This Week in Science

Science  05 Apr 2013:
Vol. 340, Issue 6128, pp. 7
  1. Reef Repair


    Coral reefs suffer mass mortality because of coral bleaching, disease, and tropical storms, but we know much more about when, where, and how rapidly these ecosystems have collapsed than we do about their recovery. Gilmour et al. (p. 69; see the Perspective by Polidoro and Carpenter) studied a highly isolated coral reef before and after a climate-induced mass mortality event that killed 70 to 90% of the reef corals. The initial recovery of coral cover involved growth and survival of remnant colonies, which was followed by increases in larval recruitment. Thus, in the absence of chronic disturbance, even isolated reefs can recover from catastrophic disturbance.

  2. Bio-Inspired Synthetic Network

    Collective behavior comes through the ability of neighboring objects to communicate and interact with each other. Villar et al. (p. 48; see the cover) produced three-dimensionally patterned, interconnected networks of lipid-bounded structures functionalized with transmembrane proteins, which allowed electrical communication along specific pathways.

  3. Cycling in Unison

    Many small mammals, especially voles, display semi-regular cycles of population boom and bust. Given the fundamental importance of small mammals as basal consumers and prey, such cycles can have cascading effects in trophic food webs. Cornulier et al. (p. 63) collated raw data from vole populations across Europe collected over the past 18 years. Reduction in winter growth rate was common across a wide variety of habitats with very different local climates, suggesting the presence of a continental-scale climatic driver of vole populations.

  4. Thwarting HIV

    Multiple genome-wide association studies have revealed that human leukocyte antigen (HLA) genes of the major histocompatibility complex locus have the strongest impact on HIV. In particular, a single-nucleotide polymorphism 35 base pairs upstream of HLA-C shows significant association with viral load and protection against HIV. How HLA-C mediates these effects is unknown. Apps et al. (p. 87) now demonstrate that increasing surface expression of HLA-C is associated with reduced viral load and reduced rate of progression to low CD4+ T cell counts in African and European Americans. High HLA-C expression likely promoted improved HIV control through a more effective cytotoxic CD8+ T cell response. In contrast to HIV infection, high HLA-C expression was associated with a higher risk of the inflammatory bowel disease, Crohn's disease.

  5. How to Make Decisions

    Recently, a number of methods to probe internal properties of nonlinear neural systems have been developed. In these methods, highly variable stimuli are used to explore the input space of the system. Neural responses are then studied using models that take advantage of the known trial-by-trial stimulus information. Brunton et al. (p. 95) adapted this combined approach to decision-making. Both in rats and humans, the diffusion constant of the drift-diffusion model of decision-making was zero, implying that the noise is all in the processing of sensory input and not in the evidence accumulator. In addition, rats gradually accumulated evidence for decision-making, with strong effects of sensory adaptation on gradual accumulation of evidence.

  6. A Light in the Dark

    Carotenoids enhance the efficiency of photosynthesis by absorbing light at wavelengths inaccessible to chlorophyll. However, the mechanism underlying transfer of the absorbed energy from these pigments to the site of chemical catalysis is unclear. Studies have debated the involvement of a “dark” state in the carotenoid electronic structure—a state inaccessible by direct absorption populated by partial relaxation of the initial excited state. Ostroumov et al. (p. 52) used two-dimensional electronic spectroscopy to provide evidence of this state.

  7. Fluorine Learns to Share

    Though halides typically coordinate to just one carbon center, their transient coordination to a second carbon (forming a positively charged bridge) explains the spatial dynamics of many reactions. However, unlike chlorine, bromine, and iodine—which can all form such halonium ions—fluorine does not appear to engage in carbon-bridging behavior, presumably because of its very high electronegativity. Struble et al. (p. 57, see the Perspective by Hennecke) synthesized a rigid molecule, particularly well-poised to manifest fluoride bridging, and provide evidence for a fluoronium intermediate in a displacement reaction.

  8. A Supernova in 3D


    When a star explodes, it expels matter at supersonic speeds. Nikolić et al. (p. 45, published online 14 February) used integral-field spectroscopy to obtain high-resolution, spatially resolved spectra of the shock waves associated with supernova 1006. The physical characteristics of the shocks around this supernova exhibit a strong spatial variation that suggests the presence of suprathermal (∼10 to 100 keV) protons, which are the potential seeds for generating cosmic rays.

  9. MicroRNA Mechanism

    MicroRNAs are small noncoding RNAs that regulate gene expression by binding complementary target messenger RNAs (mRNAs) and repressing their expression through repression of protein translation and mRNA degradation. Meijer et al. (p. 82) show that in a HeLa cell system mRNA degradation is a consequence of translational inhibition via the initiation factor eIF4A2.

  10. From Grid to Place

    Grid cells are considered one of the key sources for place-cell signals in the hippocampus. The entorhinal circuit also contains other functional cell types, but it is unclear which project to the place cells of the hippocampus. Zhang et al. (10.1126/science.1232627, see the Perspective by Poucet and Sargolini) addressed this question using optogenetics and in vivo multi-electrode electrophysiology. Hippocampal cells received input from a broad spectrum of entorhinal neuronal cell types. Grid cells represented the biggest group of spatial inputs, but border cells, head-direction cells, and a large fraction of nonspatial cells also provided inputs. Thus, hippocampal circuits have local mechanisms for processing specific types of functional input from the entorhinal cortex to generate place-specific signals.

  11. Flowers and Bees Have “Sparks”


    Plants and their pollinators have very intimate interactions, as emphasized by the many classic cases of coevolution among species of each. Such close relationships require signaling between plant and pollinator. Coordination between plant signals and pollinator perception has been shown to exist in flower color, shape, and odor. Clarke et al. (p. 66, published online 21 February) report the potential for a distinct mode of plant–pollinator communication, electric fields. Natural floral electric fields, which are impacted by visits from naturally charged bees, were easily discriminated by bees, based on their level, pattern, and structure, and improved the rate at which bees remembered the location of a nectar reward.

  12. Amorphous and More Active

    The electrochemical generation of hydrogen from water could help in the storage of energy generated by renewable resources at off-peak times. However, catalysts for the slow step of this reaction, the oxygen evolution reaction (OER), are based on oxides of noble metals (iridium and ruthenium) that have limited abundance. A strategy for improving the performance of earth-abundant elements is to explore mixed-metal oxides and to prepare these as amorphous phases. Smith et al. (p. 60, published online 28 March) developed a general method for preparing amorphous oxides, based on photodecomposition of organometallic precursors. Amorphous mixed-metal oxides of iron, nickel, and cobalt were more active than comparable crystalline materials and provided OER performance comparable to noble metal oxides.

  13. Adding to the Antiviral Arsenal

    The envelope of influenza virus contains two immunodominant glycoproteins: hemagglutinin and neuraminidase (NA). Existing antivirals like zanamivir (Relenza) and oseltamivir (Tamiflu) target NA; however, the development of drug resistance is a problem. Kim et al. (p. 71, published online 21 February) now report a different class of NA inhibitors. NA catalyzes the removal of sialic acids from the surface of host cells to initiate entry. Discovery of a NA–sialic acid intermediate led to the production of sialic acid analogs that bound covalently to NA and inhibited its enzymatic activity. These compounds showed activity against a wide variety of influenza strains, inhibited viral replication in cell culture, and were able to protect mice against influenza infection. Protection of mice was equivalent to protection seen from zanamivir. Moreover, the compounds showed activity against drug-resistant strains in vitro. These compounds represent a potentially useful addition to the arsenal of antivirals used to treat influenza infection.

  14. Putting Selenium in Proteins

    The 21st amino acid, selenocysteine (Sec), occurs in the active site of many redox enzymes. Its cognate transfer RNA (tRNA) is first loaded with Ser by seryl-tRNA synthetase and the Ser-tRNASec is then converted to Sec-tRNASec. Itoh et al. (p. 75) determined the crystal structures of the selenocysteine synthase, SelA, that is responsible for this conversion in bacteria, alone and in complex with tRNA. The decameric SelA complex binds to 10 tRNASec molecules. The structures, together with biochemistry, show how SelA discriminates tRNASec from tRNASer, give insight into the mechanism of catalysis, and show that decamerization is essential for function.

  15. Silencing Transposons

    Eukaryotic DNA is packaged onto nucleosomes, which are composed of four core histones (H2A, H2B, H3, and H4). Chromatin also contains a fifth histone, H1, which binds to both the core particles and the “linker” DNA that joins adjacent nucleosomes, where it helps chromatin to fold into higher-order structures and generally silences gene expression. In Drosophila germline and somatic cells and tissues, Lu et al. (p. 78) found that the repressive function of H1 in vivo was directed toward transposable elements and independent of small RNA silencing pathways. Instead, H1 acted through direct recruitment of the histone methyltransferase Su(var)3–9, which methylates histone H3 lysine 9, a repressive histone mark.

  16. Archaea Powered by Rocket Fuel

    Perchlorate is a ubiquitous chlorine-based compound that forms naturally in the atmosphere. It is only present in large deposits in a few locations, such as the Atacama Desert in Chile, which has been assumed to be because perchlorate-reducing bacteria normally degrade it into chloride and oxygen. Liebensteiner et al. (p. 85; see the Perspective by Nerenberg), however, found that an archeon, Archaeoglobus fulgidus, can also reduce perchlorate. The archaeal perchlorate reduction pathway shares limited similarity with bacterial perchlorate reduction: Instead of producing chloride and oxygen, enzymatically produced chlorite reacts with sulfide to produce oxidized sulfur compounds. Because hyperthermophilic anaerobic archaea similar to A. fulgidus are thought to have been among the first complex organisms to evolve on Earth, they may have started creating oxidized conditions in some habitats before the emergence of oxygen-generating photosynthesis.

  17. Neuronal Transposons

    Transposons comprise a hefty chunk of the Drosophila genome and, unregulated, can generate mutations; thus, mechanisms exist to suppress transposon activity, particularly in the germline. Perrat et al. (p. 91) investigated transposon motility in neurons of the Drosophila brain. The mushroom body of the brain, responsible for olfactory memory, contains several different types of neurons. One class of neurons, the αβ neurons, exhibited increased transposon mobility, which generated increased neuronal diversity.

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