Editors' Choice

Science  17 Nov 2006:
Vol. 314, Issue 5802, pp. 1050

    The Variation Within

    Uniparental (usually maternal) inheritance of a single type of mitochondrial genome, referred to as homoplasmy, has long been assumed to be the main mitochondrial state in eukaryotes. However, rare examples of multiple mitochondrial types within an individual, a state known as heteroplasmy, have been identified in animals, fungi, and plants.

    Previous greenhouse studies indicated that heteroplasmy can occur in the bladder campion plant (Silene vulgaris), but Welch et al. show that it can be found at frequencies of up to 26% within a natural population. Furthermore, mothers that were heteroplasmic were shown to pass it on to their offspring, and the pattern of inheritance suggested that heteroplasmy was genome-wide (in the mitochondria) and not locus-specific. Although these findings may be taken as consistent with biparental inheritance, the fact that high levels of cytoplasmic male sterility, caused by cytonuclear interactions, are known to occur in S. vulgaris suggests that heteroplasmy may be selected for within female individuals in some populations. — LMZ

    Genetics 174, 829 (2006).


    With Size Comes Stability

    The webs of interactions between producers, consumers and decomposers in natural ecosystems confront the ecologist with a bewildering complexity. Much effort has gone into exploring the structure of food webs and the forces that contribute to their stability.

    In two studies, Brose et al. estimate the consequences for food-web stability of the body-size distributions of consumer and resource species. Their theoretical simulations suggest that the population persistence of predator and prey species in food webs increases as the ratio of the predator-to-prey body-mass increases, up to a saturation point that is higher for vertebrates than invertebrates. These patterns were found to hold in a survey of body-size distributions in natural food webs, which also revealed that body-size ratios of predators and prey differed across freshwater, marine, and terrestrial ecosystems. These effects of body-size ratio on stability and complexity in food webs add an important dimension to the study of this fundamental ecological question. — AMS

    Ecol. Lett. 9, 1228 (2006); Ecology 87, 2411 (2006).


    Dipped in Oil

    The phospholipid bilayer of biological membranes is first and foremost a means of demarcating aqueous compartments by establishing a hydrophobic barrier that restricts the permeability of water-soluble compounds. One of the many additional functions of phospholipids is to provide the fatty acid substrates that can be converted into important signalling messengers, such as leukotrienes and prostaglandins. In order to cleave the linkage between the hydrophobic fatty acid and the hydrophilic headgroup, the enzyme phospholipase A2 (PLA2) attaches itself to membranes via its C2 domain, which contains binding sites for two calcium ions.

    Starting from structural and biophysical constraints, Jaud et al. have carried out a molecular dynamics simulation of the interaction between the PLA2 C2 domain and a phosphatidyl choline bilayer. They find that the neighboring lipids reorganize to form a crater-like indentation, with the alkyl chains lining the bottom and the polar headgroups around the rim. Into this depression fit the three calcium-binding loops (CBLs) and the two complexed Ca2+ ions, whose primary role seems to be to mask the negatively charged loops rather than to coordinate directly to the phosphoryl oxygens, from which they are insulated by a layer of water molecules. — GJC

    Biophys. J. 91, 10.1529/biophysj.106.090704 (2006).


    Zero Tolerance

    In carbenes, a carbon atom engages only two of its valence electrons in bonds to other atoms, leaving the remaining two electrons free to react. Over 40 years ago, a different class of divalent carbon compound was prepared, termed a carbodiphosphorane (CDP), in which the central C was bound to two phosphines in a motif that has often been represented as cumulated double bonds: R3P=C=PR3, where R is a halide, amide, or hydrocarbon substituent. More recently, several stable free and complexed CDPs have been characterized, prompting Tonner et al. to explore the valence structure more thoroughly. Using quantum chemical calculations to analyze reported as well as model compounds, they find that unlike carbenes, CDPs are best described as donor-acceptor complexes: each phosphine datively donates two electrons to a central carbon, in the zero oxidation state, that has two essentially nonbonding lone pairs. The basicity of these lone pairs is borne out in a new compound, synthesized by the authors, that links two protonated CDP moieties to a silver cation. — JSY

    Angew. Chem. Int. Ed. 45, 10.1002/anie.200602552 (2006).


    Stickier with SWNTs

    Adhesives that bond quickly and firmly to most surfaces on application of only a small amount of pressure are increasingly sought to eliminate the need for chemical activators or crosslinkers. Under tension, such pressure-sensitive adhesives form cavities that expand into fibrils, which in turn extend before detaching from the surface; it is these processes that contribute to the energy of adhesion. Wang et al. explored the adhesive properties of a poly(butyl acrylate) dispersion mixed with single-walled carbon nanotubes (SWNTs) that were functionalized with poly(vinyl alcohol) to confer hydrophilicity. They found that the SWNTs had the somewhat surprising effect of rendering the polymer both stiffer and more dissipative, two characteristics that usually vary in opposing fashion. Improved adhesive properties resulted from SWNT loading as low as 0.05 weight %, with 0.3 weight % proving optimal. During debonding, the SWNTs were found both to reduce the nucleation of cavities and to stabilize the walls between cavities, thus allowing them to absorb more energy before detachment from the substrate as fibrils. The optimized material also exhibited high optical clarity and a 10-order-of-magnitude increase in conductivity. These features bode well for eventual applications of this relatively environmentally benign material in electronics and displays. — MSL

    Adv. Mater. 18, 2730 (2006).


    Early Natural Selection

    The bright light emitted by quasars is powered by the infall of gas toward giant black holes in galactic cores. The first quasars are known to have had central black holes that comprised a billion solar masses within a region the size of a solar system. Because assembling such a massive black hole should take billions of years, astronomers have had difficulty explaining the presence of quasars in the early (billion-year-old) universe. Volonteri and Rees have modeled the growth of the first supermassive black holes, including the competing effects of gas accretion and the dynamics of black hole mergers in their calculations. Black holes may grow by accreting gas from their surroundings, but during the most efficient accretion periods, growth is slowed by increased radiation of energy. Mergers with small companion galaxies also contribute to growth but can be destructive as well. Coalescence may be prevented if merging black holes are expelled from the galaxy by recoil from asymmetric gravitational waves and multiple-body dynamics. The authors thus frame a Darwinian natural selection scenario for black hole growth in the very young universe. Rapid and efficient growth would proceed in the highest-density regions, where gas accretion was optimal and gentle mergers outcompeted recoil losses. — JB

    Astrophys. J. 650, 669 (2006).

  7. STKE

    A Complex Mode of Glucose Signaling

    In Arabidopsis, hexokinase 1 (HXK1) acts a glucose sensor to regulate gene expression and plant growth, which is a role far removed from its function in glycolysis. However, the mechanisms whereby HXK1 mediates glucose signaling have been unclear. After showing that a small fraction of Arabidopsis HXK could be found in the nucleus, Cho et al. used proteomic and two-hybrid screens to identify two proteins—vacuolar H+-ATPase B1 (VHA-B1) and the 19S regulatory particle of proteasome subunit (RPT5B)—as nucleus-specific HXK1-interacting partners that formed a complex with HXK1. Genetic analysis revealed that vha-B1 and rpt5b loss-of-function mutants resembled the HXK1 gin2 (glucose-insensitive 2 mutant): All three mutants were insensitive to repression of cotyledon expansion, chlorophyll accumulation, and leaf and root development by high glucose and showed growth retardation as compared to wild-type plants under low-light low-nutrient conditions. Like gin2, the vha-B1 and rpt5b mutants did not exhibit glucose-mediated repression of the chlorophyll a/b—binding protein (CAB) and carbonic anhydrase (CAA) genes. Chromatin immunoprecipitation analysis showed that the HXK1 complex bound to the CAB2 promoter; this binding was reduced but not abolished in the vha-B1 and rpt5b mutants. Thus, these three proteins appear to form a complex that functions in the glucose-mediated regulation of gene transcription. — EMA

    Cell 127, 579 (2006).

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