This Week in Science

Science  03 Jun 2011:
Vol. 332, Issue 6034, pp. 1124
  1. Generating Sawtooth Oscillations


    Earth's magnetosphere is subject to sawtooth oscillations as it responds to the constant battering of the solar wind, the stream of charged particles coming from the Sun. Although observations have uncovered many of the properties of this type of disturbance, the mechanism that generates it is unclear. Based on numerical simulations of the interaction of the solar wind with the magnetosphere and the ionosphere, Brambles et al. (p. 1183) now show that the flow of O+ ions from the ionosphere to the magnetosphere can produce sawtooth oscillations.

  2. Toxic Selection

    Arsenic is highly toxic to living organisms because it disrupts metabolic pathways, but, chemically, arsenate behaves in a similar way to phosphate, and it is theoretically possible for organisms to substitute one for the other under certain conditions. Wolfe-Simon et al. (p. 1163, published online 2 December) have found a living example of a bacterium that does not find arsenate poisonous (see related Editor's Note, Technical Comments, and Response p. 1149). Isolates of a halomonad bacterium, originating from the toxic and briny Mono Lake, California, were selected by successive laboratory culture in which phosphate was gradually replaced by arsenate until the bacteria were growing in the absence of the usual salt. Further analysis indicated that arsenate had substituted for phosphate in the bacterium's constituent molecules, even replacing phosphate in its DNA, as well as in its proteins and metabolites.

  3. Which Path? That Path


    A consequence of quantum mechanics and the Heisenberg uncertainty principle is that complementary variables (for example, position and momentum) cannot both be determined precisely. Measuring one variable necessarily results in loss of information about the other. The best example is the two-slit interferometer and the interference pattern that occurs when light or single photons or electrons are transmitted through it. Determining which slit the particle goes through (position) destroys the interference pattern (momentum). Kocsis et al. (p. 1170; see the News story by Cho) implement a recent theoretical proposal in which an experimental protocol involving weak measurements could answer the “which path did the photon take” question. The results may impact the foundations of quantum and classical physics and potentially find practical application in metrology.

  4. Going to Ground

    In solids with antiferromagnetic interactions, where spins on neighboring lattice sites prefer to align opposite to each other, geometric frustration occurs if the spatial arrangement of the sites is incompatible with the preferred spin orientation. In frustrated systems, some fluctuations remain even at zero temperature, and calculating the true ground state is difficult. The kagome lattice with spin 1/2 particles interacting through the Heisenberg Hamiltonian has been one of the most-studied frustrated systems, and recent numerical results indicated that its ground state may be a valence bond crystal. Now, Yan et al. (p. 1173, published online 28 April; see the cover) use the density matrix renormalization group method to reveal that a lower-energy state, that of a spin liquid, exists where magnetic order is absent even at zero temperature. Excitations of the ground state appear to be gapped, suggesting that the true ground state of this system is a gapped spin liquid.

  5. Monolayer Generates Reversible Change

    The strength and toughness of a metal or alloy can be strongly affected by surface layers that may alter the corrosion resistance, the ability of dislocations to migrate, or the formation of notches that may lead to larger cracks. Jin and Weissmüller (p. 1179; see the Perspective by Sieradzki) explore the effect of a monolayer of oxygen species on the surface of nanoporous gold on the plastic yield strength and subsequent flow strength. Nanoporous gold was filled with an electrolyte that only weakly absorbed onto its surface. Applying a small electrical potential changed the absorption and desorption of oxygen at the gold surface, allowing reversible change of the breaking strength, the stress required to initiate plastic deformation, and the amount of plastic deformation the material will tolerate.

  6. Io's Magma Revealed

    Jupiter's moon, Io, is the most volcanically active body in the solar system, but the presence of a magma ocean in its interior has long been a matter of debate. Khurana et al. (p. 1186, published online 12 May; see the Perspective by Coates) reanalyzed magnetic field measurements acquired by the Galileo spacecraft. Io's inductive response to Jupiter's rotating magnetic field indicates the existence of a globe-encircling, electrically conducting magma layer at depth.

  7. Benefits Depend on Context

    Simple genetic interactions can often result in beneficial phenotypes under selection. However, examples of complex epistastic interactions (in which multiple genes interact, resulting in a nonadditive phenotype) and their selective effects are less well known (see the Perspective by Kryazhimskiy et al.). Khan et al. (p. 1193) examined the epistatic interactions throughout an entire genome by taking the first five mutations to appear in an experimentally evolving population of bacteria, and constructed strains carrying all combinations of these mutations. For four out of these five mutations, the benefit conferred by the mutation negatively correlated with the fitness of the progenitor, which suggests that the rate of adaptation slows as fitness increases because of negative epistasis between beneficial mutations. Chou et al. (p. 1190) addressed how mutations interact with one another by estimating fitness and phenotypic measurements for the ancestor, descendent, and intermediate genotypes and found that epistastic interactions within a genome are primarily antagonistic. Together these studies suggest that adaptation involving the wider context of whole genome networks may behave entirely differently from the adaptation of single loci.

  8. DNA Computing

    One goal of so-called “synthetic biology” is to enable construction of molecular circuitry that can control biological systems or even diagnose and treat living cells from within. Qian and Winfree (p. 1196; see the Perspective by Reif) describe a system of over 100 distinct DNA strands of 15 to 30 nucleotides whose binding and replication can be controlled to perform AND, OR, NOT, NAND, and NOR logic gates logic operations. The system allowed mathematical computation of square roots in a few hours. In addition a compiler was designed that could translate a logic circuit into its equivalent circuit built of DNA sequences. The strategies used are scalable to build larger circuits, assure reliable digital behavior of the system, and suggest the possibility of embedding designed intelligent systems within biological systems.

  9. Caught in the Act

    Adenosine triphosphate (ATP)–binding cassette (ABC) transporters use the energy from ATP hydrolysis to transport substrates across a membrane against a concentration gradient. The transporters function by alternating between two conformations that expose the substrate-binding site to either side of the membrane. Oldham an Chen (p. 1202, published online 12 May) determined the structure of an intermediate pretranslocation state of the maltose transporter bound to the periplasmic maltose binding protein with maltose bound. The substrate binding induces partial closure of the interface between the two cytoplasmic ATP domains, which accelerates ATP hydrolysis and progression of the productive conformational reaction cycle of the transporter.

  10. Vibrations Beyond the Active Site


    Vibrational spectroscopy allows for probing the chemistry of enzymatic active sites with a time resolution roughly a million-fold higher than in nuclear magnetic resonance techniques. However, few studies have succeeded in applying vibrational probes to more global structural elucidation, encompassing secondary and tertiary organizational features. Remorino et al. (p. 1206) used two-dimensional vibrational echo spectroscopy to uncover the structure of a tertiary contact in the helical dimer of an integrin protein that straddles the cell membrane. The method extracts geometries by measuring distance-dependent rates of vibrational energy transfer between isotopically labeled amino acid residues at specific sites in the peptide sequence.

  11. Real Life Hif-α

    Hypoxia-inducible factor–α (Hif-α) has been linked to Notch in the regulation of stem cell maintenance, tumor cell migration, and melanoma development. However, the Hif-α/Notch interaction in vivo has largely remained uncharacterized. Using genetic tools in Drosophila, Mukherjee et al. (p. 1210) describe an in vivo role for Hif-α in the activation of full-length Notch receptor signaling. A noncanonical, ligand-independent mechanism promoted blood cell survival during both normal hematopoietic development and during a hypoxic stress response.

  12. Teaching Skills

    Students arrive at college with diverse skill sets. Studying the outcomes from large, introductory biology courses, Haak et al. (p. 1213) found that focusing on problem-solving and data analysis skills, through frequent practice, helped all the students but was particularly helpful to disadvantaged students. The change was enacted by incorporating added structure and active learning techniques to the teaching style and did not require additional funding.

  13. Whirling Swirling

    A classical fluid passing an obstacle produces waves and whirlpools. With a quantum fluid, the currents created by the presence of the obstacle give rise to the quantum counterparts of these phenomena: formation of solitons characterized by a phase slip in the fluid wave function, and the turbulent emission of quantized vortices. While these effects are difficult to observe in the usual superfluids available, Amo et al. (p. 1167) show that a highly tunable condensed-matter system can be successfully applied to the problem. The predicted quantum hydrodynamic effects were revealed in a condensate of polaritons (composite particles comprising a mixture of light and coupled negative and positive charge carriers). The tunability of the system should provide a versatile test bed for such exotic quantum fluid behavior.

  14. Setting Electron Traps

    The band theory of solid-state physics predicts that some materials are electrically conducting whereas, in fact, they are insulators. In these materials, the interactions between electrons—their Coulomb repulsions—create an energy gap and localize the conduction electrons. Singha et al. (p. 1176) created an artificial system to explore these interactions by growing a lattice of nickel disks on the surface a gallium arsenide quantum well supporting a two-dimensional electron gas. The magnetic excitations of the localized electrons, as revealed by light scattering, reveal an energy gap as well as a collective electronic ground state with a square-root dependence on applied magnetic field strength.

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