Mitochondrial membranes and mutagnesis by ethidium bromide

Induction of petite mutants in yeast Saccharomyces cerevisiae by the anticancer drug dequalinium

Dequalinium (DEQ), a drug with both antimicrobial and anticancer activity, induced the formation of petite (respiration-deficient) mutants in the yeast Saccharomyces cerevisiae. DEQ was found to be approximately 50-fold more potent than ethidium bromide (EB) at inducing petites. Analysis of the DEQ-induced petite mutants indicated a complete loss of mitochondrial DNA (<1 copy/cell). Prior to the loss of mtDNA, DEQ caused cleavage of the mtDNA into a population of fragments 30-40kbp in size suggesting that this drug causes petites by inducing a breakdown of mtDNA. The selective effect of DEQ on yeast mtDNA may underlie the antifungal activity of this chemotherapeutic agent.

Use of drug-specific antibodies to identify ethidium adducts produced in Trypanosoma brucei by photoaffinity labeling

A photoreactive azido analog of the trypanocide ethidium bromide, 3-amino-8-azido-5-ethyl-6-phenylphenanthridinium chloride, attached covalently to calf thymus DNA (CT DNA) by photoaffinity labeling, was used to generate antibodies for the drug analog. The specificity of the antiserum was tested using enzyme-linked immunoadsorbant assays (ELISA) against immobilized antigen (photoaffinity labeled DNA) and by both the avidin-biotin peroxidase reaction and indirect immunofluorescence performed on smears of drug treated trypanosomes. The reaction of the antiserum with the covalently bound drug adduct was diminished effectively by prior incubation with an excess of ethidium monoazide, ethidium diazide, and ethidium bromide, and to a lesser extent by the DNA-ethidium complex, the diazide-DNA or RNA adduct, and the monoazide-RNA adduct. DNA which had been photoaffinity labeled with either the propidium or the acridine moiety did not react. The antiserum recognition of DNA photoaffinity labeled with ethidium monoazide was based on the substituted phenanthridinium ring system of the parent ethidium, as evidenced by competition binding studies involving the free monoazido analog (EA1), the diazido analog (EA2), and the parent compound, ethidium bromide (EB). This approach and the sensitivity it provides should prove useful for identifying the distribution and fate of covalently bound drugs resulting from antiparasitic drug treatment, and for studying their roles in antiparasitic action.

Petite induction in Saccharomyces cerevisiae by ethidium analogs. Action on mitochondrial genome

Petite induction of ethidium analogs was examined in both resting and growing yeast cells. All of the analogs used in these experiments were active in dividing cells of Saccharomyces cerevisiae; only the parent ethidium bromide was mutagenic under resting conditions. Incorporation of adenine into mitochondrial DNA appeared to be prevented completely by ethidium and partially inhibited by other analogs. Treatment of growing cells with analogs affected fragmentation of pre-existing DNA as seen by the loss of a mitochondrial antibiotic resistance marker. The rates of elimination of the marker were different; ethidium generated greater loss than the monoamino analogs (3-amino and 8-amino-); and the deaminated analog was least effective. However, in resting yeast the marker was partially eliminated only with treatment of the parent ethidium. The degradation of the mitochondrial DNA by exposure to ethidium compounds was confirmed by agarose gel electrophoresis. Electrophoretic patterns of the mitochondrial DNA treated with each of the analogs under growing conditions and only with ethidium under resting conditions showed degradation of the mitochondrial DNA.

Petite and sectored induction in Saccharomyces cerevisiae by propidium iodide: synergistic effect of sodium dodecyl sulfate

Sodium dodecyl sulfate (SDS) was examined for its effect on petite and sectored colony induction in Saccharomyces cerevisiae by propidium iodide (PI) and ethidium bromide (EB). 4-h cultivation with 100 microM PI and 100 micrograms/ml SDS resulted in virtually all plated cells growing as sectored colonies with no decrease in viability. Sectored colonies are mixed colonies comprised of respiratory deficient and competent cells believed to be derived from an unstable respiratory deficient cell. Further cultivation with PI and SDS prior to plating led to induction of complete petite colonies with a rapid decrease in viable cells. PI alone at this concentration exhibited weak induction of sectored colonies (maximum 12.3% at 8 h) and petite colonies (maximum 10.8% at 12 h), but SDS alone caused induction of neither. 50 microM PI had almost the same activity as 100 microM except for a delay in the induction of sectored colonies in the initial stage, and a decreased rate of petite colony induction. The effects of 20 microM PI and SDS were much lower than that by 50 microM and no inhibition of growth was observed. 10 microM PI was quite inactive even in the presence of SDS. Under resting conditions, 10 approximately 100 microM PI and 100 micrograms/ml SDS induced about 60% sectored colonies at 12 h incubation and more than 60% petite colonies at 24 h. After 6 h incubation, decrease in survival was also observed.

Comparison of petite induction in yeast by acridines, ethidium and their photoaffinity probes

The production of petite mutations by different acridine analogs was studied in Saccharomyces cerevisiae. Compounds with amino substituents at the 2 and 3 positions of the acridine nucleus and methylation at position 10 were effective for petite induction in growing cells but not in resting cells, while those with chloro, nitro and methoxy substituents were not effective in either resting or growing cells. Photosensitive azido derivatives of the acridines were tested to evaluate the role of covalent drug attachment for mutagenesis in resting cells. Photolysis of resting cells with 9-axido, 3-azido-6-amino-, 9-azido-10-methyl-, or 3-azido-6-amino-10-methyl-acridine was highly toxic. 3-Azido-6-amino-acridine, and especially 3-azido-10-methyl-, and 3-azido-6-amino-10-methyl-acridine, were effective petite inducers in resting cells. Thus, the photosensitive (azido) group at position 9 produced only cell killing while the azido group at position 3 and/or 6 led to effective petite induction in resting cells. In contrast, petite induction was observed only for growing cells, for dark control experiments with these compounds or with the monoazide precursor compounds.

Induction of petite "mutants" in an ethidium-resistant strain of Saccharomyces cerevisiae by photoaffinity labeling. Distinction between early and late steps

A strain of Saccharomyces cerevisiae (MH41-7B/011) was resistant to petite induction by ethidium bromide at 30 degrees, but was sensitive to induction by photolabeling with ethidium monoazide. These results suggested a defect in the mutant in metabolic activation of ethidium to account for its resistance. Synchronized cultures of both the mutant and the normal parent strains showed a substantial reduction in petite response to photolabeling in stationary phase cells which could not be accounted for by changes in cell penetration of the drug. The use of photolabeling with normal and mutant cells suggested that petite induction can be divided into early and late steps.

Petite induction in Saccharomyces cerevisiae by ethidium analogs: distinction between resting and growing cells

The importance of specific substituents, especially amino azide groups, for ethidium induction of petites was evaluated in resting and dividing cells of Saccharomyces cerevisiae through the study of a series of ethidium analogs. The structural requirements in resting and growing cells were found to be different, suggesting that at least two mechanisms are responsible for induction. The significance of particular substituents in the induction processes were recognized by: (1) a dependence upon the ethyl substituent at the ring nitrogen in both actively growing and in resting cells; and (2) the implication that amino substituents are important for the effect in dividing cells and especially in resting cells. Photolytic enhancement of petite induction (via a nitrene which forms a covalent linkage to a biological site) was observed for 3 of the azide analogs, which emphasizes the likelihood that metabolic activation of ethidium to a covalent complex is responsible for its effectiveness. Furthermore, these studies indicate that these monoazide analogs should be ideal probes for examining the mitochondrial mutagenic processes.

Reversal of protection by light of the ethidium bromide induced petite mutation in yeast

An intermediate in the ethidium bromide (EB) induced petite mutation pathway may be destabilized by daylight light to cause a reversion to the normal grande phenotype. Starved cells preincubated in the dark for up to 6 h with 100 microgram/ml EB could be reverted to grandes after one hour of light exposure, whereas similarly treated cells maintained in the dark expressed the petite mutation in more than 80 percent of the population. In addition, the production of petite mutants by EB in buffer could be prevented if cell suspensions were exposed to light immediately upon the addition of EB. Photoreversal of the EB-derived petite mutation in growing cells was less efficient presumably because the availability of an energy source caused a continuation of mutation events beyond the light revertible step to a non-reversible fixation of the mutation. Cells treated with EB in growth media at 4 degrees C were more responsive to light protection and reversal of the mutation. This may be due to the cold inhibition of an enzyme which comes into play beyond the light sensitive step in the mutation pathway.

Studies on the induction of petite mutants in yeast by analogues of berenil. Characterization of three mutants resistant to the compound Hoe 15,030

Compound Hoe 15,030 is an analogue of berenil which is as effective as berenil in inducing petite mutants in Saccharomyces cerevisiae. Hoe 15,030 has greater stability than berenil in aqueous solution, and is less toxic to yeast at high drug concentrations. Mutants of S. cerevisiae strain J69-1B have been isolated which are resistant to the petite inducing effects of Hoe 15,030. Three mutant strains (HR7, HR8 and HR10) were characterized and each was shown to carry a recessive nuclear mutation determining resistance to Hoe 15,030. The degree of resistance to Hoe 15,030 is different for each mutant, and each was found to be co-ordinately cross-resistant both to berenil and to another analogue of berenil, Hoe 13,548. However, the three mutants show no cross-resistance to other unrelated petite inducing drugs, including ethidium bromide, euflavine and 1-methyl phenyl neutral red. Further studies on the mutants revealed that each strain exhibits characteristic new properties indicative of changes in mitochondrial membrane functions concerned with the replication (and probably also repair) of mitochondrial DNA. Thus, mutant HR7 is hypersensitive to petite induction by the detergent sodium dodecyl sulphate under conditions where the parent J69-1B is unaffected by this agent. Mutant HR8 is even more sensitive to sodium dodecyl sulphate than is HR7, and additionally shows a markedly elevated spontaneous petite frequency. Isolated mitochondria from strains HR8 and HR10 (but not HR7) show resistance to the inhibitory effects of Hoe 15,030 on the replication of mitochondrial DNA in vitro.

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.

The action of structural analogues of ethidium bromide on the mitochondrial genome of yeast

We have studied the effects on the yeast mitochondrial genome of four analogues of ethidium bromide, in which the phenyl moieyt has been replaced by linear alkyl chains of lengths varying from seven to fifteen carbon atoms. These analogues are more efficient than ethidium bromide in inducing petite mutants in Saccharomyces cervisiae. The drugs also cause a loss of mtDNA from the cells in vivo; however these analogues are in fact less effective inhibitors of mitochondrial DNA replication per se, as shown by direct in vitro studies. It is concluded that these analogues are more efficient than ethidium bromide in causing the fragmentation of mitochondrial DNA in S. cervisiae.

A novel class of Saccharomyces cerevisiae mutants specifically UV-sensitive to "petite" induction

A mutant of Saccharomyces cerevisiae has been isolated which, though exhibiting a normal response to nuclear genetic damage by ultraviolet light (UV), is more sensitive than its wild type specifically in the production of the cytoplasmic (rho-) mutation by this agent. Some of the features of this mutation which has been designated uvsrho 5 are: i) The mutation is recessive, it exhibits a Mendelian, and hence presumably nuclear, pattern of segregation, but manifests its effects specifically and pleiotropically on mitochondrial functions. ii) Mutant cells resemble their wild type parents in a) growth characteristics on glucose; b) in their UV induced dose response to lethality or nuclear mutation and c) the ability of their mitochondrial genome, upon mating with appropriate testers, of transmitting and recombining various markers, albeit with enhanced efficiency. Similarly, d) they are able to modulate the expression of mitochondrial mutagenesis by ethidium bromide. Thus their mitochondrial DNA appears genetically as competent as that of the wild type. iii) Mutant cells differ from their wild type parents in a) growth characteristics on glycerol; b) susceptibility to induction of the mitochondrial (rho-) mutation by various mutagens, in that the rate of spontaneous mutation is slightly and that by UV is significantly enhanced, whild that by ethidium bromide is greatly diminished. Conversely, c) modulating influences resulting in the repair of initial damage are diminished fro UV and stimulated in the case of Berenil. iv) The amount of mitochondrial DNA per cell appears elevated in the mutant, relative to wild type, and its rate of degradation subsequent to a mutagenic exposure to either UV or ethidium bromide is diminished. v) A self-consistent scheme to account for this and all other information so far available for the induction and modulation of the (rho-) mutation is presented. In a previous study it was shown that some nuclear mutants of Saccharomyces cerevisiae, more sensitive to lethal damage induced by ultraviolet light (rad) than their parent wild type (RAD), also exhibit a concomitant modification in sensitivity to both nuclear and cytoplasmic genetic damage (Moustacchi, 1971). However, another class of rad mutants respond to the induction of the cytoplasmic "petite" also designated as rho- (or rho-) mutation by UV in a manner indistinguishable from that of the RAD strain. One possible interpretation of this last observation is that some of the steps in the expression of the UV damage on mitochondrial (mt)DNA may be governed by other nuclear and cytoplasmic genetic determinants, the products of which may then act specifically on mitochondrial lesions. If this assumption is correct, it should be possible to find mutants with a normal response to nuclear damage but specifically UV-sensitive towards induction of (rho-)...

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.

Protein synthesis and the recovery of both survival and cytoplasmic "petite" mutation in ultraviolet-treated yeast cells. II. Mitochondrial protein synthesis

The contribution of mitochondrial proteins in the repair of UV-induced lethal and cytoplasmic genetic damages was studied in dark liquid held exponential and stationary phase yeast cells. This was performed by using the specific inhibitors, erythromycin (ER) anc chloramphenicol (CAP). It was shown that mitochondrial proteins are involved in the recovery of stationary phase cells. Mitochondrial proteins are partly implicated in the mechanisms leading to the restoration of the (see article) genotype in UV-irradiated dark liquid held exponential phase cells. Here again, in stationary phase cells, mitochondrial enzymes do not seem to participate in the negative liquid holding (NLH) process for the (see article) induction, as shown by inhibiting mitochondrial protein synthesis or both mitochondrial and nuclear protein synthesis. When cells are grown in glycerol, the response after dark liquid holding of UV-treated cells in the different growth stages are similar to that found for glucose-grown cells. In other words, the fate of cytoplasmic genetic damage, in particular, is not correlated with the repressed or derepressed state of the mitochondria.

Coupling between mitochondrial mutation and energy transduction

Upon incubation with ethidium bromide (Etd Br) isolated mitochondria of Saccharomyces cerevisiae have been shown to be capable of performing five novel reactions: (a) a single scission of their DNA (mtDNA) coincident with (b) the incorporation of Etd Br into the product (mtDNA --> 2 mtDNA'-Etd Br); (c) an energy- (and probably ATP-) requiring degradation of mtDNA'-Etd Br to acid-labile products. These reactions acting in series generate (d) a DNase dependent on both Etd Br and an energy supply with mtDNA'-Etd Br as an obligatory intermediate. Coincident with (d) there occurs (e) an activation of adenosinetriphosphatase. Experiments with specific inhibitors suggest that the enzyme responsible is the mitochondrial adenosinetriphosphatase complex itself, and that it and the ability to carry out reactions (a) through (e) are tightly coupled to the energy-transducing functions of the particle.

Accumulation of replicative intermediates of mitochondrial DNA in Tetrahymena pyriformis grown in ethidium bromide

The effect of growth of Tetrahymena pyriformis in ethidium bromide (EthBr) on the structure and synthesis of mitochondrial DNA (mtDNA) has been investigated. During the first 5 h of growth in EthBr, mtDNA synthesis is inhibited 95% or more. After 10-15 h, this block is partially released and large numbers of replicating molecules accumulate, indicating that inhibition by EthBr primarily affects the rate of chain growth and not the initiation of new rounds of replication. The accumulated molecules sediment more rapidly than normal Tetrahymena mtDNA and do not contain enough single-strand regions to distinguish them from normal Tetrahymena mtDNA when banded in buoyant CsCl or NaI gradients. Electron microscopy shows that the predominant species in this rapidly sedimenting DNA is a linear molecule containing one symmetrical double-stranded replication loop of varying size located at its center. No degradation of mtDNA from cells grown in EthBr was detected in alkaline velocity gradients.

Mitochondrial genetics. VI. The petite mutation in Saccharomyces cerevisiae: interrelations between the loss of the p+ factor and the loss of the drug resistance mitochondrial genetic markers

The survival of the rho(+) factor and of Drug(R) mitochondrial genetic markers after exposure to ethidium bromide has been studied. A technique allowing the determination of Drug(R) genetic markers among a great number of both grande and petite colonies has been developed. The results have been analyzed by the target theory. The survival of the rho(+) factor is always less than the survival of any Drug(R) genetic marker. The survivals of C(R) and E(R) are similar to each other, while that of O(R) is greater than that of the other two Drug(R) markers. All possible combinations of Drug(R) markers have been found among the rho(-) petite cells induced, while the only type found among the grande colonies is the preexisting one. The loss of the C(R) and E(R) genetic markers was found to be the most frequently concomitant, while the correlation between the loss of the O(R) marker and the other two Drug(R) markers is less strong. Similar results have been obtained after U.V. irradiation. Interpretations concerning the structure of the yeast mitochondrial genome are given and hypotheses on the mechanism of petite mutation discussed.

Cytoplasmic "petite" induction in recombination-deficient mutants of Saccharomyces cerevisiae

As compared to the original wild type, the induction of the cytoplasmic "petite" mutation by ultraviolet light and by the intercalating dye, ethidium bromide, is reduced in two mutants (rec4 and rec5) of Saccharomyces cerevisiae. These mutants are blocked in X rays or ultraviolet light-induced intragenic recombination. It then appears that the products of nuclear genes necessary for the completion of nuclear intragenic recombination events are also involved in steps of the metabolic chain which leads to the mitochondrial mutation, rho(-).