This Week in Science

Science  26 Feb 2010:
Vol. 327, Issue 5969, pp. 1055
  1. Difference, Not Diversity


    In tropical forests, as in the ocean plankton, thousands of species may compete for the same resources. How they succeed in coexisting remains one of the central paradoxes in the study of biodiversity. Theory shows that coexisting species must partition the environment, but such partitioning is not obvious. Using data from coexisting forest trees in the southeastern United States, Clark (p. 1129) show that individual variation between members of the same species allows them to avoid direct competition: One plant may differ significantly from another in its requirements for light, nutrients, or moisture, yet remain within the general spectrum of features displayed by its conspecifics.

  2. Critical Mediators

    Helper T cells are the immune system's ringmasters, having a multiplicity of functions that mediate the body's immune responses to infections. Depending on the type of infection, CD4+ helper T cells respond by secreting specific patterns of cytokines, which provide important cues to other subsets of immune cells. CD4+ T cells with distinct cytokine profiles have been viewed classically as separate lineages; however, there is mounting evidence that these cells may not be terminally differentiated but are in fact quite plastic. O'Shea and Paul (p. 1098) review the current understanding of CD4+ T cell subset differentiation and the underlying mechanisms that drive cell-lineage commitment.

  3. Slip, Tripped, and Faulted

    Earthquake risk assessment can be improved if we were able to quantify the recurrence and magnitude of slip events. Until recently though, a lack of sophisticated seismometers has forced us to rely on anecdotal evidence from those who survived major earthquakes or to look for clues in the landscape. Zielke et al. (p. 1119, published online 21 January; see the Perspective by Scharer) analyzed high-resolution images of the San Andreas Fault in southern California. The data showed that major surface ruptures, such as the 1857 Fort Tejon earthquake, resulted from slips of only about 5 meters; much less than previously thought. In a study that lends support to this discovery, Grant Ludwig et al. (p. 1117, published online 21 January; see the Perspective by Scharer) suggest from analysis of the geomorphic features of this region that several smaller earthquakes have occurred during recent centuries rather than infrequent but larger movements. The Perspective by Scharer (p. 1089) discusses how paleoseismological studies like these may be valuable for feeding data into earthquake prediction.

  4. Lactic Fuels

    In the quest to find sustainable alternatives to petrochemicals, a small cyclic ester, γ-valerolactone, derived from cellulose offers promising raw material. Bond et al. (p. 1110) show that carbon dioxide can be catalytically excised from the lactone efficiently at high pressure, leaving a mixture of butanes. In a second-stage reactor, the butanes can be strung together to form heavier hydrocarbons similar to those found in automotive and jet fuels. The method simultaneously yields fuel and a relatively pure stream of pressurized carbon dioxide amenable to sequestration or further chemical modification.

  5. Cations in the Veins

    Major events in Earth's history, from climate change to tectonic activity, can be revealed by reconstructing past conditions of the oceans. Clues from ancient ocean chemistry can be found in the cation content of fossilized microorganisms, marine carbonates, or salt deposits from old coastal zones. As these proxies are prone to inconsistencies between samples and methodologies, Coggon et al. (p. 1114, published online 4 February; see the Perspective by Elderfield) estimated past seawater composition from the geochemistry of resistant carbonate veins precipitated within fresh basalts on the sea floor. The sudden rise to modern-day levels of ocean magnesium:calcium and strontium:calcium ratios occurred about 24 million years ago, and can be explained by a decrease in seafloor hydrothermal activity combined with a decrease in river discharge.

  6. Spin into Control


    Spintronics—the use of the spin direction of subatomic particles to control on and off states, instead of electric charge—has the potential to create low-power electronics, because less energy is needed to flip spin states than to flip switches to create voltage barriers. Theoretical work hints that spin-polarized electrons from a ferromagnetic electrode can be controlled by a change in polarization created in a ferroelectric thin film. Garcia et al. (p. 1106, published online 14 January) fabricated an iron-barium titanate junction on a lanthanum strontium manganate substrate that acts as a spin detector. Local control of spin polarization was observed in the ferroelectric layer, which retained its polarization without any applied power.

  7. Recombinant Infectious Prions

    Prion diseases are a group of fatal neurodegenerative disorders that include Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy in cows. The prion hypothesis states that the infectious agent of these diseases is an aberrant conformational isoform of the normal prion protein (PrPC), a glycosylphosphatidylinositol-anchored cell surface protein enriched in the central nervous system. The final proof for the prion hypothesis is to convert bacterially expressed recombinant PrP into an infectious prion, but this has been difficult to achieve. F. Wang et al. (p. 1132, published online 28 January; see the Perspective by Supattapone) put recombinant PrP purified from bacteria into mice and obtained all the characteristics of the infectious agent in prion disease. The recombinant form is not only resistant to proteinase-K, but also shows infectivity in cultured cells and causes rapid disease progression in wild-type mice, yielding both the behavioral and the neuropathological symptoms.

  8. Tripartite Toxin

    Luminescent bacterial symbionts of nematode worms that attack insects have long stirred interest in their possibilities for biological control. The bacteria produce a family of toxins composed of at least three subunits that resemble a widely occurring class of bacterial toxins also produced by human pathogens. Lang et al. (p. 1139) have elucidated the mode of action and structural interactions of some of these tripartite protein toxins and found that they poison the cell's actin cytoskeleton by catalyzing unusual reactions. One toxin mediated adenosine diphosphate (ADP)–ribosylation at threonine-148 to cause actin polymerization, another ADP-ribosylated Rho protein at glutamine-63, and both synergized to cause actin clustering and cell paralysis.

  9. Legume Symbiosome


    Leguminous plants (peas and beans) are major players in global nitrogen cycling by virtue of their symbioses with nitrogen-fixing bacteria that are harbored in specialized structures, called nodules, on the plant's roots. Van de Velde et al. (p. 1122) show that the host plant, Medicago truncatula produces nodule-specific cysteine-rich peptides, resembling natural plant defense peptides. The peptides enter the bacterial cells and promote its development into the mature symbiont. In a complementary study, D. Wang et al. (p. 1126), have identified the signal peptidase, also encoded by the plant, that is required for processing these specialized peptides into their active form.

  10. Bursty, Infrequent Noise

    In gene regulatory networks, positive feedback loops can give rise to bistability and hysteresis in gene expression, thereby allowing switching mechanisms and memory effects. To and Maheshri (p. 1142; see the Perspective by Levens and Gupta) eschew the commonly held idea that sigmoidal promoter responses are required to achieve a steady-state bimodal response in a positive feedback loop. Instead, using a model and data from an experiment, they favor noisy gene expression and multiple, noncooperative transcription factor binding as an explanation for the bimodal response, and they expect that similar noisy systems are widespread in biology.

  11. Inflexible Timing for Flexibility

    During critical periods in early life, sensory experience molds circuits in the brain. In the visual cortex, blurring or occluding vision in one eye triggers a rapid reorganization of neuronal responses known as ocular dominance plasticity. The critical period for this plasticity depends on inhibitory neurotransmission. Southwell et al. (p. 1145) show that by transplanting embryonic precursors of inhibitory neurons into mice, a period of ocular dominance plasticity can be induced after the end of the normal critical period. These observations suggest that transplantation of inhibitory neurons has therapeutic potential for brain repair and for treating neurological disorders and inducing periods of brain plasticity.

  12. Supernova Remnant Observations

    The sources of the cosmic rays that bombard Earth have been mysterious, but more recent explanations invoke high-energy acceleration of protons by the remnants of stellar explosions. Using the Fermi Large Area Telescope, Abdo et al. (p. 1103, published online 5 January) obtained an image of the supernova remnant W44, which shows associated gamma-ray emissions in the order of gigaelectronvolts, conforming with models indicating local proton and nuclei acceleration.

  13. Limiting Inflammatory Signaling

    Dysregulation of the transcription factor NF-κB causes chronic inflammation, autoimmunity, and malignancy. The zinc-finger protein, A20, is vital for regulating NF-κB signaling, so much so that mice lacking A20 die young of multiorgan failure and cachexia. Similar deficiencies have been seen in Crohn's disease, rheumatoid arthritis, and other inflammatory diseases of humans. But how A20 works has remained unknown. Shembade et al. (p. 1135; see the Perspective by Sriskantharajah and Ley) have discovered that the A20 protein acts through the ubiquitin labeling system of the cell required for normal protein turnover, and its lack prevents NF-κB recycling, resulting in chronic inflammation. A human T cell lymphotropic virus possesses an oncoprotein that subverts this pathway to the virus' benefit by inhibiting A20 interactions with the ubiquitin pathway to promote NF-κB signaling and cell immortalization.