Expression in Saccharomyces cerevisiae of a gene associated with cytoplasmic male sterility from maize: respiratory dysfunction and uncoupling of yeast mitochondria

Novel Antarctic yeast adapts to cold by switching energy metabolism and increasing small RNA synthesis

The novel extremophilic yeast Rhodotorula frigidialcoholis, formerly R. JG1b, was isolated from ice-cemented permafrost in University Valley (Antarctic), one of coldest and driest environments on Earth. Phenotypic and phylogenetic analyses classified R. frigidialcoholis as a novel species. To characterize its cold-adaptive strategies, we performed mRNA and sRNA transcriptomic analyses, phenotypic profiling, and assessed ethanol production at 0 and 23 °C. Downregulation of the ETC and citrate cycle genes, overexpression of fermentation and pentose phosphate pathways genes, growth without reduction of tetrazolium dye, and our discovery of ethanol production at 0 °C indicate that R. frigidialcoholis induces a metabolic switch from respiration to ethanol fermentation as adaptation in Antarctic permafrost. This is the first report of microbial ethanol fermentation utilized as the major energy pathway in response to cold and the coldest temperature reported for natural ethanol production. R. frigidialcoholis increased its diversity and abundance of sRNAs when grown at 0 versus 23 °C. This was consistent with increase in transcription of Dicer, a key protein for sRNA processing. Our results strongly imply that post-transcriptional regulation of gene expression and mRNA silencing may be a novel evolutionary fungal adaptation in the cryosphere.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Heterologous expression of an alternative oxidase from Moniliophthora perniciosa in Saccharomyces cerevisiae: Antioxidant function and in vivo platform for the study of new drugs against witches' broom disease

The fungus Moniliophthora perniciosa is the causal agent of witches' broom disease (WBD), one of the most devastating diseases of cacao, the chocolate tree. Many strategies to control WBD have been tested so far, including the use of agrochemicals such as the strobilurins. Strobilurins are fungicides of the QoI family, and they are used in the control of a wide array of fungal diseases in many different crops, including cereals, field crops, fruits, tree nuts, and vegetables. These drugs act by specifically inhibiting fungal respiration at the Qo site of complex III, which is a component of the main mitochondrial respiratory chain. However, M. perniciosa is resistant to this family of chemicals. It has been postulated that this resistant phenotype is, at least in part, a result of the strong ability of this fungus to counteract the oxidative stress generated by the impairment of the main mitochondrial respiratory chain, through the activation of an alternative oxidase (Mp-AOX). To test this hypothesis, we expressed functional mitochondria-localized Mp-AOX in the model yeast Saccharomyces cerevisiae. We demonstrated that heterologous expression of Mp-AOX strongly inhibits hydrogen peroxide production by mitochondria. It also diminishes the total cell amount of oxidized glutathione (GSSG), resulting in a fifty-fold higher GSH/GSSG ratio in cells expressing Mp-AOX than in wild type cells. In addition, Mp-AOX activity decreases yeast growth rate and leads to low biomass production. Therefore, we propose the use of this heterologous expression system to direct the development of new inhibitors of fungal AOX by comparing the differences in optical density of Mp-AOX-expressing cells in the presence and absence of potential AOX inhibitors. Together, our results confirm the antioxidant role of Mp-AOX and provide an in vivo platform to be used in the screening of new fungicides based on Mp-AOX inhibition.

Heterologous expression of mitochondrial nicotinamide adenine dinucleotide transporter (Ndt1) from Aspergillus fumigatus rescues impaired growth in Δndt1Δndt2 Saccharomyces cerevisiae strain

Our understanding of nicotinamide adenine dinucleotide mitochondrial transporter 1 (Ndt1A) in Aspergillus fumigatus remains poor. Thus, we investigated whether Ndt1A could alter fungi survival. To this end, we engineered the expression of an Ndt1A-encoding region in a Δndt1Δndt2 yeast strain. The resulting cloned Ndt1A protein promoted the mitochondrial uptake of nicotinamide adenine dinucleotide (NAD⁺), generating a large mitochondrial membrane potential. The NAD⁺ carrier utilized the electrochemical proton gradient to drive NAD⁺ entrance into mitochondria when the mitochondrial membrane potential was sustained by succinate. Its uptake has no impact on oxidative stress, and Ndt1A expression improved growth and survival of the Δndt1Δndt2 Saccharomyces cerevisiae strain.

Proteomic analysis reveals strong mitochondrial involvement in cytoplasmic male sterility of pepper (Capsicum annuum L.)

Although cytoplasmic male sterility (CMS) is widely used for developing pepper hybrids, its molecular mechanism remains unclear. In this study, we used a high-throughput proteomics method called label-free to compare protein abundance across a pepper CMS line (A-line) and its isogenic maintainer line (B-line). Data are available via ProteomeXchange with identifier PXD006104. Approximately 324 differentially abundant protein species were identified and quantified; among which, 47 were up-accumulated and 140 were down-accumulated in the A-line; additionally, 75 and 62 protein species were specifically accumulated in the A-line and B-line, respectively. Protein species involved in pollen exine formation, pyruvate metabolic processes, the tricarboxylic acid cycle, the mitochondrial electron transport chain, and oxidative stress response were observed to be differentially accumulated between A-line and B-line, suggesting their potential roles in the regulation of pepper pollen abortion. Based on our data, we proposed a potential regulatory network for pepper CMS that unifies these processes.

BIOLOGICAL SIGNIFICANCE

Artificial emasculation is a major obstacle in pepper hybrid breeding for its high labor cost and poor seed purity. While the use of cytoplasmic male sterility (CMS) in hybrid system is seriously frustrated because a long time is needed to cultivate male sterility line and its isogenic restore line. Transgenic technology is an effective and rapid method to obtain male sterility lines and its widely application has very important significance in speeding up breeding process in pepper. Although numerous studies have been conducted to select the genes related to male sterility, the molecular mechanism of cytoplasmic male sterility in pepper remains unknown. In this study, we used the high-throughput proteomic method called "label-free", coupled with liquid chromatography-quadrupole mass spectrometry (LC-MS/MS), to perform a novel comparison of expression profiles in a CMS pepper line and its maintainer line. Based on our results, we proposed a potential regulated protein network involved in pollen development as a novel mechanism of pepper CMS.

Cloning and functional expression of the mitochondrial alternative oxidase ofAspergillus fumigatusand its induction by oxidative stress

Aspergillus fumigatus possesses a branched mitochondrial electron transport chain, with both cyanide-sensitive and -insensitive oxygen-consumption activities. Mitochondrial reactive oxygen species mediate signaling for alternative oxidase (AOX) expression. A 1173 bp-long Afaox gene encoding a 40 kDa protein has been cloned and identified. Recombinant constructs containing the Afaox ORF were transformed into Escherichia coli and Saccharomyces cerevisiae for heterologous expression. In A. fumigatus, AOX activity and mRNA expression were both induced with menadione or paraquat, suggesting an important role of AOX under oxidative stress. Therefore, positive transformants showed a cyanide-resistant and salicylhydroxamic acid-sensitive respiration, whereas in control cells the oxygen uptake was completely inhibited after KCN addition.

Functional expression of the maize mitochondrial URF13 down-regulates galactose-induced GAL1 gene expression in Saccharomyces cerevisiae

Genes for the enzymes that metabolize galactose in Saccharomyces cerevisiae are strongly induced by galactose and tightly repressed by glucose. Because glucose also represses mitochondrial activity, we examined if derepression of the GAL1 galactokinase gene requires physiologically active mitochondria. The effect of mitochondria on the expression of GAL1 was analyzed by a novel approach in which the activity of the organelles was altered by functional expression of URF13, a mitochondrial protein unique to the Texas-type cytoplasmic male sterility phenotype in maize. Mitochondrial targeting and functional expression of the URF13 protein in yeast result in a decrease of the mitochondrial membrane potential similar to those observed in cells treated with mitochondrial inhibitors such as antimycin A or sodium azide. Activation of URF13 in galactose-induced cells results in the inhibition of GAL1 expression in the absence of repressing concentrations of glucose. Our data reveal the existence of a regulatory pathway that connects the derepression of the GAL1 gene with mitochondrial activity.

Role of Light and Malate in the Decreased Sensitivity of cms-T Cytoplasm Maize Leaves to Bipolaris maydis Race T Toxin

ABSTRACT Leaf segments from Texas male sterile (cms-T) cytoplasm maize isolines exposed to light (50 muM s(-1) m(-2)) for 8 h or more before or after being infiltrated with the Bipolaris maydis race T toxin (T-toxin) leaked significantly less electrolytes when immersed in distilled water (DW) for 24 to 48 h than did dark-treated leaf segments. No comparable effect of light on toxin-induced electrolyte leakage was observed with normal (N) cytoplasm isolines. Toxin-treated cms-T leaf segments incubated in DW for three consecutive 12-h periods of alternating light and dark showed significantly greater electrolyte leakage than leaf segments incubated in continuous light for 36 h and significantly less leakage than segments incubated in continuous dark for 36 h.Exposure of cms-T, but not N, cytoplasm leaves to 25 or 50 muM malic acid decreased their sensitivity to T-toxin in the dark to a level similar to that observed when leaves were incubated in the light without malic acid. The potency of T-toxin appeared to be unaffected by its exposure to light. The loss of electrolytes from T-toxin-treated cms-T cytoplasm leaf segments was at approximately the level seen with light or malate when 25 muM H(2)O(2) was added to the DW bathing solution. Evaluation of the data points to the possibility that H(2)O(2) might be involved with the altered sensitivity of cms-T cytoplasm leaves to T-toxin caused by either light or malate.

Asymmetrical distribution of cardiolipin in yeast inner mitochondrial membrane triggered by carbon catabolite repression

Transmembrane asymmetry of cardiolipin in yeast was monitored during the switch from fermentative to gluconeogenic growth and the reverse. As soon as cells used ethanol as an electron donor to produce ATP by oxidative phosphorylation, rapid and abundant cardiolipin synthesis was observed on the matrix side of the inner mitochondrial membrane followed by a transverse rearrangement between the two leaflets. The cardiolipin distribution changed from about 20:80 (in/out) to 70:30 (in/out), and after translocation towards the outer leaflet it finally became 37:63 (in/out). At the same time, cytochrome c oxidase activity remained stable, then increased as a possible result of the topographical rearrangement. During the reverse process from gluconeogenic to fermentative growth, the amount of cardiolipin rapidly decreased by half, its bilayer distribution apparently changing to a monolayer organization before the 20:80 (in/out) asymmetry of repressed cells was re-established. Experimental impairment of cardiolipin topography by antibiotic inhibition of gene expression or in situ dissipation of mitochondrial membrane potential produced data that prove that the amount and transmembrane distribution of the phospholipid are two specific parameters of the mitochondrial inner membrane organization in both fermentative (2.2 fmol/cell and 20:80, in/out) and gluconeogenic (4.2 fmol/cell and 37:63, in/out) growing yeast cells. Finally, the inner mitochondrial membrane topography of cardiolipin appeared to be closely associated with the transmembrane redox potential.

In vitro mutagenesis of the mitochondrial leucyl tRNA synthetase of Saccharomyces cerevisiae shows that the suppressor activity of the mutant proteins is related to the splicing function of the wild-type protein

The NAM2 gene of Saccharomyces cerevisiae encodes the mitochondrial leucyl tRNA synthetase (mLRS), which is necessary for the excision of the fourth intron of the mitochondrial cytb gene (bI4) and the fourth intron of the mitochondrial coxI gene (aI4), as well as for mitochondrial protein synthesis. Some dominant mutant alleles of the gene are able to suppress mutations that inactivate the bI4 maturase, which is essential for the excision of the introns aI4 and bI4. Here we report mutagenesis studies which focus on the splicing and suppressor functions of the protein. Small deletions in the C-terminal region of the protein preferentially reduce the splicing, but not the synthetase activity; and all the C-terminal deletions tested abolish the suppressor activity. Mutations which increase the volume of the residue at position 240 in the wild-type mLRS without introducing a charge, lead to a suppressor activity. The mutant 238C, which is located in the suppressor region, has a reduced synthetase activity and no detectable splicing activity. These data show that the splicing and suppressor functions are linked and that the suppressor activity of the mutant alleles results from a modification of the wild-type splicing activity.

Mutator-induced mutations of the rf1 nuclear fertility restorer of T-cytoplasm maize alter the accumulation of T-urf13 mitochondrial transcripts

Dominant alleles of the rf1 and rf2 nuclear-encoded fertility restorer genes are necessary for restoration of pollen fertility in T-cytoplasm maize. To further characterize fertility restoration mediated by the Rf1 allele, 123,500 gametes derived from plants carrying the Mutator transposable element family were screened for rf1-mutant alleles (rf1-m) Four heritable rf1-m alleles were recovered from these populations. Three rf1-m alleles were derived from the progenitor allele Rf1-IA153 and one was derived from Rf1-Ky21. Cosegregation analysis revealed 5.5- and 2.4-kb Mu1-hybridizing EcoRI restriction fragments in all of the male-sterile and none of the male-fertile plants in families segregating for rf1-m3207 and rf1-m3310, respectively. Mitochondrial RNA gel blot analyses indicated that all four rf1-m alleles in male-sterile plants cosegregated with the altered steady-state accumulation of 1.6- and 0.6-kb T-urf13 transcripts, demonstrating that these transcripts are Rf1 dependent. Plants carrying a leaky mutant, rf1-m7323, revealed variable levels of Rf1-associated, T-urf13 transcripts and the degree of pollen fertility. The ability to obtain rf1-m derivatives from Rf1 indicates that Rf1 alleles produce a functional gene product necessary for the accumulation of specific T-urf13 transcripts in T-cytoplasm maize.

Molecular interactions ofBipolaris maydisT-toxin and maize

The toxins (T-toxins) produced by the fungal pathogens Bipolaris maydis race T (BmT) and Phyllosticta maydis (Pm) target the mitochondrial receptor, URF13, in maize (Zea mays L.) plants containing the Texas male-sterile cytoplasm (cms-T). URF13, a 13-kDa protein, is the product of the maize mitochondrial gene T-urf13, which is found only in the mitochondrial genome of cms-T maize and is thought to be responsible for cytoplasmically inherited male sterility and disease susceptibility. Pm-toxin binds specifically to URF13 in a cooperative manner, and Pm- and BmT-toxins compete for the same, or overlapping, binding sites. The binding of T-toxin to URF13 causes rapid permcabilization of the inner mitochondrial membrane, which results in leakage of NAD+and other ions from the matrix. A pore consisting of at least six transmembrane α-helices is required for NAD+leakage. Cross-linking experiments showed that URF13 oligomers are present in the mitochondrial membrane. A model of the secondary structure of URF13 proposes that each monomer contains three transmembrane α-helices. Studies combining site-directed mutagenesis and chemical cross-linking of URF13 expressed by Escherichia coli cells indicate that the oligomers are composed of a central core of helices II that line the center of the URF13 pores. Key words: maize cytoplasmic male sterility, URF13, mitochondrial pores, T-toxin receptor, Bipolaris maydis race T, Phyllosticta maydis, Helminthosporium maydis.

URF13, a ligand-gated, pore-forming receptor for T-toxin in the inner membrane of cms-T mitochondria

URF13 is the product of a mitochondrial-encoded gene (T-urf13) found only in maize plants containing the Texas male-sterile cytoplasm (cms-T), and it is thought to be responsible for both cytoplasmic male sterility and the susceptibility of cms-T maize to the fungal pathogens Bipolaris maydis race T and Phyllosticata maydis. Mitochondria isolated from cms-T maize are uniquely sensitive to pathotoxins (T-toxin) produced by these fungi and to methomyl (a commercial insecticide). URF13 acts as a receptor that specifically binds T-toxin to produce hydrophilic pores in the inner mitochondrial membrane. When expressed in Escherichia coli cells, URF13 also forms hydrophilic pores in the plasma membrane if exposed to T-toxin or methomyl. Topological studies established that URF13 contains three membrane-spanning alpha-helices, two of which are amphipathic and can contribute to pore formation. Chemical cross-linking of URF13 was used to demonstrate the existence of URF13 oligomers in cms-T mitochondria and E. coli cells. The ability of the carboxylate-specific reagent, N,N'-dicyclohexycarbodiimide, to cross-link URF13 was used in conjunction with site-directed mutagenesis to establish that the URF13 tetramer has a central core consisting of a four-alpha-helical bundle which undergoes a conformational change after interaction with T-toxin or methomyl. Overall, the experimental evidence indicates that URF13 functions as a ligand-gated, pore-forming T-toxin receptor in cms-T mitochondria.

The relationship between the mitochondrial gene T-urf13 and fungal pathotoxin sensitivity in maize

Mitochondria isolated from maize containing cms-T cytoplasm are specifically sensitive to pathotoxins (T-toxins) produced by the fungi Bipolaris maydis race T and Phyllosticta maydis. T-toxins interact with a 13 kDa membrane-bound toxin receptor protein, URF13, to produce hydrophillic pores in the membrane. Expression of URF13 in Escherichia coli produces bacterial cells that form hydrophillic pores in the plasma membrane when exposed to T-toxin or methomyl. Topological studies have established that URF13 contains three membrane-spanning alpha-helices, two of which are amphipathic and may contribute to pore formation. URF13 specifically binds T-toxin in a cooperative manner. Oligonucleotide-directed mutagenesis of URF13 led to the isolation of methomyl/T-toxin-resistant mutations at 39 separate positions throughout the URF13 primary sequence. Chemical cross-linking of URF13 demonstrated the presence of URF13 oligomers and established that the pore-forming species is oligomeric. The ability of the carboxylate-specific reagent, dicyclohexycarbodiimide to cross-link URF13 has been used in conjunction with site-directed mutagenesis to establish that the URF13 tetramer has a central core consisting of a four-alpha-helical bundle that may undergo a conformational change after T-toxin or methomyl binding. Experimental evidence indicates that URF13 acts as a ligand-gated, pore-forming T-toxin receptor.

Targeting the maize T-urf13 product into tobacco mitochondria confers methomyl sensitivity to mitochondrial respiration

The URF13 protein, which is encoded by the maize mitochondrial T-urf13 gene, is thought to be responsible for pathotoxin and methomyl sensitivity and male sterility. We have investigated whether T-urf13 confers toxin sensitivity and male sterility when expressed in another plant species. The coding sequence of T-urf13 was fused to a mitochondrial targeting presequence, placed under the control of the cauliflower mosaic virus 35S promoter, and introduced into tobacco by Agrobacterium tumefaciens-mediated transformation. Plants expressing high levels of URF13 were methomyl sensitive. Subcellular analysis indicated that URF13 is mainly associated with the mitochondria. Adding methomyl to isolated mitochondria stimulated NADH-linked respiration and uncoupled oxidative phosphorylation, indicating that URF13 was imported into the mitochondria, and conferred toxin sensitivity. Most control plants, which expressed the T-urf13c construct lacking the mitochondrial presequence, were methomyl sensitive and contained URF13 in a membrane fraction. Subcellular fractionation by sucrose gradient centrifugation showed that URF13 sedimented at several positions, suggesting the protein is associated with various organelles, including mitochondria. No methomyl effect was observed in isolated mitochondria, however, indicating that URF13 was not imported and did not confer toxin sensitivity to the mitochondria. Thus, URF13 confers toxin sensitivity to transgenic tobacco with or without import into the mitochondria. There was no correlation between the expression of URF13 and male sterility, suggesting either that URF13 does not cause male sterility in transgenic tobacco or that URF13 is not expressed in sufficient amounts in the appropriate anther cells.

Heat shock protein HSP60 can alleviate the phenotype of mitochondrial RNA-deficient temperature-sensitive mna2 pet mutants

mna2, which belongs to the class I temperature-sensitive pet mutants that lose mitochondrial (mt)RNA at restrictive temperature, was shown by complementation and sequence determination to correspond to the gene coding for HSP60. Both mna2-1 and mna2-2, the two available alleles of mna2, have conservative single amino acid substitutions in the HSP60 gene. Valine substitutes for an alanine (position 47) in mna2-1, and an isoleucine substitutes for a valine (position 77) in mna2-2. These substitutions result in defects in respiration and in steady-state mtRNA accumulation. Wild-type hsp60 alleviates the mtRNA phenotype completely, while partially relieving the respiratory deficiency.

MRG1-1, a dominant allele that confers methomyl resistance in yeast expressing the cytoplasmic male sterility T-urf13 gene from maize

We have previously described a eukaryotic heterologous expression system, with the urf13TW gene in yeast, which mimics the disease susceptibility associated with the Texas cytoplasmic male sterility in maize. This yeast model was used to isolate yeast nuclear mutants conferring methomyl resistance. The genetic strategy we have developed focused on screening for nuclear dominant yeast mutations which restore methomyl resistance. MRG1-1, a yeast nuclear dominant allele, was identified as a methomyl-resistance restorer. We have shown that methomyl resistance co-segregated with a pleiotropic phenotype in the heterozygous MRG1-1/MRG1 diploids, detectable even in the absence of the maize-derived mitochondrial protein and/or methomyl. We observed an increase in oxygen uptake, a significant decrease of the levels of cytochrome aa3, and a decrease in the growth yield. This phenotype is influenced by the carbon source and the results suggest a defect in the adaptation to the respiratory pathway in MRG1-1 yeast cells.

Mapping complementary genes in maize: positioning the rf1 and rf2 nuclear-fertility restorer loci of Texas (T) cytoplasm relative to RFLP and visible markers

There are three major groups of cytoplasmic male-sterile cytoplasms in maize; C (Charrua), S (USDA), and T (Texas). These cytoplasms can be classified by the unique nuclear genes that suppress the male-sterility effects of these cytoplasms and restore pollen fertility. Typically, plants that carry Texas (T) cytoplasm are male fertile only if they carry dominant alleles at two unlinked nuclear restorer loci,rf1 andrf2. To facilitate analysis of T-cytoplasm-mediated male sterility and fertility restoration, we have mappedrf1 andrf2 relative to closely-linked RFLP markers using five populations. Therf1 locus and/or linked visible markers were mapped in four populations; therf2 locus was mapped in two of the populations. Data from the individual populations were joined with the aid of JoinMap software. The resulting consensus maps placerf1 between umc97 and umc92 on chromosome 3 andrf2 between umc153 andsus1 on chromosome 9. Markers that flank therf1 andrf2 loci have been used to identify alleles atrf1 andrf2 in segregating populations. These analyses demonstrate the possibility of tracking separate fertility restorer loci that contribute to a single phenotype.

The NAM1 protein (NAM1p), which is selectively required for cox1, cytb and atp6 transcript processing/stabilisation, is located in the yeast mitochondrial matrix

The NAM1 nuclear gene was shown to control the stability and/or processing of mitochondrial transcripts of the cytochrome b, cytochrome oxidase subunit I and ATP synthase subunit VI genes [Groudinsky O., Bousquet I., Wallis M. G., Slonimski, P. P. & Dujardin G. (1993) Mol. Gen. Genet. 240, 419-427]. In order to better understand the mode of action of the NAM1 gene product, we have examined its intracellular fate. A fusion plasmid enabling bacterial over-expression of the corresponding protein-A-NAM1 cognate was constructed and subsequently employed as an antigen to raise polyclonal antibodies. These antibodies specifically recognise a 50-kDa protein which purifies along with the mitochondria and corresponds to NAM1p. Submitochondrial localisation experiments show that NAM1p is a soluble protein, located interior to the mitoplasts. Matricial location is a strong argument in favour of a direct interaction of NAM1p with particular mitochondrial transcripts and leads us to propose a model in which NAM1p could be an RNA-convoying protein stabilising and directing mitochondrial transcripts towards the inner face of the inner membrane where translation and assembly seem to occur.

Recovery of heritable, transposon-induced, mutant alleles of the rf 2 nuclear restorer of T-cytoplasm maize

T (Texas) cytoplasm is associated with a mitochondrial disruption that is phenotypically expressed during microsporogenesis resulting in male sterility. Restoration of pollen fertility in T-cytoplasm maize is controlled by dominant alleles at two unlinked, complementary, nuclear-encoded genes, rf1 and rf2. As a first step in the molecular isolation of the rf2 gene, 178,300 gametes derived from plants that carried the Mutator, Cy or Spm transposon families were screened for rf2 mutant alleles (rf2-m) via their inability to restore pollen fertility to T-cytoplasm male-sterile maize. Seven heritable rf2-m alleles were recovered from these transposon populations. Pedigrees and restriction fragment length polymorphism (RFLP)-based analyses indicated that all seven rf 2-m alleles were derived independently. The ability to obtain rf 2-m derivatives from Rf2 suggests that Rf2 alleles produce a functional product necessary to restore pollen fertility to cmsT. Molecular markets flanking the rf1 and rf2 loci were used to decipher segregation patterns in progenies segregating for the rf2-m alleles. These analyses provided preliminary evidence of a weak, third restorer gene of cmsT that can substitute for Rf1.

Translation in a wheat germ cell-free system of RNA from mitochondria of the normal and Texas male-sterile cytoplasms of maize (Zea mays L.)

RNA isolated from etiolated seedling shoot mitochondria of maize (Zea mays L.) with normal (N) or Texas male-sterile (T) cytoplasm stimulated the incorporation of [35S]-methionine into protein when added to a cell-free protein-synthesizing system from wheat germ. Discrete polypeptides with molecular masses of up to approximately 67 kDa were synthesized, and the pattern of bands was distinct from that obtained with total RNA. Products of translation of T-urf13 RNA were identified by immunoprecipitation, and of atpA, coxI, and coxII RNA by hybrid arrest of translation by the cloned gene. Several polypeptides were differentially synthesized from N and T mitochondrial RNA; these differences were more extensive than those found when isolated, intact, N and T mitochondria are allowed to synthesize proteins.

Baculovirus expression of the maize mitochondrial protein URF13 confers insecticidal activity in cell cultures and larvae

The URF13 protein, which is encoded by the mitochondrial gene T-urf13, is responsible for cytoplasmic male sterility and pathotoxin sensitivity in the Texas male-sterile cytoplasm (cms-T) of maize. Mitochondrial sensitivity to two host-specific fungal toxins (T toxins) is mediated by the interaction of URF13 and T toxins to form pores in the inner mitochondrial membrane. A carbamate insecticide, methomyl, mimics the effects of T toxins on isolated cms-T mitochondria. URF13 was expressed in Spodoptera frugiperda (fall army-worm) cells (Sf9) in culture and in Trichoplusia ni (cabbage looper) larvae with a baculovirus vector. In insect cells, URF13 forms oligomeric structures in the membrane and confers T toxin or methomyl sensitivity. Adding T toxin or methomyl to Sf9 cells producing URF13 causes permeabilization of plasma membranes. In addition, URF13 is toxic to insect cells grown in culture without T toxins or methomyl; even a T-toxin-insensitive mutant form of URF13 is lethal to cell cultures. Baculoviruses expressing URF13 are lethal to T. ni larvae, at times postinjection comparable to those obtained by injecting a baculovirus expressing an insect neurotoxin. This result suggests that URF13 could be useful as a biological control agent for insect pests. Our data indicate that URF13 has two independent mechanisms for toxicity, one that is mediated by T toxin and methomyl and one that is independent of these toxins. Similarly, male sterility and toxin sensitivity in cms-T maize may be due to independent mechanisms.

Mitochondrial dysfunction in yeast expressing the cytoplasmic male sterility T-urf13 gene from maize: analysis at the population and individual cell level

The urf13TW gene, which is derived from the mitochondrial T-urf13 gene responsible for Texas cytoplasmic male sterility in maize, was expressed in Saccharomyces cerevisiae by targeting its translation product into mitochondria. Analysis by oxygraphy at the population level revealed that in the presence of methomyl the oxygen uptake of intact yeast cells carrying the targeted protein is strongly stimulated only with ethanol as respiratory substrate and not with glycerol, lactate, pyruvate, or acetate. When malate is the substrate oxidized by isolated mitochondria, interaction between the targeted protein and methomyl results in significant inhibition of oxygen uptake. This inhibition is eliminated and oxygen uptake is stimulated by subsequent addition of NAD+. Using 3,3'-dihexyloxacarbocyanine iodide [DiOC6(3)] as probe, interactive laser scanning and flow cytometry, which permit analysis at the individual cell level, demonstrated that specific staining of the mitochondrial compartment is obtained and that DiOC6(3) fluorescence serves as a measure of the membrane potential. Finally, it was shown that, as in T cytoplasm maize mitochondria, HmT toxin and methomyl dissipate the membrane potential of yeast mitochondria that carry the foreign protein. Furthermore, the results suggest that the HmT toxin and methomyl response is related to the plasmid copy number per cell and that the deleterious effect induced by HmT toxin is stronger than that of methomyl.

Genetic analysis of nuclear control of T-urf13/orf221 transcription in T cytoplasm maize

The mitochondrial gene T-urf13 in T cytoplasm maize is associated with sensitivity to disease toxins and with cytoplasmic male sterility. T-urf13 is co-transcribed with an open reading frame designated orf221. We have detected alterations in the transcription of the T-urf13/orf221 region that are affected by nuclear genotype. There are multiple mRNA transcripts generated from the T-urf13/orf221 region, one of which is a processed 1538-nucleotide (nt) transcript. This 1538-nt transcript is present in Wf9 (T), but was not found in mitochondrial RNAs (mtRNAs) from maize inbreds B14A (T) and 33-16 (T). For B14A (T) a 1500-nt transcript was detected and for 33-16 (T) a 1400-nt transcript was detected. In F1 progeny of the cross of Wf9 (T) x 33-16 (N), only the 1400-nt transcript was present. Genetic analyses revealed this processing event is nuclear controlled with dominant gene action and is independent of nuclear restorer gene Rf1-associated processing events. T-urf13/orf221 transcriptional patterns were shown to vary in both sterile and fertile states. Segregation analysis of a 1100-nt orf221-specific transcript indicated that the genetic basis of nuclear control for the presence of this transcript was relatively simple. Analysis of the A188 (T4) tissue culture mutant, which has reverted to male fertility but displays the same T-urf13/orf221 transcript pattern as A188 (T), indicated no DNA sequence differences between T4-orf221 and T-orf221. Presence of the nuclear gene Rf2 was not necessary for expression of the T4 cytoplasm-associated malefertile phenotype.

Differences in amino acid transport in isonuclear lines of cytoplasmic male-sterile and male-fertile petunia

Two pairs of isonuclear lines of cytoplasmic male-sterile (CMS) and fertile (F) petunia cells grown in suspension culture in the presence or absence of amino acid sources were examined for uptake of 11 amino acids and adenosine. Cells from CMS lines exhibited a significant lower rate of uptake than F cells. These differences, for various amino acids, are a result of lower affinity (high Km) values and of lower maximal velocities. Although the uptake of most of the amino acids examined was affected by the availability of energy in the cell, the differences in uptake seem to be less dependent on the energy status of the cell.

Functional expression of the rat brown adipose tissue uncoupling protein in Saccharomyces cerevisiae

The uncoupling protein (UCP) from mammalian brown adipose tissue is an integral component of the mitochondrial inner membrane where it dissipates the proton electrochemical gradient. UCP is transported into mitochondria from the cytosol but lacks a cleavable targeting peptide. We have expressed the rat UCP in Saccharomyces cerevisiae and shown that this protein, which is not normally found in yeast, is targeted to the mitochondria where it disrupts mitochondrial function, probably by uncoupling oxidative phosphorylation. The observed growth defect is dependent upon the level of expression of UCP. When the unmodified UCP cDNA is expressed in yeast under the control of the GAL10 promoter no defect in growth is observed. We have inserted the UCP coding sequence behind the strong phosphoglycerate kinase promoter under the control of the GAL1-10 upstream activation site and introduced a yeast consensus sequence (ATAATG) at the translation start site. We have found that UCP expressed in S. cerevisiae is targeted to mitochondria and that its expression induces a marked growth defect on non-fermentable carbon sources in a manner dependent on induction with galactose.

URF13, a maize mitochondrial pore-forming protein, is oligomeric and has a mixed orientation in Escherichia coli plasma membranes

URF13, an inner mitochondrial membrane protein of the maize Texas male-sterile cytoplasm (cms-T), has one orientation in the inner membrane of maize mitochondria but two topological orientations in the plasma membrane when expressed in Escherichia coli. Antibodies specific for the carboxyl terminus of URF13 and for an amino-terminal tag fused to URF13 in E. coli were used to determine the location of each end of the protein following protease treatments of right-side-out and inside-out vesicles derived from cms-T mitochondria and the E. coli plasma membrane. Cross-linking studies indicate that a portion of the URF13 population in mitochondria and E. coli exists in membranes in an oligomeric state and, in combination with proteolysis studies, show that individual subunits within a given multimer have the same orientation. A three-membrane-spanning helical model for URF13 topology is presented.

Transfer of methomyl and HmT-toxin sensitivity from T-cytoplasm maize to tobacco

The mitochondrial gene, T-urf13, which is unique to the T-cytoplasm of maize, has been expressed in tobacco plants using the Cauliflower Mosaic Virus 35S promoter. Tobacco plants expressing T-urf13 exhibit a variety of responses to methomyl. Leaf discs and petiole sections bleach when exposed to methomyl or HmT-toxin; this effect increases with the age of the tissue. The bleaching effect is not however observed when light is excluded. Plants homozygous for T-urf13 exhibit extreme sensitivity when sprayed with methomyl. The growth of seedling which are either homozygous or heterozygous for T-urf13 is inhibited by methomyl and by kanamycin, whereas seedlings from untransformed tobacco or tobacco which has lost the T-urf13 gene through segregation are sensitive to kanamycin but develop normally when exposed to methomyl. The results demonstrate that T-URF13 need not be specifically targeted to the mitochondrion for it to induce methomyl or HmT-toxin sensitivity in tobacco.