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

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.

Structural and functional mapping of Rtg2p determinants involved in retrograde signaling and aging of Saccharomyces cerevisiae

In Saccharomyces cerevisiae mitochondrial dysfunction induces retrograde signaling, a pathway of communication from mitochondria to the nucleus that promotes a metabolic remodeling to ensure sufficient biosynthetic precursors for replication. Rtg2p is a positive modulator of this pathway that is also required for cellular longevity. This protein belongs to the ASKHA superfamily, and contains a putative N-terminal ATP-binding domain, but there is no detailed structural and functional map of the residues in this domain that accounts for their contribution to retrograde signaling and aging. Here we use Decomposition of Residue Correlation Networks and site-directed mutagenesis to identify Rtg2p structural determinants of retrograde signaling and longevity. We found that most of the residues involved in retrograde signaling surround the ATP-binding loops, and that Rtg2p N-terminus is divided in three regions whose mutants have different aging phenotypes. We also identified E137, D158 and S163 as possible residues involved in stabilization of ATP at the active site. The mutants shown here may be used to map other Rtg2p activities that crosstalk to other pathways of the cell related to genomic stability and aging.

Transformation and analysis of tobacco plant var Petit havana with T-urf13 gene under anther-specific TA29 promoter

T-urf13, a well-documented cms-associated gene from maize, has been shown to render methomyl sensitivity to heterologous systems like rice, yeast and bacteria when expressed constitutively. Since these transgenic plants were fertile, it was hypothesized that T-urf13 gene if expressed in anthers may result in male sterility that could be used for hybrid seed production. Hence, this work was aimed at analysing whether T-urf13 gene when expressed in anthers can result in male sterile plants or requires methomyl treatment to cause male sterility (controllable). This is the first report of transformation of tobacco with T-urf13 gene under anther-specific promoter (TA29) with or without mitochondrial targeting sequence. Most of the transgenic plants obtained were fertile; this was surprising as many male sterile plants were expected as T-urf13 gene is a cms associated gene. Our results suggest that it may not be possible to obtain male sterility by expressing URF13 in the anther by itself or by methomyl application.

Mitochondrial membrane potential and ageing in Podospora anserina

Some filamentous fungi exhibit a limited vegetative growth with modifications in the mitochondria, suggesting the involvement of mitochondria in the process of ageing. Nevertheless, the relationship between the ability to grow or the fate of these cells relative to their mitochondrial membrane potential (Psi(mt)) level has not been investigated. Using flow cytometric analysis, we have assessed Psi(mt) in young and senescent cultures of wild type strains and mitochondrial or nuclear mutant strains of Podospora anserina that present very long or brief life span. When we compared two distinct populations of cells obtained from the same strain, we can show a correlation not only between Psi(mt) and ageing, but also between Psi(mt) and the frequency of regeneration and/or the life span. However, this relationship is not observed when we compared the cells obtained from different physiological states or mutants strains. These results allow us to suggest that the Psi(mt) modifications during senescence could be only one of the possible consequences of the process and are not the factor driving towards death. We also show that the driving force of Psi(mt) is principally maintained by the alternative pathway during ageing, suggesting a role of the alternative oxidase pathway.

DiOC6(3): a useful dye for staining the endoplasmic reticulum

The present review discusses the fluorescent organelle probe, DiOC6(3), with reference to its structure, chemistry, availability, spectral properties, labeling procedures, vital staining characteristics, and major applications in cellular and molecular biology. The specificity of dye for endoplasmic reticulum is summarized. We examine the simplicity and advantages of the fluorescent dye system for evaluating structure and function of endoplasmic reticulum. Other significant uses of the dye are also discussed.

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.

Slow fluorescent indicators of membrane potential: a survey of different approaches to probe response analysis

Basic tenets related to the use of three main classes of potentiometric redistribution fluorescent dyes (carbocyanines, oxonols, and rhodamines) are discussed in detail. They include the structure/function relationship, formation of nonfluorescent (H-type) and fluorescent (J-type) dimers and higher aggregates, probe partitioning between membranes and medium and binding to membranes and intracellular components (with attendant changes in absorption and emission spectra, fluorescence quantum yield and lifetime). The crucial importance of suitable probe-to-cell concentration ratio and selection of optimum monitored fluorescence wavelength is illustrated in schematic diagrams and possible artifacts or puzzling results stemming from faulty experimental protocol are pointed out. Special attention is paid to procedures used for probe-response calibration (potential clamping by potassium in the presence of valinomycin, use of gramicidin D in combination with N-methylglucamine, activation of Ca-dependent K-channels by A23187, the null-point technique). Among other problems treated are dye toxicity, interaction with mitochondria and other organelles, and possible effects of intracellular pH and the quantity of cytosolic proteins and/or RNA on probe response. Individual techniques using redistribution dyes (fluorescence measurements in cuvettes, flow cytometry and microfluorimetry of individual cells including fluorescence confocal microscopy) are discussed in terms of reliability, limitations and drawbacks, and selection of suitable probes. Up-to-date examples of application of slow dyes illustrate the broad range of problems in which these probes can be used.

The Thom Award address. Industrial mycology and the new genetics

The genetic investigation of fungi has been extended substantially by DNA-mediated transformation, providing a supplement to more conventional genetic approaches based upon sexual and parasexual processes. Initial transformation studies with the yeast Saccharomyces cerevisiae provided the model for transformation systems in other fungi with regard to methodology, vector construction and selection strategies. There are, however, certain differences between S. cerevisiae and filamentous fungi with regard to type of genomic insertion and the availability of shuttle vectors. Single-site linked insertions are common in yeast due to the high level of homology required for recombination between vectored and genomic sequences, whereas mycelial fungi often show a high frequency of heterologous and unlinked insertions, often in the form of random and multiple-site integrations. While extrachromosomally-maintained or replicative vectors are readily available for use with yeasts, such vectors have been difficult to construct for use with filamentous fungi. The development of vectors for replicative transformation with these fungi awaits further study. It is proposed that replicative vectors may be inherently less efficient for use with mycelial fungi relative to yeasts, since the mycelium, as an extended and semicontinuous network of cells, may delimit an adequate diffusion of the vector carrying the selectable gene, thus leading to a high frequency of abortive or unstable transformants.

Commitment to apoptosis is associated with changes in mitochondrial biogenesis and activity in cell lines conditionally immortalized with simian virus 40

Rodent embryo cells immortalized with temperature-sensitive mutants of simian virus 40 large tumor (T) antigen have a proliferative potential that depends on temperature. At the restrictive temperature, heat-inactivation of large T antigen causes p53 release, growth arrest, and cell death. Morphological and molecular analysis indicate that the induced cell death corresponds to apoptosis. Flow cytometric analysis using a combination of forward light scatter and side scatter allows a discrimination of cells committed to apoptosis within the whole population. These cells display a reduction in cell size and a higher cellular density, confirming the apoptotic nature of the cell death. When cells exhibiting the morphological features of apoptosis were stained with a fluorescent probe of the mitochondrial membrane potential, a decreased accumulation of the dye was recorded. Measures of cellular respiration, performed with whole-cell populations, showed that the lower mitochondrial membrane potential (delta psi m) correlates, as expected, with an uncoupling of electron transport from ATP production and is linked to the induction of apoptosis. We also show that this decrease in delta psi m is associated with a decrease in the rate of mitochondrial translation. These events are detected at early stages of the apoptotic process, when most of the cells are not irreversibly committed to death, suggesting that mitochondria could be a primary target during apoptosis.

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.