This Week in Science

Science  25 Sep 2015:
Vol. 349, Issue 6255, pp. 1500
  1. Quantum Simulation

    Visualizing edge states in atomic systems

    1. Jelena Stajic

    Illustration of atoms in a synthetic lattice


    Visualizing edge states in atomic systems Simulating the solid state using ultracold atoms is an appealing research approach. In solids, however, the charged electrons are susceptible to an external magnetic field, which curves their trajectories and makes them skip along the edge of the sample. To observe this phenomenon with cold atoms requires an artificial magnetic field to have a similar effect on the neutral atoms (see the Perspective by Celi and Tarruell). Stuhl et al. obtained skipping orbits with bosonic atoms using a lattice that consisted of an array of atoms in one direction and three internal atomic spin states in the other. In a complementary experiment, Mancini et al. observed similar physics with fermionic atoms.

    Science, this issue pp. 1514 and 1510; see also p. 1450

  2. Mitochondrial Import

    Dissecting the mitochondrial entry portal

    1. Stella M. Hurtley

    Mitochondria, the powerhouses of the cell, are mainly composed of proteins made in the cytosol. These newly synthesized proteins need to be imported across the organelle's membrane through dedicated protein import machinery. Shiota et al. have worked out the architecture and mechanism of the mitochondrial protein import channel.

    Science, this issue p. 1544

  3. Protein Folding

    Interactions that slow protein folding

    1. Valda Vinson

    As proteins fold, they diffuse over an energy barrier that separates unfolded and folded states. The transition path defines how a single protein crosses the barrier and so contains key information on the mechanism of folding. Transition paths have not yet been experimentally observed, but Chung et al. have discovered which structural features of the protein affect the duration of the transition. As the protein folds, non-native salt bridges form and break, slowing diffusion along the transition path.

    Science, this issue p. 1504

  4. Nanomaterials

    Flat perovskite crystals

    1. Phil Szuromi

    Bulk crystals and thick films of inorganic-organic perovskite materials such as CH3NH3PbI3 have shown promise as active material for solar cells. Dou et al. show that thin films—a single unit cell or a few unit cells thick—of a related composition, (C4H9NH3)2PbBr4, form squares with edges several micrometers long. These materials exhibit strong and tunable blue photoluminescence.

    Science, this issue p. 1518

  5. Astrophysics

    Placing bounds on gravitational wave detection

    1. Ian S. Osborne

    Gravitational waves are expected to be generated by the interaction of the massive bodies in black-hole binary systems. As gravitational waves distort spacetime, it should be possible to verify their existence as they interfere with the pulses emitted by millisecond pulsars. However, after monitoring 24 pulsars with the Parkes radio telescope for 12 years, Shannon et al. found no detectable variation in pulsar records. This nondetection result indicates that a new detection strategy for gravitational waves is needed.

    Science, this issue p. 1522

  6. Transposons

    Parasitic DNA targets a genomic home

    Long-terminal-repeat (LTR) retrotransposons are a form of parasitic DNA that can jump around within the host's genome. To avoid damaging resident genes, they have been selected to integrate away from protein-coding sequences. For instance, the fission yeast LTR retrotransposon Tf1 inserts at nucleosome-free regions in gene promoters. Jacobs et al. show that Tf1 is directed to these insertion sites by a specific DNA binding protein, Sap1, which forms DNA replication–fork barriers.

    Science, this issue p. 1549

  7. Immunology

    Mitochondria signal ‘eat me’

    1. John F. Foley

    Cardiolipin becomes exposed if mitochondria become damaged. Under these circumstances it becomes an “eat me” signal. Cardiolipin is a phospholipid found in the inner mitochondrial membrane and in bacterial membranes. Balasubramanian et al. show that cardiolipin stimulates macrophages to phagocytose the damaged cells. But it also inhibits cytokine production from macrophages when they encounter bacterial lipopolysaccharide, which resembles cardiolipin in structure.

    Sci. Signal. 8, ra95 (2015).

  8. Evolutionary Ecology

    Climate change decoupling mutualism

    1. Sacha Vignieri
    Generalist bees replace specialists as climate warmsPHOTO: CANDACE GALEN

    Many coevolved species have precisely matched traits. For example, long-tongued bumblebees are well adapted for obtaining nectar from flowers with long petal tubes. Working at high altitude in Colorado, Miller-Struttmann et al. found that long-tongued bumblebees have decreased in number significantly over the past 40 years. Short-tongued species, which are able to feed on many types of flowers, are replacing them. This shift seems to be a direct result of warming summers reducing flower availability, making generalist bumblebees more successful than specialists and resulting in the disruption of long-held mutualisms.

    Science, this issue p. 1541

  9. Virology

    Therapeutic opportunity knocks

    1. Angela Colmone

    The urinary tract of most adults harbors JC polyomavirus (JCV) asymptomatically but persistently. In immunocompromised individuals, JCV can opportunistically infect the brain to cause the debilitating and frequently fatal disease progressive multifocal leukoencephalopathy (PML). No treatments are currently available for PML, but two papers have identified and exploited a gap in immune responses to JCV. Ray et al. report that JCV strains found in the cerebrospinal fluid of PML patients have mutations that prevent antibody neutralization and that these blind spots can be overcome by vaccination. Jelcic et al. suggest that broadly neutralizing antibodies derived from a patient who recovered from PML can also be used therapeutically.

    Sci. Transl. Med. 7, 306ra151, 306ra150 (2015).

  10. Aging

    Transcriptional control of cell senescence

    1. L. Bryan Ray

    Senescent cells that have stopped proliferating secrete molecules that influence the cells around them. Prevention of this senescence-activated secretory phenotype seems to slow organismal aging. Kang et al. explored the regulatory process behind cell senescence and found that DNA damage led to stabilization of the transcription factor GATA4 (see the Perspective by Cassidy and Narita). Increased activity of GATA4 in senescent cells stimulated genes encoding secreted factors. GATA4 also accumulates in the brains of aging mice or humans.

    Science, this issue p. 10.1126/science.aaa5612; see also p. 1448

  11. Biocatalysis

    A clean and green approach to amines

    1. Jake Yeston

    Enzymes evolved to operate in water and to modify their substrates using comparatively nontoxic reagents. Thus, a major advantage of applying enzymes to synthetic chemistry is their compatibility with environmentally benign conditions. Mutti et al. report that two enzymes—alcohol and amine dehydrogenases—can operate in tandem to convert alcohols to amines. The reaction proceeds with ammonium as the only input and water as the only byproduct. The mechanism relies on consecutive oxidation and reduction steps, with hydrogen shuttled by a nicotinamide coenzyme.

    Science, this issue p. 1525

  12. Batteries

    A solution for scalable-flow batteries

    1. Marc S. Lavine

    Flow batteries, in which the redox active components are held in tanks separate from the active part of the cell, offer a scalable route for storing large quantities of energy. A challenge for their large-scale development is to avoid formulations that depend on toxic transition metal ions. Lin et al. show that quinones can be dissolved in alkaline solutions and coupled with ferricyanides to make a flow cell battery (see the Perspective by Perry). This gives scope for developing flow cells with very low costs, high efficiencies at practical power densities, simplicity of operation, and inherent safety.

    Science, this issue p. 1529; see also p. 1452

  13. Catalysis

    A trio helps activate C-H bonds in alcohols

    1. Jake Yeston

    Enzymes can accelerate chemical reactions by activating specific portions of a molecule through well-placed H bonds. Jeffrey et al. showcase the power of H-bonding in a synthetic context. Here, reactivity at the C centers of alcohols is selectively induced by using an H-bonding catalyst to bind the hydroxyl group of the alcohol. The adjacent C-H bonds now become susceptible to a reaction accelerated by another pair of catalysts. In combination, the trio of catalysts promotes C-C bond formation at the alcohol C within an array of competing sites.

    Science, this issue p. 1532

  14. Climate

    Extreme events under climate change

    1. Julia Fahrenkamp-Uppenbrink

    Extreme climate events, such as heatwaves and droughts, can have devastating consequences for human populations. Many of these events are likely to become more common under climate change. But is it correct to say that an individual climate event has been caused by climate change? In his Perspective, Solow argues that even if the event frequency (e.g., of drought conditions in Europe) increases, it is not possible to say whether an individual event would have happened without climate change. The effects of climate change have become so pervasive that it may not even make sense to ask the question.

    Science, this issue p. 1444

  15. Paleoceanography

    Flushing the deep ocean

    1. H. Jesse Smith

    Have changes in ocean circulation contributed to the sudden increases in the concentration of atmospheric carbon dioxide that occurred during the last deglaciation? Chen et al. provide a high-resolution radiocarbon record for that time, derived from deep sea corals. This record shows that two deep ocean “flushing” events were accompanied by abrupt rises in carbon dioxide and Northern Hemispheric warming. There is a clear connection between these ocean processes and the atmosphere during this interval.

    Science, this issue p. 1537

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