Mitochondrial genetics in perspective: the derivation of a genetic and physical map of the yeast mitochondrial genome

Why are most organelle genomes transmitted maternally?

Why the DNA-containing organelles, chloroplasts, and mitochondria, are inherited maternally is a long standing and unsolved question. However, recent years have seen a paradigm shift, in that the absoluteness of uniparental inheritance is increasingly questioned. Here, we review the field and propose a unifying model for organelle inheritance. We argue that the predominance of the maternal mode is a result of higher mutational load in the paternal gamete. Uniparental inheritance evolved from relaxed organelle inheritance patterns because it avoids the spread of selfish cytoplasmic elements. However, on evolutionary timescales, uniparentally inherited organelles are susceptible to mutational meltdown (Muller's ratchet). To prevent this, fall-back to relaxed inheritance patterns occurs, allowing low levels of sexual organelle recombination. Since sexual organelle recombination is insufficient to mitigate the effects of selfish cytoplasmic elements, various mechanisms for uniparental inheritance then evolve again independently. Organelle inheritance must therefore be seen as an evolutionary unstable trait, with a strong general bias to the uniparental, maternal, mode.

Genetic transfer in plants through interspecific protoplast fusion

Protoplasts of sexually incompatible species have been fused and in some combinations have given rise to somatic hybrid plants. Partial elimination of parental chromosomes from either species is common in such hybrids, but total chromosome loss has generally occurred only with phylogenetically unrelated pairings. Genetic function of one parent may be retained despite a complete loss of its chromosomes, suggesting that genetic introgression is possible in the absence of complete donor chromosomes. A model interspecific combination for such studies is the potato-tomato somatic hybrid for which numerous phenotypes and karyotypes are encountered at the outset, with a broader range observed in the second somatic generation.

The thankless task of playing genetics with mammalian mitochondrial DNA: a 30-year review

The advances obtained through the genetic tools available in yeast for studying the oxidative phosphorylation (OXPHOS) biogenesis and in particular the role of the mtDNA encoded genes, strongly contrast with the very limited benefits that similar approaches have generated for the study of mammalian mtDNA. Here we review the use of the genetic manipulation in mammalian mtDNA, its difficulty and the main types of mutants accumulated in the past 30 years and the information derived from them. We also point out the need for a substantial improvement in this field in order to obtain new tools for functional genetic studies and for the generation of animal models of mtDNA-linked diseases.

Mitochondrial genome evolution and the origin of eukaryotes

Recent results from ancestral (minimally derived) protists testify to the tremendous diversity of the mitochondrial genome in various eukaryotic lineages, but also reinforce the view that mitochondria, descendants of an endosymbiotic alpha-Proteobacterium, arose only once in evolution. The serial endosymbiosis theory, currently the most popular hypothesis to explain the origin of mitochondria, postulates the capture of an alpha-proteobacterial endosymbiont by a nucleus-containing eukaryotic host resembling extant amitochondriate protists. New sequence data have challenged this scenario, instead raising the possibility that the origin of the mitochondrion was coincident with, and contributed substantially to, the origin of the nuclear genome of the eukaryotic cell. Defining more precisely the alpha-proteobacterial ancestry of the mitochondrial genome, and the contribution of the endosymbiotic event to the nuclear genome, will be essential for a full understanding of the origin and evolution of the eukaryotic cell as a whole.

The petite mutation in yeasts: 50 years on

Fifty years ago it was reported that baker's yeast, Saccharomyces cerevisiae, can form "petite colonie" mutants when treated with the DNA-targeting drug acriflavin. To mark the jubilee of studies on cytoplasmic inheritance, a review of the early work will be presented together with some observations on current developments. The primary emphasis is to address the questions of how loss of mtDNA leads to lethality (rho 0-lethality) in petite-negative yeasts and how S. cerevisiae tolerates elimination of mtDNA. Recent investigation have revealed that rho 0-lethality can be suppressed by specific mutations in the alpha, beta, and gamma subunits of the mitochondrial F1-ATPase of the petite-negative yeast Kluyveromyces lactis and by the nuclear ptp alleles in Schizosaccharomyces pombe. In contrast, inactivation of genes coding for F1-ATPase alpha and beta subunits and disruption of AAC2, PGS1/PEL1, and YME1 genes in S. cerevisiae convert this petite-positive yeast into a petite-negative form. Studies on nuclear genes affecting dependence on mtDNA have provided important insight into the functions provided by the mitochondrial genome and the maintenance of structural and functional integrity of the mitochondrial inner membrane.

The universality of bioenergetic disease. Age-associated cellular bioenergetic degradation and amelioration therapy

During the present century there has been a dramatic change in life expectancy in advanced societies, now exceeding 80 years. As distinct from life expectancy, life potential is said to be at least 120 years, so that the continuing increase in knowledge has the potential for further major changes in the survival of humans conceivably in the near future. This presentation will be concerned with one aspect of the development of biomedical advances related in part to a concept of an "age-related universality of bioenergetic disease," and its potential amelioration and proposed impact on age-related disease and lifestyle. Aging is a complex biological process associated with a progressive decline in the physiological and biochemical performance of individual tissues and organs, leading to age-associated disease and senescence. Consideration of the progressive accumulation of mitochondrial DNA mutation with age and the tissue/cellular bioenergy decline associated with the aging process has led us to the proposal of a "universality of bioenergetic disease" and the potential for a redox therapy for the condition. This concept envisages that a tissue-bioenergetic decline will be intrinsic to various diseases of the aged and thereby contribute to their pathology, in particular, heart failure, degenerative brain disease, muscle and vascular diseases, as well as other syndromes. The information and concepts embodied in this proposal will be reviewed under the following headings: (1) mitochondrial DNA deletion mutation in some tissue is very extensive and shows mosaicism; (2) age-associated tissue/cellular bioenergy mosaic closely corresponds to the mtDNA profile; (3) cellular bioenergy as a function of mitochondrial bioenergy, glycolysis, and plasma membrane oxidoreductase; (4) redox therapy for the reenergization of cells, tissues, and whole organs. A redox therapy based on coenzyme Q10 has demonstrated profound alteration in heart function of old rats; no significant effect was observed with young rats.

The universality of bioenergetic disease and amelioration with redox therapy

Overt mitochondrial diseases associated with mitochondrial DNA mutations are characterized by a decline in mitochondrial respiratory function. Similarly, a progressive decline in mitochondrial respiratory function associated with mitochondrial DNA mutations is clearly evidenced in aged human subjects. This communication is concerned with the development of a rat model for the study of bioenergy decline associated with the ageing process and overt mitochondrial diseases. The model involves the treatment of young rats with AZT to induce skeletal and cardiac myopathies. It has shown that there is a decline in soleus muscle function in vivo and that this decline is mirrored in the capacity of heart sub-mitochondrial particles to maintain bioenergy function. Coenzyme Q10 and several analogs were administered with AZT as potential therapeutics for the re-energization of affected tissues. Coenzyme Q10 and especially decyl Q were found to be therapeutically beneficial by both in vivo improvement in soleus muscle function and in vitro cardiac mitochondrial membrane potential capacity. Sub-mitochondrial particles were also prepared from heart mitochondria of young and aged rats. The particles prepared from the aged rats were found to have a decreased ability to maintain membrane potential as compared to those derived from the young rats.

Mitochondrial DNA mutation associated with aging and degenerative disease

Previous theories of aging based on somatic mutation neglected mtDNA, which has a high propensity for mutational error. Knowledge of yeast mtDNA mutations and their functional effects, and of human mtDNA mutations identified in the mitochondrial cytopathies, provides for a concept of aging based on the cumulative effect of mutations affecting human mtDNA. An essential feature of this concept is heteroplasmy, representing mixtures of normal and mutant mtDNA at the cellular and mitochondrial level, resulting in a "tissue mosaic" of focal bioenergetic deficits. Direct evidence for the concept is provided by (i) focal loss of staining for mitochondrially encoded enzymes, such as cytochrome c oxidase, in tissues of aged individuals (humans and rats) and (ii) an age-related increase in deletional mutations in mtDNA demonstrable by application of the polymerase chain reaction to DNA templates from individuals of different ages.

Mitochondrial DNA mutations as an important contributor to ageing and degenerative diseases

The human mitochondrial genome is very small and economically packed; the expression of the whole genome is essential for the maintenance of mitochondrial bioenergetic function. Mutation occurs at a much higher rate in the mitochondrial DNA (mtDNA) than in chromosomal DNA. Transient heteroplasmy of mtDNA occurs after a mutational event; the random pattern of cytoplasmic segregation that occurs during subsequent growth gives rise to a mosaic of cells. The variable proportion of mutant mitochondrial genomes per cell results in cells with a range of bioenergetic capacities. It is proposed that the accumulation of mitochondrial mutations and the subsequent cytoplasmic segregation of these mutations during life is an important contributor both to the ageing process and to several human degenerative diseases. Replacement therapy and pharmacological support may be possible for the amelioration of such disorders by means of appropriate redox compounds. Moreover, new compounds with desired redox potentials can be rationally designed for clinical use.

Anti-mitochondrial autoantibodies of primary biliary cirrhosis as a novel probe in the study of the biosynthetic regulation of the yeast 2-oxo acid dehydrogenase complexes

Autoantibodies present in the disease primary biliary cirrhosis react by immunoblotting with four major yeast mitochondrial antigens of 58 kDa, 55 kDa, 52 kDa and 45 kDa, tentatively identified as the lipoate acetyl transferases (E2) of the pyruvate dehydrogenase, component X of E2 pyruvate dehydrogenase, E2 of 2-oxo glutarate dehydrogenase and E2 of branched-chain 2-oxo acid dehydrogenase complexes respectively. The synthesis of these antigens is sensitive to catabolite repression. The reactive antigens are present in mit- mutants of yeast which have specific defects in the mitochondrial apocytochrome b, cytochrome oxidase subunit II and H+ -ATPase subunits 8 and 9, and in mtDNA-less rho O petite mutants, but a significant increase in the sensitivity to catabolite repression was observed in these mutants in particular in the mtDNA-less strains.

Effects of mycoplasma contamination on phenotypic expression of mitochondrial mutants in human cells

HeLa cells sensitive to the mitochondrial protein synthesis inhibitors erythromycin (ERY) and chloramphenicol (CAP) and HeLa variants resistant to the effects of these drugs were purposefully infected with drug-sensitive and -resistant mycoplasma strains. Mycoplasma hyorhinis and the ERY-resistant strain of Mycoplasma orale, MO-ERYr, did not influence the growth of HeLa and ERY-resistant ERY2301 cells in the presence or absence of ERY. M. hyorhinis also did not affect the growth of HeLa and CAP-resistant Cap-2 cells in the presence or absence of CAP. However, both HeLa and Cap-2 cells infected with the CAP-resistant strain of M. hyorhinis, MH-CAPr, were more sensitive to the cytotoxic effect of CAP. This may be due to the glucose dependence of the cells, which was compromised by the increased utilization of glucose by MH-CAPr in these infected cell cultures. In vitro protein synthesis by isolated mitochondria was significantly altered by mycoplasma infection of the various cell lines. A substantial number of mycoplasmas copurified with the mitochondria, resulting in up to a sevenfold increase in the incorporation of [3H]leucine into the trichloroacetic acid-insoluble material. More importantly, the apparent drug sensitivity or resistance of mitochondrial preparations from mycoplasma-infected cells reflected the drug sensitivity or resistance of the contaminating mycoplasmas. These results illustrate the hazards in interpreting mitochondrial protein synthesis data derived from mycoplasma-infected cell lines, particularly putative mitochondrially encoded mutants resistant to inhibitors of mitochondrial protein synthesis.

Inherent resistance of HeLa cell derivatives to paromomycin

The human tumor-derived cell line HeLa S3 and nuclear and mitochondrial gene mutants derived from it are resistant to the aminoglycoside antibiotic, paromomycin (PAR). Other carcinoma-derived cells, SV40-transformed cells, and four human diploid fibroblast cell lines are all sensitive to PAR. Sensitivity is dependent on cell density, and at cell numbers greater than 400/cm2 sensitive cells will proliferate in PAR. The resistance to PAR is inherited in a dominant manner in cell-to-cell fusion hybrids, but is not transferred in cytoplast-to-cell fusions. PAR resistance is therefore encoded by a nuclear gene(s). Resistance to PAR is not caused by changes in the response of mitochondrial or cytoplasmic protein synthesis to PAR in vitro. The uptake of PAR is similar in resistant and sensitive cells, and dimethyl sulfoxide does not render resistant cells more sensitive. Thus, HeLa cell PAR resistance is unlike previously reported ribosomal mutations and may derive from differences in the intracellular metabolism of PAR.

The oli1 gene and flanking sequences in mitochondrial DNA of Saccharomyces cerevisiae: the complete nucleotide sequence of a 1.35 kilobase petite mitochondrial DNA genome covering the oli1 gene

As part of our genetic and molecular analysis of mutants of Saccharomyces cerevisiae affected in the oli1 gene (coding for mitochondrial ATPase subunit 9) we have determined the complete nucleotide sequence of the mtDNA genome of a petite (23-3) carrying this gene. Petite 23-3 (1,355 base pairs) retains a continuous segment of the relevant wild-type (J69-1B) mtDNA genome extending 983 nucleotides upstream, and 126 nucleotides downstream, of the 231 nucleotide oli1 coding region. There is a 15-nucleotide excision sequence in petite 23-3 mtDNA which occurs as a direct repeat in the wild-type mtDNA sequence flanking the unique petite mtDNA segment (interestingly, this excision sequence in petite 23-3 carries a single base substitution relative to the parental wild-type sequence). The putative replication origin of petite 23-3 is considered to be in its single G,C rich cluster, which differs in just one nucleotide from the standard oriS sequence. The DNA sequences in the intergenic regions flanking the oli1 gene of strain J69-1B (and its derivatives) have been systematically compared to those of the corresponding regions of mtDNA in strains derived from the D273-10B parent (sequences from the laboratory of A. Tzagoloff). The nature and distribution of the sequence divergences (base substitutions, base deletions or insertions, and more extensive rearrangements) are considered in the context of functions associated with mitochondrial gene expression which are ascribed to specialized sequences in the intergenic regions of the yeast mitochondrial genome.

Characterization of chloramphenicol- and 8-azaguanine-resistant mutants isolated from a continuous rat-liver epithelial cell line

2 non-tumorigenic, chloramphenicol- and 8-azaguanine-resistant strains have been isolated from the rat-liver cell line K-22, by a 2-step mutagenesis procedure. Their chromosome composition and growth properties have been characterized. Failure of chloramphenicol to inhibit mitochondrial protein synthesis in one of the clones, F1, strongly suggests that resistance to the antibiotic in this strain is due to a mutation in mitochondrial DNA.

Mitochondrial variants of Neurospora intermedia from nature

From a sample of 122 natural isolates of Neurospora intermedia collected recently from around the world, five variants had erratic stop-start growth patterns reminiscent of the phenotype of "stopper" laboratory extranuclear mutants of Neurospora crassa. Like laboratory isolated mutants, the natural "stopper" variants were sterile as protoperithecial parents and transmitted the variant growth phenotypes very inefficiently, if at all, as male parents. Heterokaryon tests could not be made because of strain incompatibilities. Four of the variants have mitochondrial cytochrome aa3 and b deficiencies. These four variants are all defective in mitochondrial ribosome assembly and have abnormal ratios of large to small subunits. Restriction enzyme analyses revealed some similarity of N. intermedia to N. crassa mtDNA. One normal and four variant strains had additional DNA in comparison to a standard normal strain. Cumulatively, the results indicate that the genetic alterations which cause stopper phenotypes of these natural isolates of N. intermedia are of mitochondrial rather than nuclear origin.

Transport of newly synthesized proteins into mitochondria - a review

This review examines the mechanism of translocation of cytoplasmically synthesized proteins into mitochondria. Approximately 10% of the mitochondrial proteins are synthesized within the organelles while most mitochondrial proteins are coded for by nuclear genes and synthesized on cytoplasmic ribosomes. Those mitochondrial proteins synthesized on cytoplasmic ribosomes have to be transferred at some point into one of the mitochondrial compartments, a process which would require their insertion through one or both mitochondrial membranes. Data accumulated during the past five years indicate that the cytoplasmically synthesized mitochondrial proteins are synthesized on free polysomes then released into the cytoplasm. Most of the proteins examined so far are synthesized in the cytoplasm as larger precursors whose conformations may differ from the conformations of their respective mature forms. These precursor proteins become translocated into mitochondrial post-translationally and processed to their mature forms either during or immediately following translocation into the organelles. The translocation step appears to require mitochondrial ATP. Some processing activities have been localized in the matrix fractions of mitochondria from liver and yeast and they appear to be associated with soluble endopeptidases which act selectively on precursors of mitochondrial proteins. Although it is not clear how the precursor proteins interact with or recognize mitochondrial membranes, studies in yeast indicate that the interactions occur at specific regions on the other mitochondrial membranes.

Assignment of an oligomycin-resistance locus to human chromosome 10

An oligomycin-resistant variant of human fibrosarcoma HT1080 was isolated and characterized as nuclear and codominant. The mutant was stable, was not cross-resistant to respiratory inhibitors, and it contained a mitochondrial ATPase which was less sensitive to oligomycin. Hybrids formed between the human mutant and a mouse cell line expressed the resistance phenotype. By a detailed karyotypic analysis of these hybrids using trypsin-Giemsa banding it was found that resistance to oligomycin correlated with the retention of two human chromosomes 10. The hybrid lines contained only mouse mitochondrial DNA as shown by analyses of mitochondrially synthesized proteins and mitochondrial DNA. The study assigns an ATPase oligomycin-resistance locus to human chromosome 10 and suggests that mouse and human subunits can combine in a functional enzyme complex.

Transfer of heritable properties by cell hybridization: specificity and the role of selective pressure

Enucleated chloramphenicol (CAP) resistant mouse L-cells (LEA-2A) were fused with the mouse hepatoma cells (BW1J). The resultant cybrids expressed CAP resistance (the property used in selection of the cybrids), and also expressed the hepatic-specific functions of the BW1J parent. Hybrids between these same cells, on the other hand, exhibited chloramphenicol resistance and extinction of the hepatoma-specific properties. Cybrids were also prepared between enucleated rat hepatoma cells (FT-2) and mouse erythroleukemia cells (C19TK). The resultant cybrids selected in tyrosine-free medium expressed phenylalanine hydroxylase, an enzyme normally appeared to be the result of activation of the previously silent gene of the C19TK cells. These cybrids, however, did not express any other liver-specific functions present in the FT-2 cytoplast donor. These experiments suggest that the transfer of heritable properties by cell cybridization is selection specific and that activation or extinction observed in hybrids may not occur in cybrids of the same cells.

Effects of mycoplasma contamination on phenotypic expression of mitochondrial mutants in human cells

HeLa cells sensitive to the mitochondrial protein synthesis inhibitors erythromycin (ERY) and chloramphenicol (CAP) and HeLa variants resistant to the effects of these drugs were purposefully infected with drug-sensitive and -resistant mycoplasma strains. Mycoplasma hyorhinis and the ERY-resistant strain of Mycoplasma orale, MO-ERYr, did not influence the growth of HeLa and ERY-resistant ERY2301 cells in the presence or absence of ERY. M. hyorhinis also did not affect the growth of HeLa and CAP-resistant Cap-2 cells in the presence or absence of CAP. However, both HeLa and Cap-2 cells infected with the CAP-resistant strain of M. hyorhinis, MH-CAPr, were more sensitive to the cytotoxic effect of CAP. This may be due to the glucose dependence of the cells, which was compromised by the increased utilization of glucose by MH-CAPr in these infected cell cultures. In vitro protein synthesis by isolated mitochondria was significantly altered by mycoplasma infection of the various cell lines. A substantial number of mycoplasmas copurified with the mitochondria, resulting in up to a sevenfold increase in the incorporation of [3H]leucine into the trichloroacetic acid-insoluble material. More importantly, the apparent drug sensitivity or resistance of mitochondrial preparations from mycoplasma-infected cells reflected the drug sensitivity or resistance of the contaminating mycoplasmas. These results illustrate the hazards in interpreting mitochondrial protein synthesis data derived from mycoplasma-infected cell lines, particularly putative mitochondrially encoded mutants resistant to inhibitors of mitochondrial protein synthesis.

Characterization of mitochondrial DNA in chloramphenicol-resistant interspecific hybrids and a cybrid

We have examined the restriction endonuclease cleavage patterns exhibited by the mitochondrial DNAs (mtDNA) of four chloramphenicol-resistant (CAPR) human x mouse hybrids and one CAPR cybrid derived from CAPR HeLa cells and CAPS mouse RAG cells. Restriction fragments of mtDNAs were separated by electrophoresis and transferred by the Southern technique to diazobenzyloxymethyl paper. The covalently bound DNA fragments were hybridized initially with 32P-labeled complementary RNA (cRNA) prepared from human mtDNA and, after removal of the human probe, hybridized with mouse [32P]cRNA prepared from mouse mtDNA. Three hybrids which preferentially segregated human chromosomes and the cybrid exhibited mtDNA fragments indistinguishable from mouse cells. One hybrid, ROH8A, which exhibited "reverse" chromosome segregation, contained only human mtDNA. The pattern of chromosome and mtDNA segregation observed in these hybrids and the cybrid support the hypothesis that a complete set of human chromosomes must be retained if a human-mouse hybrid is to retain human mitochondrial DNA.

Restoration of fertility in cytoplasmic male sterile (CMS) Nicotiana Sylvestris by fusion with X-irradiated N. tabacum protoplasts

Restoration of male fertility was achieved by fusing protoplasts from male sterile (CMS) Nicotiana sylvestris plants with X-irradiated protoplasts derived from fertile N. tabacum plants. The CMS N. sylvestris plants were derived from a previous somatic hybridization experiment and contained alien (Line 92) cytoplasm. About one quarter of the regenerated plants were found to be cybrids. i.e. they consisted of N. sylvestris nuclei combined with all or some components of N. tabacum cytoplasm. In one half of these cybrids male fertility was restored to different levels. The chloroplasts of the two parental donors differ in respect to tentoxin sensitivity: chloroplasts of CMS N. sylvestris are sensitive while those of N. tabacum are insensitive. It could therefore be demonstrated that there was an independent segregation of chloroplast type and male fertility/sterility: several somatic cybrids were male fertile but tentoxin sensitive and others were tentoxin insensitive yet they were male sterile. Only in about one half of the somatic cybrids was male fertility restored together with restoration to tentoxin insensitivity.

Erythromycin resistance in mouse L cells

The sensitivity of mouse cell lines in culture to the macrolide antibiotic, erythromycin stearate, was investigated. Both resistant and sensitive lines were found. Experiments indicated that in sensitive cells erythromycin stearate inhibits mitochondrial protein synthesis. Mutants resistant to erythromycin stearate were selected from the line LM(TK-), and these are also less sensitive to other macrolide antibiotics such as carbomycin and spiramycin. Attempts to transfer the erythromycin resistance of either the mutants or naturally resistant lines by fusion of cytoplasts with sensitive cells were unsuccessful, and it is concluded that resistance to erythromycin stearate is controlled by nuclear genetic factors.

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.

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.