Coupling between mitochondrial mutation and energy transduction

Isolation and Quantification of DNA

Purifying DNA is the key to successful cloning. The cleaner the final preparation of DNA, the more efficient will be the enzymatic reactions that use the DNA as a template or a substrate. In the 1930s and 1940s, the scientific literature began to accumulate methods to release DNA from cells and to remove cellular constituents that inhibit or act as competitors on enzymatically catalyzed reactions. Since then, thousands of protocols for purification of DNA from a wide variety of organisms, tissues, and bodily fluids have been published. This introduction provides an overview of methods for isolation and quantification of DNA.

Interaction of drugs with extranuclear genetic elements and its consequences

Bacterial ancestry of mitochondria and plastids is now generally accepted. Both organelles contain their own DNA and transcription-translation apparatus of a prokaryotic type. Due to this fact these systems carry bacteria-like properties. Thus organellar DNA and ribosomes are essentially different from nuclear DNA and cytoplasmic ribosomes in physical as well as in functional respects. Due to the bacterial character of both types of organelles they are susceptible to various antibacterial chemicals. Inhibitors of bacterial protein synthesis inhibit mitochondrial (plastidial) biogenesis. Therefore the cellular content of mitochondria (plastids)-made proteins decreases during cytoplasmic turnover or cell division in the presence of these drugs. Such drug activity consequently leads to a reduced capacity for oxidative phosphorylation or photosynthesis. Organellar genomes are less stable and more sensitive to mutagenesis as compared to nuclear genome. It means also that genotoxic agents induce various disorders of mitochondrial (plastidial) functions. Impairments in the respiratory chain are associated with structural as well as functional abnormalities of mitochondria. These are clinically expressed mostly in tissues with a high demand for ATP: brain, heart, skeletal muscle, and retina. On the other hand, some antibacterial inhibitors of mitochondrial biogenesis (e.g., tetracyclines) inhibit selectively tumor cell proliferation. Therefore they may be considered for use in anticancer therapy. The article summarizes the response of mitochondria and plastids in various organisms to drugs and environmental xenobiotics. Various model organisms suitable for detection of xenobiotic effect on mitochondria (plastids) are presented as well as the possible consequences of such interaction.

Synthesis of DNA in excised watermelon cotyledons grown in water and benzyladenine

Excised watermelon cotyledons were grown in water and benzyladenine, which greatly promotes growth, breakdown of reserves and development of organelles. In order to investigate the involvement of DNA synthesis in these benzyladenine-induced effects, [(3)H]thymidine was applied continuously (for 3 d) or administered briefly (5 h) to excised cotyledons at various stages of development. Autoradiographic analysis of squashed and sectioned cotyledons showed that both the cytoplasm (mainly in the region of the plastids) and most of the nuclei were labelled. Both types of labelling were promoted by benzyladenine treatment. The highest percentage of labelled nuclei was found in the early stages of growth (first day after excision of cotyledons), long before the burst of enzymatic activities involved in the germination processes. The possible meaning of the increase of nuclear DNA, apart from the normal replicative synthesis preceding cell division, is discussed.

Synthesis and assembly of adenosinetriphosphatase in synchronous cultures of yeast

Maximal respiration and expression of mitochondrial enzymes are found at the late-S phase of yeast cells growing synchronously in glucose medium. Adenosinetriphosphatase (ATPase) activity follows a similar pattern. However, the cytosolically synthesized F1-ATPase and also that released from the membrane accumulate in the cytosol during the G1 and early-S phases. After the mid-S phase, when the mitochondrially synthesized membrane factors are available, the enzyme migrates to the membrane and is integrated.

Deletion of mitochondrial genetic markers in yeast by ethidium and the photoaffinity probe, ethidium azide

Induction of petite (cytoplasmic-respiration-deficient, rho-,rho-) mutations in yeast and deletion of mitochondrial drug-resistance genetic markers were compared after after treatment with ethidium and the corresponding photoaffinity probe, ethidium azide. Deletion of mitochondrial drug-resistance markers for chloramphenicol, erythromycin and oligomycin in these petite mutants was observed during prolonged treatment times with ethidium and with ethidium azide in the dark. A similar loss of drug-resistance markers was also observed in petites produced by photolytic treatment with the azide analogue, although the rate of loss appeared to be somewhat less. These results confirmed the usefulness of photoaffinity labeling with ethidium monoazide for studies of mitochondrial mutations.

Basis for slow growth on the non-fermentable substrates by a Saccharomyces cerevisiae mutant UV-sensitive for rho- production

The mutant uvsrho 72 of Saccharomyces cerevisiae UV-sensitive for rho- production displays slower growth on media containing non-fermentable carbon sources such as glycerol or lactate. The slower growth on glycerol is not due to any deficiency in glycerol catabolism or mitochondrial oxidative phosphorylation. No modifications of the sensitivity to ethidium bromide of the mitochondrial ATPase activity could be detected. A mathematical model is presented which accounts for slower growth of uvsrho 72 on the sole basis of the continuous and elevated rho- production in the mutant strain. This model, which estimates the rate of mutation from the rate of growth and vice versa, has been verified experimentally in the case of of usvrho 72. The model has been generalised, so that it can be used for any microbial population subject to constant and high rates of any type of mutation providing that the mutant is stable, and either unable to grow or able to grow at this own rate different from that of the parental strain.

Respiratory repression and the stability of the mitochondrial genome

The variation in sensitivity of the mitochondrial genome of Saccharomyces cerevisiae to ethidium bromide-induced petite mutation in response to changes in glucose concentration has been studied. Growth in high glucose considerably depressed the mutation rate, whilst small variations are observed in response to step-up or step-down in glucose concentration. Variations in mitochondrial DNA and respiratory activity during the mutagenic process are described. Effects of non-metabolizable sugars which repress mitochondrial biogenesis and a number of antimitochondrial drugs have been investigated. The results are discussed in terms of possible mechanisms of modulation of the mutation rates.

Integration and regulation of mitochondrial assembly in yeast

The interactions between the mitochondrial and nucleocytoplasmic systems required for mitochondriogenesis have been investigated at several different levels. Those involved in the formation of functional enzyme complexes have been studied using cytochrome oxidase: this multimeric (2 X 7 and 2 X 6 subunits for enzymes from yeast and beef heart respectively) has been resolved, and the mitochondrial contribution has been shown to be dispensible for catalytic function proper. Using novel mutants, with a mitochondrial mode of inheritance, a mitochondrial gene product localized in the oligomycin-sensitive ATPase has been implicated in the assembly not only of this complex, but of cytochrome oxidase as well. Interactions required for the genetic competence of the mitochondrial system have become apparent as a result of studies in the mechanism of action of the highly effective mitochondrial mutagen ethidium bromide. This agent first becomes covalently inserted into mitochondrial DNA and, after its excision, eventually results in extensive degradation of the macromolecule. The excision reaction has now been shown to be performed by a complex between the oligomycin-sensitive ATPase and a DNA-binding protein presumably involved in recognizing the damage. On the level of replication and expression of the mitochondrial genome studies using thermolabile mutants have demonstrated that these processes appear independent of the replication of nuclear DNA but not of its expression.

Effect of caffeine on the rho- -induction with ethidium bromide in Saccharomyces cerevisiae

It is shown that caffeine antagonizes petite-induction with ethidium bromide under non-growth conditions when administered during or after mutagenic treatment. Caffeine itself is shown to be a petite-inducing agent when cells are grown in liquid glucose-complete-medium in the presence of the drug. A possible mode of action of caffeine in the ethidium bromide induced petite-mutagenesis is discussed.

Factors affecting petite induction and the recovery of respiratory competence in yeast cells exposed to ethidium bromide

When growing cultures of S. cerevisiae are treated with high concentrations of ethidium bromide (greater than 50 mug/ml), three phases of petite induction may be observed: I. the majority of cells are rapidly converted to petite, II. subsequently a large proportion of cells recover the ability to form respiratory competent clones, and III. slow, irreversible conversion of all cells to petite. The extent of recovery of respiratory competence observed is dependent on the strain of S. cerevisiae employed and the temperature and the carbon source used in the growth medium. The effects of 100 mug/ml ethidium bromide are also produced by 10 mug/ml ethidium bromide in the presence of the detergent, sodium dodecyl sulphate, and recovery is also observed when cells are treated with 10 mug/ml ethidium bromide under starvation conditions. Genetic analysis of strain differences indicates that a number of nuclear genes influence petite induction by ethidium bromide. In one strain, S288C, petite induction by 100 mug/ml ethidium bromide is extremely slow under certain conditions. Mitochondria isolated from from S288C lack the ethidium bromide stimulated nuclease activity found in D243-4A, a strain which shows triphasic kinetics of petite formation. This enzyme may, therefore, be responsible for the initial phase of rapid petite formation.

Molecular and genetic events accompanying petite induction and recovery of respiratory competence induced by ethidium bromide

The treatment of yeast cells with high levels of ethidium bromide causes a rapid induction of respiratory deficient mutants followed by a period of recovery to respiratory competence in 60 to 70% of the cells. Prolonged exposure then results in a final irreversible phase of petite formation. Sucrose gradient sedimentation analysis of 3H-adenine labelled mtDNA indicates that limited fragmentation (to about 16-18S) occurs during the initial phase of petite induction followed by a reassembly of the fragments during the period corresponding to the recovery of respiratory competence. The reassembly is associated with an ethidium bromide insensitive incorporation of 3H-adenine into mtDNA at a level consistent with repair synthesis. Genetic analyses, based on the transmission of five markers carried on the mtDNA of "repaired rho+" clones, suggests that reassembly occurs with a high degree of fidelity, though in two of a total of twenty five clones differences in marker transmission frequency were observed which could possibly reflect an altered gene order. In addition, a description is given of the marked changes in the suppressive nature of the treated cells and the temporary reduction in the capacity for marker transmission seen to accompany the transitory fragmentation of the mtDNA. The final phase of petite induction is an energy dependent degradation of the mtDNA to produce a rho degrees culture.

Biogenesis of mitochondria. 43. A comparative study of petite induction and inhibition of mitochondrial DNA replication in yeast by ethidium bromide and berenil

The action of ethidium bromide and berenil on the mitochondrial genome of Saccharomyces cerevisiae has been compared in three types of study: (i) early kinetics (up to 4 h) of petite induction by the drugs in the presence or absence of sodium dodecyl sulphate; (ii) genetic consequences of long-term (8 cell generations) exposure to the drugs; (iii) inhibition of mitochondrial DNA replication, both in whole cells and in isolated mitochondria. The results have been interpreted as follows. Firstly, the early events in petite induction differ markedly for the two drugs, as indicated by differences in the short-term kinetics. After some stage a common pathway is apparently followed because the composition of the population of petite cells induced after long-term exposure are very similar for both ethidium bromide and berenil. Secondly, both drugs probably act at the same site to inhibit mitochondrial DNA replication, in view of the fact that a petite strain known to be resistant to ethidium bromide inhibition of mitochondrial DNA replication was found to have simultaneously acquired resistance to berenil. From consideration of the drug concentrations needed to inhibit mitochondrial DNA replication in vivo and in vitro it is suggested that in vivo permeability barriers impede the access of ethidium bromide to the site of inhibition of mitochondrial DNA replication, whilst access of berenil to this site is facilitated. The site at which the drugs act to inhibit mitochondrial DNA replication may be different from the site(s) involved in early petite induction. Binding of the drugs at the latter site(s) is considered to initiate a series of events leading to the fragmentation of yeast mitochondrial DNA and petite induction.