The intracellular site of formation of the mitochondrial protein synthetic system

Inhibition of mitochondrial protein synthesis in regenerating rat liver stimulates mitochondrial transcription

Partially hepatectomized rats were treated in vivo with thiamphenicol for 3 days to block mitochondrial protein synthesis. Protein synthesis, RNA synthesis and the steady-state levels of individual transcripts were measured in mitochondria in vitro in the absence of thiamphenicol. Incorporation of [35S]methionine and [3H]UTP into protein and RNA, respectively, was increased 2-3-fold in isolated mitochondria from thiamphenicol-treated animals, indicating increased rates of synthesis of both. Electrophoretic analysis of transcripts labelled with [32P]UTP suggests that synthesis of all the transcripts is increased. The steady-state concentrations of mitochondrial transcripts, measured by Northern blotting using nick-translated cloned EcoRI fragments of rat liver mtDNA, were also elevated 2-4-fold in thiamphenicol-treated animals. The data suggest that mitochondrial transcription is under control of a mitochondrial factor which, in turn, is dependent upon mitochondrial protein synthesis.

Defective assembly of the mitochondrial ribosomes in yeast cells grown in the presence of mitochondrial protein synthesis inhibitors

The involvement of mitochondrial protein synthesis in the assembly of the mitochondrial ribosomes was investigated by studying the extent to which the assembly process can proceed in the presence of mitochondrial protein synthesis inhibitors erythromycin and chloramphenicol. Yeast cells grown in the presence of erythromycin (2 mg/ml) do not appear to contain any detectable amounts of the mitochondrial small (37 S) ribosomal subunit. Instead, a ribonucleoparticle with a sedimentation coefficient of 30 S was observed; this particle could be shown to be related to the mitochondrial small ribosomal subunit by two-dimensional gel electrophoretic analysis of its protein components. Since the var1 protein is the only mitochondrial translation product known to be associated with the mitochondrial ribosome, our results suggest that this protein is essential for the assembly of the mature small subunit, and that the var1 protein enters the pathway for the assembly of the small subunit at a late step. In at least one strain of yeast the accumulation of the 30-S particle appears to be very sensitive to catabolite repression. When yeast cells are grown in the presence of chloramphenicol instead of erythromycin, assembly of the small subunit appears to be only partially inhibited, and the presence of the 30-S particle could not be clearly demonstrated. This observation is consistent with the fact that in yeast, chloramphenicol inhibits mitochondrial protein synthesis by about 95% only and that the synthesis of the var1 protein appears to be the least sensitive to this inhibition.

Selective loss of mitochondrial genome can be caused by certain unsaturated fatty acids

Various unsaturated fatty acids had different effectiveness for maintaining the continued replication of functional mitochondria in an unsaturated fatty acid auxotroph of Saccharomyces cerevisiae (KD115). Certain isomers of octadecenoic acid (i.e., cis-9) and eicosatrienoic acid (i.e.,cis-8,11,14) permitted continued replication of mitochondria and provided cultures that contained only 4 to 5% cells that formed petite colonies. On the other hand, cultures grown with cis-12- or cis-13-octadecenoic acid or cis-11,14,17-eicosatrienoic acid, produced a 12- to 16-fold greater frequency of petite mutants (50-60%) after 8 to 10 generations of growth. The production of the petite mutants occurred despite adequate incorporation of these unsaturated fatty acids into cellular phospholipids and an apparently normal ability to undergo the initial steps in the induction of cellular respiration. The evidence suggests that some cellular processes necessary for continued mitochondrial replication depend on the structural features of the fatty acyl chains as well as the overall content of unsaturated fatty acids in membrane phospholipids. Impairment of that process by certain inadequate fatty acids or by an inadequate supply of a suitable fatty acid leads to a permanent loss of the mitochondrial genome from the cells of subsequent generations.

Biogenesis of mitochondria 51: biochemical characterization of a mitochondrial mutation in Saccharomyces cerevisiae affecting the mitochondrial ribosome by conferring resistance to aminoglycoside antibiotics

An examination of the effect of the aminoglycoside antibiotics paromomycin and neomycin on mitochondrial ribosome function in yeast has been made. Both antibiotics are potent inhibitors of protein synthesis in isolated mitochondria. With isolated mitochondrial ribosomes programmed with polyuridylic acid (poly U), the drugs are shown to inhibit polyphenylalanine synthesis at moderately high concentrations (above 100 microgram/ml). At lower concentrations (about 10 microgram/ml), paromomycin and neomycin cause a 2-3 fold stimulation in the extent of misreading of the UUU codons in poly U, over and above the significant level of misreading catalyzed by the ribosomes in the absence of drugs. Comparative studies have been made between a paromomycin sensitive strain D585-11C and a mutant strain 4810P carrying the par l-r mutation in mtDNA, which leads to high resistance to both paromomycin and neomycin in vivo. A high level of resistance to these antibiotics is observed in strain 4810P at the level of mitochondrial protein synthesis in vitro. Whilst the degree of resistance of isolated mitochondrial ribosomes from strain 4810P judged by the inhibition of polyphenylalanine synthesis by paromomycin and neomycin is not extensive, studies on misreading of the poly U message promoted by these drugs demonstrate convincingly the altered properties of mitochondrial ribosomes from the mutant strain 4810P. These ribosomes show resistance to the stimulation of misreading of the codon UUU brought about by paromomycin and neomycin in wild-type mitochondrial ribosomes. Although strain 4810P was originally isolated as being resistant to paromomycin, in all the in vitro amino acid incorporation systems tested here, the 4810P mitochondrial ribosomes show a higher degree of resistance to neomycin than to paromomycin. It is concluded that the parl-r mutation in strain 4810P affects a component of the mitochondrial ribosome, possibly by altering the 15S rRNA or a protein of the small ribosomal subunit. The further elucidation of the functions in the ribosomes that are modified by the parl-r mutation was hampered by the inability of current preparations of yeast mitochondrial ribosomes to translate efficiently natural messenger RNAs from the several sources tested.

Cytoplasmic and mitochondrial protein synthesis in the "petite-negative" yeast Kluyveromyces lactis. Cytoplasmic inheritance of resistance to tetracycline

The growth of two strains of the "petite-negative" yeast Kluyveromyces lactis is inhibited by Tetracycline in different ways under the same culture conditions. Tetracycline resistant mutants of one strain have been isolated which can tolerate doses as high as 3000 microgram/ml of antibiotic. The segregation pattern of this character obtained by random spore analysis of the ascospores derived from the cross of the two strains strongly suggests that the resistance to tetracycline is under mitochondrial control.

Sterol biosynthesis in yeast. 3-Hydorxy-3-methylglutaryl-Coenzyme A reductase as a regulatory enzyme

Anaerobically and aerobically grown yeast contains 3-hydroxy-3-methylglutaryl-CoA reductase, which is located in the mitochondrial fraction of the cell. Anaerobically grown yeast has a low specific activity of 3-hydroxy-3-methylglutaryl-CoA reductase and a low sterol content. Aeration of this yeast in buffer, without growth, results in an increase in the specific activity of the enzyme, which is paralleled by an increase in the sterol content. This induction has an oscillatory profile with yeast grown anaerobically for 24 h and a linear pattern with cells grown anaerobically for 72 h. With the latter type of yeast, glucose is necessary for an induction, whereas with the other yeast an induction occurs with and without glucose. By an anaerobic incubation in buffer of the yeast grown anaerobically for 24 h, the oscillatory profile can be transformed into a linear one. The extent of induction of the reductase is strictly dependent on the concentration of glucose present. Sterols increase in whole cells, but they do not increase in the mitochondrial fraction. The induction of 3-hydroxy-3-methylglutaryl-CoA reductase is strongly inhibited by cycloheximide, but is not affected by chloramphenicol. The induction of the enzyme is closely connected with the glucose metabolism of the cell; fructose, mannose, and ethanol can also induce the reductase.

Biogenesis of mitochondria. The effects of physiological and genetic manipulation of Saccharomyces cerevisiae on the mitochondrial transport systems for tricarboxylate-cycle anions

1. Kinetic and equilibrium parameters for the uptake of l-malate, succinate, citrate and alpha-oxoglutarate by fully functional mitochondria of Saccharomyces cerevisiae were determined. 2. The uptake of l-malate and succinate is mediated by a common carrier, and two other distinct carriers mediate the uptake of citrate and alpha-oxoglutarate. 3. The properties of the carrier systems for l-malate, succinate and citrate closely resemble those of mammalian mitochondria, but the alpha-oxoglutarate carrier differs from the mammalian system in minor respects. 4. The composition of the yeast mitochondria was extensively manipulated by (a) anaerobiosis, (b) catabolite repression, (c) inhibition of mitochondrial protein synthesis and (d) elimination of mitochondrial DNA by mutation. 5. The carrier systems for l-malate, succinate, citrate and alpha-oxoglutarate are essentially similar in the five different types of mitochondria. 6. It is concluded that all the protein components of the carrier systems for l-malate, succinate, citrate and alpha-oxoglutarate are coded by nuclear genes and synthesized extramitochondrially by cell-sap ribosomes.

The intracellular site of synthesis of mitochndrial ribosomal proteins in Neurospora crassa

The intracellular site of synthesis of mitochondrial ribosomal proteins (MRP) in Neurospora crassa has been investigated using three complementary approaches. (a) Mitochondrial protein synthesis in vitro: Tritium-labeled proteins made by isolated mitochondria were compared to (14)C-labeled marker MRP by cofractionation in a two-step procedure involving isoelectric focusing and polyacrylamide gel electrophoresis. Examination of the electrophoretic profiles showed that essentially none of the peaks of in vitro product corresponded exactly to any of the MRP marker peaks. (b) Sensitivity of in vivo MRP synthesis to chloramphenicol: Cells were labeled with leucine-(3)H in the presence of chloramphenicol, mitochondrial ribosomal subunits were subsequently isolated, and their proteins fractionated by isoelectric focusing followed by gel electrophoresis. The labeling of every single MRP was found to be insensitive to chloramphenicol, a selective inhibitor of mitochondrial protein synthesis. (c) Sensitivity of in vivo MRP synthesis to anisomycin: We have found this antibiotic to be a good selective inhibitor of cytoplasmic protein synthesis in Neurospora. In the presence of anisomycin the labeling of virtually all MRP is inhibited to the same extent as the labeling of cytoplasmic ribosomal proteins. On the basis of these three types of studies we conclude that most if not all 53 structural proteins of mitochondrial ribosomal subunits in Neurospora are synthesized by cytoplasmic ribosomes.

Cooperation of mitochondrial and nuclear genes specifying the mitochondrial genetic apparatus in Neurospora crassa

Enzymes involved in the expression of the mitochondrial genome in Neurospora crassa are induced by chloramphenicol and ethidium bromide, which block transcription and translation of mitochondrial DNA. It is concluded that most, if not all, proteins of the mitochondrial genetic apparatus are coded by nuclear genes, synthesized on cytoplasmic ribosomes, and controlled by a repressor-like mitochondrial gene product. A model explaining the coordination of nuclear and mitochondrial division cycles by repressor control is discussed.

Effects of chloramphenicol on chloroplast and mitochondrial ultrastructure in Ochromonas danica

The effect of chloramphenicol (CAP) on cell division and organelle ultrastructure was studied during light-induced chloroplast development in the Chrysophyte alga, Ochromonas danica. Since the growth rate of the CAP-treated cells is the same as that of the control cells for the first 12 hr in the light, CAP is presumed to be acting during that interval solely by inhibiting protein synthesis on chloroplast and mitochondrial ribosomes. CAP markedly inhibits chloroplast growth and differentiation. During the first 12 hr in the light, chlorophyll synthesis is inhibited by 93%, the formation of new thylakoid membranes is reduced by 91%, and the synthesis of chloroplast ribosomes is inhibited by 81%. Other chloroplast-associated abnormalities which occur during the first 12 hr and become more pronounced with extended CAP treatment are the presence of prolamellar bodies and of abnormal stacks of thylakoids, the proliferation of the perinuclear reticulum, and the accumulation of dense granular material between the chloroplast envelope and the chloroplast endoplasmic reticulum. CAP also causes a progressive loss of the mitochondrial cristae, which is paralleled by a decline in the growth rate of the cells, but it has no effect on the synthesis of mitochondrial ribosomes. We postulate that one or more chloroplast ribosomal proteins are synthesized on chloroplast ribosomes, whereas mitochondrial ribosomal proteins are synthesized on cytoplasmic ribosomes.

The effects of inhibitors of RNA and protein synthesis on chloroplast structure and function in wild-type Chlamydomonas reinhardi

Wild-type cells of the unicellular green alga Chlamydomonas reinhardi have been grown for several generations in the presence of rifampicin, an inhibitor of chloroplast DNA-dependent RNA polymerase, spectinomycin and chloramphenicol, two inhibitors of protein synthesis on chloroplast ribosomes, and cycloheximide, an inhibitor of protein synthesis on cytoplasmic ribosomes. The effects of cycloheximide are complex, and it is concluded that this inhibitor cannot give meaningful information about the cytoplasmic control over the synthesis of chloroplast components in long-term experiments with C. reinhardi. In the presence of acetate and at the appropriate concentrations, the three inhibitors of chloroplast protein synthesis retard growth rates only slightly and do not affect the synthesis of chlorophyll; however, photosynthetic rates are reduced fourfold after several generations of growth. Each inhibitor produces a similar pattern of lesions in the organization of chloroplast membranes. Only rifampicin prevents the production of chloroplast ribosomes.

Genetic evidence for 'Darwinian' selection at the molecular level. II. Genetic analysis of cytoplasmically-inherited high and low suppressitivity in Saccharomyces cervisiae

A method was devised for the genetic analysis of cytoplasmically-inherited high and low suppressitivity in S. cervisiae thus enabling a test of the prediction, by the model of 'Darwinian' selection of mitochondrial DNA, that abnormal mitochondrial DNA of a high suppressitivity strain has a replicative advantage over abnormal mitochondrial DNA of a low suppressitivity strain. Support for the model was indicated by the ability of the suppressive factor resident in the high suppressitivity strain to control the phenotypic expression of suppressitivity in zygotes formed by crossing a low and high suppressitivity strain.