subunits display rounded profiles bearing a conspicuous knoblike projection, reminiscent of the large bacterial subunit. The 36S subunits show a variety of angular profiles. Mitoribosomal subunits are subject to artifactual dimerization at high Mg(2+) concentration Under these conditions, a supplementary 80S peak arises. Electron microscopic observation of the 80S peak reveals closely paired particles of the 50S type Buoyant density determinations after glutaraldehyde fixation show a single peak at rho = 1.48 for mitoribosomes and 1.53 for cytoribosomes In the presence of ethylenediaminetetraacetate (EDTA), two species of RNA, 21S and 16S, are obtained from mitoribosomes, while 25S and 17S RNA are obtained from cytoribosomes It is established that the small and large RNA species are derived from the 36S and 50S subunits, respectively, by extraction of the RNA from each subunit The G + C content of the RNA is lower for mitoribosomes (33%) than for cytoribosomes (50%). Incubation of C utilis mitochondria with leucine-(14)C results in the labeling of 72S mitoribosomes. The leucine-(14)C incorporation is inhibited by chloramphenicol and resistant to cycloheximide Puromycin strips the incorporated radioactivity from the 72S mitoribosomes, which is consistent with the view that leucine-(14)C is incorporated into nascent polypeptide chains at the level of mitoribosomes

Studies on mitochondria. II. Mitochondrial DNA of thoracic muscles of Schistocerca gregaria

The buoyant densities of the nuclear and mitochondrial DNA from the thoracic muscles of Schistocerca gregaria were found to be 1 702 and 1.689 g/cm(3), respectively, corresponding to guanine plus cytosine (G + C) content of 42.2 and 30% A preliminary treatment of the mitochondrial pellet with DNase (25 degrees C, 20 min) is necessary to eliminate the contaminating nuclear DNA. The mitochondrial DNA renatures readily after heat denaturation and incubation at 65 degrees C. The DNA released from the mitochondrial pellet by osmotic shock consists of circular open and closed molecules with a contour length around 5 micro The instability of insect mitochondria in in vitro preparations is discussed.

The effects of erythromycin and chloramphenicol on the ultrastructure of mitochondria in sensitive and resistant strains of Paramecium

The effects on cell structure of 12 hr to 6 days of exposure to erythromycin or chloramphenicol, two antibiotics known to inhibit specifically the mitochondrial protein synthesizing system, have been studied in the ciliate Paramecium aurelia. A wild type strain (sensitive to both antibiotics) and three mutant strains carrying cytoplasmically inherited mutations conferring resistance to one or the other antibiotic have been used In sensitive cells both antibiotics lead to a progressive and profound alteration of mitochondrial structure evidenced by an elongation of the organelle, a considerable decrease in the number of cristae, and the appearance of some abnormal lamellar cristae and of rigid plates of periodic structure. The modifications of cell structure, then, are mainly restricted to mitochondrial cristae. The three resistant mutants studied, on the contrary, retain normal or nearly normal mitochondrial structure in the presence of the antibiotic to which they are resistant. This fact is in good agreement with the postulated location in the mitochondrial DNA of the resistance mutations studied. The results are discussed in the light of present knowledge concerning the function of the mitochondrial protein-synthesizing system.

Selective inhibition of nuclear DNA synthesis by 9- -D-arabinofuranosyl adenine in rat cells transformed by Rous sarcoma virus

The drug, 9-beta-D-arabinofuranosyl adenine, selectively inhibits the synthesis of nuclear DNA without affecting extrachromosomal DNA synthesis in rat cells transformed by Rous sarcoma virus (B-mix K-44/6). The inhibition was linear with respect to drug concentration over the range of 37-600 muM. Mitosis and total synthesis of DNA per cell were also depressed. DNA synthesis was determined by measurement of [(3)H]thymidine incorporation into DNA. Covalently-closed circular DNA was extracted by the Hirt procedure and separated from residual chromosomal DNA by buoyant density gradient ultracentrifugation in CsCl-propidium diiodide. On the basis of buoyant density and sedimentation velocity centrifugation, the covalently-closed circular DNA formed in the presence of the drug was indistinguishable from that formed in its absence.