This Week in Science

Science  29 Apr 2011:
Vol. 332, Issue 6029, pp. 511
  1. Can You Feel Me Now?


    Communication within a species is most effective when it is exclusive, so it is beneficial to recognize signals from your own kind and exclude those made by others. Some fish species living in muddy rivers overcome the challenge of living in a low-visibility environment by electrical signaling. While studying the distinctive patterns of discharges from African electrical fish, Carlson et al. (p. 583) discovered that the evolution of key sensory processing regions in the brain allowed for rapid diversification in one group but was minimal in a closely related group that had diverged prior to the evolution of the sensory innovations. Species within the diversified clade possess receptor morphology that is more efficient at transmitting and discerning complex electric signals than those of the less diverse group, implying that key innovations in brain development can lead to speciation because of improved perception and species recognition.

  2. Putting the F in Friedel-Crafts

    The Friedel-Crafts class of reactions, among the oldest and most broadly applied in organic chemistry, form carbon-carbon bonds between aromatic rings and a variety of non-aromatic substituents, such as alkyl groups. Generally, a metal complex is used to activate chlorinated or brominated precursors of these substituents, but by using silicon-based reagents to activate a fluorinated precursor, Allemann et al. (p. 574) extend the reaction to coupling of two different aromatic sites, leading to efficient formation of elaborate polycyclic structures. The method relies on the unusual strength of silicon-fluorine bonds as a driving force.

  3. Insulator to Order

    Unlike simple band-gap insulators, bulk topological insulators permit movement of charges on their boundaries while acting as insulators in their interior. Both types of insulators usually occur in different families of compounds. Now, Xu et al. (p. 560, published online 31 March; see the Perspective by Fiete) can promote transition between a direct band gap and topological insulator within a bulk bismuth-containing solid by changing the relative ratio of its sulfur and selenium content. This results in the tuning of the spin-orbit interaction and a topological phase transition in which the material TlBi(S1–δSeδ)2 exhibits a 180-degree inversion of its Fermi surface spin pattern as the Fermi energy is tuned through the zero-electron density point. This spin texture inversion may lead to devices with novel quantum properties.

  4. Changing as Little as Possible

    Phase-change materials can be used in memory devices because they change a physical property, such as resistivity or optical reflectivity, depending on whether they are in an amorphous or crystalline state. These changes are thermally driven, but, for memories based on resistivity changes, one hurdle has been to reduce the amount of power needed to melt these materials. Xiong et al. (p. 568, published online 10 March; see the Perspective by Salinga and Wuttig) created gaps in single-walled and small-diameter multiwalled carbon nanotubes and then coated these electrodes with a thin film of the phase-change material Ge2Sb2Te5. Phase changes occurred only for the material embedded in the gaps, which, when compared with current device configurations, helps reduce the power required by two orders of magnitude.

  5. Improving Organic LEDs

    Organic light-emitting diodes (OLEDs) could form the basis of faster, lower-powered displays, by comparison with liquid crystal displays that require a lighting source and have inherent viewing-angle limitations. However, OLEDs require higher driving voltages, and this has limited the number of materials that can be used for the active matrix backplane. McCarthy et al. (p. 570) show that single-walled carbon nanotubes can be used to fabricate vertical field-effect transistors to integrate with OLEDs and generate efficient light emission across the color spectrum.

  6. Frequency Combs on a Chip


    Precisely spaced spectral lines that span a broad range of wavelengths to form a frequency comb have been used in a variety of applications, including precision metrology and spectroscopy, molecular fingerprinting, and astronomical observations. These combs generally have been produced using pulsed laser systems, which require expensive equipment. Kippenberg et al. (p. 555) review recent developments using microresonators to generate frequency combs. This alternative technology means that not only can the comb source be reduced to chip-size scale, the frequency ranges can be controlled and tuned by the size and shape of the resonators, thereby opening up the range of possible applications.

  7. Changing Vortex

    Like Earth's atmosphere, the atmosphere of Venus features large-scale cyclonic circulation patterns, or vortices, at its poles. Based on long-term, high-resolution data from the European Space Agency Venus Express mission, Luz et al. (p. 577, published online 7 April) show that the southern polar atmospheric circulation is more complex than was previously thought. The center of the south polar vortex is slightly displaced from the solid planet's pole of rotation, and it moves around the pole with a period of 5 to 10 Earth days. These findings should contribute to improved models of the global atmospheric circulation on Venus.

  8. Top-to-Bottom Action

    Hydrothermal vents supply large amounts of heat and chemical components to the deep ocean and support unique communities of organisms. Winds transfer great quantities of energy to the ocean and can help create eddies that extend far beneath the surface. Adams et al. (p. 580) suggest that these eddies can influence deep ocean flow below 2 kilometers and can help transfer heat and chemicals from deep ocean vents, as well as dispersing larval stages of organisms. Time-series observations of larval organisms, currents, and chemical and mass fluxes near the East Pacific Rise indicate that transport is associated with the passage of mesoscale eddies above. Interactions between surface winds and deep-sea currents may thus enable long-distance dispersal of larvae to supply isolated, ephemeral habitats and communities of organisms and provide a seasonal component to deep-sea processes.

  9. No Fusion Confusion

    Placental syncytins and Caenorhabditis elegans fusion family (FF) proteins, EFF-1 and AFF-1, can mediate eukaryotic cell-to-cell fusion. EFF-1 must be expressed in both fusing cells and must act via homotypic interactions. This fusion process has a hemifusion intermediate resembling viral and intracellular membrane fusion processes. Now, Avinoam et al. (p. 589, published online 24 March) show that FF homologs, from the human parasitic nematode Trichinella spiralis and the chordate Branchiostoma floridae, induce fusion in mammalian cells, indicating that the FFs are a family of conserved fusogens. Furthermore, AFF-1 can complement a fusion-deficient enveloped virus, rescuing its fusion defect and altering its infection mechanism.

  10. Disrupting the Grid


    Grid cells are neurons in the medial entorhinal cortex that have multiple firing peaks that organize into a highly regular hexagonal pattern in two-dimensional space. Koenig et al. (p. 592) and Brandon et al. (p. 595) used different drugs to inhibit neuronal firing in the medial septum (the “pleasure zone”), which caused a reduction of theta rhythm oscillations and prevented the grid cells from firing. However, cells that sense head-direction and cells with mixed grid and head direction properties continued to fire appropriately, and the cells in the hippocampus that sense location, the place cells, were not disrupted either. Thus, input from the medial septum is not necessary for all aspects of spatial processing.

  11. Signal Stealer

    Cytotoxic T lymphocyte antigen 4 (CTLA-4) is expressed by activated and regulatory T cells and helps to prevent T cell–mediated immune responses from getting out of control. The costimulatory pair of ligands bind CTLA-4 and are expressed on antigen-presenting cells. CD80 and CD86 also bind to CD28 expressed on T cells, which is critical for T cell activation. Qureshi et al. (p. 600, published online 7 April; see the Perspective by Sakaguchi and Wing) find that CTLA-4, expressed by both human and mouse T cells, plucks CD80 and CD86 from the surface of antigen-presenting cells and internalizes them through trans-endocytosis. These findings suggest an extrinsic cell mechanism for the negative regulation of T cell immunity by CTLA-4.

  12. Orbital Ordering in Iron Pnictide Superconductors

    The origin of superconductivity in the iron pnictides is usually attributed to antiferromagnetism associated with a structural transition that gives rise to spin-density waves. However, there have been suggestions that orbital ordering effects may play a role. Shimojima et al. (p. 564, published online 7 April) used laser excitation to obtain angle-resolved photoemission spectra from the subsurface regions of two iron pnictides, BaFe2(As0.65P0.35)2 and Ba0.6K0.4Fe2As2. The Fermi surfaces of the hole charge carrier in each material have the same superconducting gap magnitude, which suggests that magnetism-induced pairing between orbitals, orbital fluctuations, or both effects may play a role in creating the superconducting state.

  13. Coordinating Regeneration

    The activation of stem cells is vital for organ and whole animal survival, but for regeneration in large organs, such as the skin, stem cell growth needs to be coordinated. Plikus et al. (p. 586) report that upon activating hair stem cells in mice, diverse regenerative wave patterns form over the entire skin, and hair regeneration spreads like a chain reaction. The waves are governed by simple patterns of activator and inhibitor signaling.

  14. Complement Receptor Complex Structure

    Complement receptor 2 (CR2), found in B cells and follicular dendritic cells, binds to a ligand, C3d, which is bound to antigen. The antigen simultaneously binds to the B cell antigen recognition complex such that this co-ligation enhances the antibody response. A previously published crystal structure of the CR2:C3d complex possibly represented a nonphysiological interaction. Hence, van den Elsen and Isenman (p. 608) report a structure in which the molecular details of the interface are more consistent with biochemical data, supporting physiological relevance and providing a foundation for rational design of therapeutics against autoreactive B cells.

  15. Dissecting Methylation Reactions

    Methylation of proteins and nucleic acids by the enzymatic transfer of a methyl group from S-adenosyl-l-methionine (SAM) is important for many biological processes. Hypothetically, transfer to a non-nucleophilic substrate can be achieved by cleavage of SAM to give a radical. Grove et al. (p. 604, published online 17 March; see the Perspective by Stubbe) report a mechanism for the methylation of two sites in bacterial ribosomal RNA, one of which is important in antibiotic resistance. In both cases a methyl group from one SAM molecule is first appended to a cysteine in the relevant enzyme before a second SAM molecule is cleaved. The resulting radical initiates transfer of the methyl from the enzyme to the RNA.

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