Sedimentation properties of RNA species homologous to mitochondrial DNA in HeLa cells

Fate of mitochondrial DNA in human-mouse somatic cell hybrids (density gradient centrifugation-ethidium bromide-karyotype)

Several hybrid lines between human and mouse somatic cells, containing one or two complements of mouse chromosomes and a reduced complement of human chromosomes, have been examined for the presence of mouse and human mitochondrial DNAs. For this analysis, advantage was taken of the fact that these two types of mitochondrial DNA have a buoyant density difference in CsCl gradients of 0.008 g/cm(3). In all the hybrid clones analyzed, which retained an average number of human chromosomes estimated conservatively to vary from 5 to 23, only mitochondrial DNA of mouse character was detected. It seems likely that either repression of relevant human genes by the mouse genome or loss of human chromosomes is responsible for these results. If the latter explanation is true, since chromosome loss under the conditions used here was substantially a random process, one would have to assume that the activity of nuclear genes distributed in many chromosomes is required for the survival of mitochondrial DNA.

Ribosomal-type ribonucleic acid from rodent mitochondria

1. Highly purified mitochondria containing 3.0mug of RNA/mg of mitochondrial protein were prepared from rat liver by differential centrifugation. 2. RNA, labelled with [(32)P]P(i) or [(3)H]orotate, was isolated from these mitochondria by a phenol extraction method. The RNA sedimented at 15S and 13S on sucrose density gradients. Its nucleotide composition was 23% uridylate, 30% adenylate, 22% guanylate and 25% cytidylate. 3. RNA from mouse L cells was labelled with [(3)H]-uridine in the presence of 0.1mug of actinomycin D/ml to suppress the synthesis of cytoplasmic rRNA. The RNA isolated from crude L-cell mitochondria by a cold-phenol-sodium dodecyl sulphate method had components sedimenting at 15S and 12.5S. These components had an electrophoretic mobility on agarose-acrylamide gels of 21 and 12S(E) compared with 28 and 18S(E) for cytoplasmic rRNA. The nucleotide composition was 26% uridylate, 34% adenylate, 18% guanylate and 22% cytidylate. 4. RNA extracted from crude L-cell mitochondria by a hotphenol-sodium dodecyl sulphate method had an additional component sedimenting at 21S and having an electrophoretic mobility of 18S(E). It was probably DNA because of its sensitivity to deoxyribonuclease and its insensitivity to ribonuclease and alkali. It was present in nuclear fragments contaminating the crude mitochondrial fraction and could be removed by deoxyribonuclease or isopycnic-gradient centrifugation.

Amino acid incorporation by isolated rat liver mitochondria during liver regeneration

Intact mitochondria, isolated from regenerating rat liver 2-3 days after partial hepatectomy, are 2.5-3 times more active in amino acid incorporation than mitochondria from control livers. Liver mitochondria from sham-operated animals showed normal amounts of incorporation. Sterile procedures insured low levels of bacterial contamination; cycloheximide was used to eliminate any contribution by contaminating microsomes. Mitochondria from control and 3-day-old regenerating livers were nearly identical in their concentration of several respiratory chain components, P/O ratios, specific O(2) consumption, and cytochrome c oxidase activities. Small differences were observed in respiratory control ratios but these were shown to be unrelated to the differences observed in amino acid incorporating ability. Differential contamination by lysosomes and differences in lysosome fragility were also shown not to be factors in the increased incorporation by regenerating liver mitochondria. Thus, mitochondria from rapidly growing and dividing mammalian tissues are more active in protein synthesis than mitochondria from tissues that grow and divide more slowly.

Mitochondrial RNA from cultured animal cells. II. A comparison of the high molecular weight RNA from mouse and hamster cells

Discrete RNA fractions sedimenting slightly slower than 18s ribosomal RNA have been found in mitochondrial preparations from both hamster (BHK-21) and mouse (L-929) cells. This RNA could be separated into two components, present in approximately equimolar amounts, by prolonged zonal centrifugation or acrylamide gel electrophoresis. The hamster components had sedimentation constants averaging 16.8 and 13.4, and molecular weights (estimated by gel electrophoresis) averaging 0.74 and 0.42 x 10(6) daltons. Mixed labeling experiments showed that the mouse components sedimented and electrophoresed 3-6% more slowly than the corresponding hamster components. The RNA from both cell lines resembled mitochondrial ribosomal RNA from yeast and Neurospora in being GC poor, and in addition the larger and smaller components resembled each other in base composition. These results, taken with those of other recent studies, are compatible with the idea that our high molecular weight mitochondrial RNA is ribosomal; such RNA would then constitute a uniquely small size-class of ribosomal RNA.