Editors' Choice

Science  23 Sep 2011:
Vol. 333, Issue 6050, pp. 1680
  1. Physics

    Spin Control

    1. Jelena Stajic

    Unlike regular electronic circuitry, spintronic devices foster current based on the electron's spin, leading to much higher efficiencies as a consequence of reduced heat loss. However, spintronics is not yet ready to hit the shelves, mainly because manipulating spin is difficult and often requires very low temperatures. A crucial ingredient is the electric control of spin, achievable through spin-orbit interaction (SOI), which couples the electron's motion to its spin. One candidate for this control is a type of SOI called the Rashba effect, which leads to an energy splitting of electronic states with opposite spins—the bigger the splitting, the better. King et al. use angle-resolved photoemission spectroscopy (ARPES) to observe a very large Rashba spin-splitting effect in the material Bi2Se3, which has lately attracted attention as a promising topological insulator. The authors find that the residual adsorbates on the material surface affect the band structure, creating a quantum well, which hosts a two-dimensional electron gas (2DEG). When the concentration of these adsorbates is controllably varied, the amount of spin splitting in the 2DEG varies too: The Rashba effect is not only large, it is also tunable. Because electric gating would be expected to have the same tuning effect, which persists to room temperature, this material holds potential for realistic spintronic applications.

    Phys. Rev. Lett. 107, 96802 (2011).

  2. Microbiology

    Chlamydia Transformed

    1. Caroline Ash

    The single most important pathogen causing blindness in humans is the intracellular bacterium Chlamydia trachomatis. Despite its clinical importance, this bacterium has not succumbed to the now-standard genetic techniques required for probing the biology of pathogens. Starting with the knowledge that penicillin is not recommended for treatment of Chlamydia and allied sexually transmitted diseases (because it merely temporarily arrests replication of Chlamydia), Wang et al. have developed plasmid vectors composed of standard shuttle vectors carrying antibiotic resistance genes, ligated to Chlamydia's own plasmid. They then used penicillin selection to produce clones of the pathogen bearing the plasmid construct. They took their verified new shuttle vector to transform Chlamydia strains lacking plasmids, and en route elucidated the puzzle of the native plasmid's function: glycogen biosynthesis.

    PLoS Pathog. 7, 10.1371/journal.ppat.1002258 (2011).

  3. Sociology

    How to Spread the Word

    1. Barbara R. Jasny

    How do behaviors spread in social networks? This is of interest to the corporate world, where companies want to get the word out about their products in the most efficient way possible. It is thought that active, personalized messaging to selected friends is more persuasive than broadcasting passively to an entire network. Aral and Walker compared these two approaches by conducting a randomized field experiment on users of Facebook. A free application was designed that allowed users to share information about movies, actors, directors, and the film industry. When users downloaded the application, they were randomly designated to have personalized invitation and/or broadcast capability or neither capability. The application could record use of the messaging capabilities as well as the rate of adoption by members of the experimental subject's social network (“peers”). Although there was a greater likelihood that a peer would adopt the application when the message was personalized, many more messages were sent out by broadcast notification. Surprisingly, the users who had the capability to actively invite their peers used the application more than those who could only passively broadcast, who, in turn, used it more than those who had neither capability. By modeling and analyses, the authors concluded that when more of a user's friends adopt the application, a positive feedback loop is created, so the application is used more.

    Manage. Sci. 57, 10:1287/mnsc.1110.1421 (2011).

  4. Immunology

    Deadly Interfer(on)ence in Malaria

    1. Kristen L. Mueller

    Malaria is a devastating disease that infects more than 250 million people worldwide, and in a small percentage of individuals, many of whom are children, the infection can progress to life-threatening cerebral malaria. The host response to Plasmodium falciparum, the parasite that causes cerebral malaria, is thought to contribute to disease pathogenesis. In order to better understand this phenomenon, Sharma et al. performed global gene expression profiling and found that type I interferons, which are cytokines typically associated with antiviral immunity, were enriched in cells isolated from febrile patients with malaria. Further analysis demonstrated that AT-rich regions of the P. falciparum genome, which number over 6000, induced type I interferon production by human cells. Surprisingly, none of the known nucleic acid sensors of the host immune system were required for parasite-induced type I interferon production. This response, however, did depend on STING and TBK1, proteins known to signal downstream other DNA sensors. Because type I interferons can cause immune pathology, the role of the STING-TBK1-type I interferon pathway was examined in a mouse model of cerebral malaria. Genetic ablation of this pathway in mice resulted in protection from disease. Whether this pathway is also an important driver of cerebral malaria in humans will need to be examined.

    Immunity 35, 194 (2011).

  5. Cell Biology

    Divide to Survive

    1. Stella M. Hurtley

    Mitochondria are critical cellular sources of energy in the form of ATP within metazoan cells. During cell division, it is imperative that each daughter cell receive a competent complement of mitochondria—for which the mitochondria need to be separated by a fission process. Fission is promoted by a large GTPase, DRP1, itself a target of a mitotic kinase cyclin B–CDK1. Kashatus et al. asked how mitochondrial fission is linked to cell division in human tissue culture cells. They found that, during mitosis, another mitotic kinase, Aurora A, phosphorylates a Ras-like GTPase, RALA, which localizes to the mitochondria, in turn recruiting its effector RALBP1 and DRP1. Interfering with this process by reducing the amounts of RALA or RALBP1 impaired mitochondrial fission at mitosis and led to improper segregation of mitochondria to daughter cells during cytokinesis. Under these circumstances, cell numbers and levels of ATP were reduced. This coordination of mitochondrial fission and segregation during cell division is thus a key factor in maintaining cellular health.

    Nat. Cell Biol. 13, 1108 (2011).

  6. Microbiology

    Protect and Serve

    1. Nicholas S. Wigginton

    Uranium-contaminated groundwater is a concern in regions with naturally high levels of uranium-rich sediments, and also in areas that have a legacy of nuclear weapons use or production. One possible, relatively noninvasive strategy for cleaning up these former sites of mobile U(VI) ions is the use of certain types of bacteria to promote the reductive precipitation of tiny U(IV) mineral grains that are insoluble and presumably immobile. However, much remains unknown about how these metal-reducing bacteria reduce U(VI) and what U(IV) phases ultimately form, especially in light of the recent discovery that some bacteria can pass electrons across long distances away from the cell through extended hairlike pili structures. Cologgi et al. discovered that for one particular species, Geobacter sulfurreducens, the pili, not redox-active cytochrome proteins, do most of the work reducing U(VI) to U(IV). When pili growth was encouraged, solid-phase U(IV) aggregates tended to form extensive networks away from the cell and along the pili. Because this reduction strategy prevents U(IV) encrustations from enveloping the cell or forming in the periplasm, this species may use its pili to reduce U(VI) as a cellular protection mechanism.

    Proc. Natl. Acad. Sci. U.S.A. 108, 15248 (2011).

  7. Chemistry

    Bubbling Biomass

    1. Jake Yeston

    What exactly happens when plants burn? The question is likely among the oldest pondered by humanity, and yet it's drawing newfound attention in the context of how biomass might be productively transformed into a sustainable source of bulk chemical feedstock in place of petroleum. Though chemists have a reasonable grasp of the web of reactions underlying combustion, the source of certain nonvolatile compounds in the product stream—particularly those that enter the gas phase—remains uncertain. Teixeira et al. combine high-speed photography of heated cellulose with extensive theoretical fluid modeling to uncover a plausible mechanism for ejection of this product class. Specifically, they put forward a process termed reactive boiling ejection, whereby bubbles form and then rapidly collapse in a short-lived molten cellulose phase. The collapse expels streams of aerosols that contain nonvolatile material. They argue that a deeper understanding of this process should lead to more control over product distributions.

    Energy Environ. Sci. 4, 10.1039/c1ee01876k (2011).

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