This Week in Science

Science  20 Feb 2009:
Vol. 323, Issue 5917, pp. 979
  1. Variation Comes in Threes

    Triplet repeat expansions underlie many human genetic disorders and phenotypic variation in microbes, but there are no natural models for experimental studies. Sureshkumar et al. (p. 1060, published online 15 January) found a wild isolate of Arabidopsis collected in Ireland bearing an extended triplet repeat in an intron of the GREENWORMS gene, which reduced its expression at high temperatures and resulted in growth defects. This model could be valuable for testing natural selection operating on the variation found in extreme repeat expansions.

  2. Etch-A-Sketch Oxide Electronics

    The interface between two transition-metal oxides has been shown to give rise to insulating, metallic, and even superconducting behavior. The thickness of the oxides, down to the difference of a single atomic layer, is critical to determining which type of behavior occurs. Until now, fabrication of metal oxide devices required complex lithographic processes. In a simpler approach, Cen et al. (p. 1026; see the Perspective by Reiner et al.) used a conducting atomic force microscope to pattern the devices into forms including tunnel junctions and field-effect transistors, thus paving the way to make more complex oxide electronic circuitry.

  3. Measuring Martian Methane

    Mars' atmosphere is highly oxidizing, so the presence of any methane would imply active geological, or perhaps even biological, processes. Hints of methane have been detected with both space-based and Earth-based spectrometers. While using Earth-based telescopes, Mumma et al. (p. 1041, published online 15 January) detected transient methane plumes on Mars originating from rocks rich in clay minerals. The amount of methane emitted is comparable to massive hydrocarbon seeps found on Earth.

  4. Damping Decomposition

    Catalysts not only lower the barrier for converting reactants to products, they can also catalyze decomposition of the product. Usually, this reverse reaction can be minimized by rapid product removal—or in the case of heterogeneous catalysts, by inhibiting product readsorption. Edwards et al. (p. 1037) found that the rate of decomposition of H2O2 occurring during its direct synthesis from H2 and O2 on supported gold-palladium nanoparticles could be reduced by acid pretreatment of the carbon support. The treatment resulted in smaller nanoparticles of the catalyst that were less active for H2O2 decomposition.

  5. Ancient Microbial Metabolism

    Different metabolic pathways produce different, and in some cases diagnostic, isotope fractionations. Measurements of stable isotopes in rocks can thus provide clues to the early evolution of microbial metabolisms.Garvin et al. (p. 1045) studied nitrogen and carbon isotopes from the 2.5-billion-year-old McRae Shale in western Australia to trace microbial nitrogen-processing as the planet oxidized. The data indicated that a nitrogen biochemical cycle had developed by this time and that at least one of the three modern groups of nitrogen-processing microbes had evolved.

  6. Mid-Atlantic Ridge Crust Cooling

    Plate tectonics on Earth involves the formation of new oceanic crust at mid-ocean ridges and the return into the mantle of crust at subduction zones. The formation of crust can be roughly dated using magnetic signatures, but the detailed mechanism has been difficult to discern. Recent analytical advances have allowed the dating of zircons discovered in ocean crust; using these, Lissenberg et al. (p. 1048, published online 29 January; see the Perspective by Michael and Cheadle) have been able to establish detailed dates of the magmatic bodies formed at the Mid-Atlantic Ridge. The age range of the magmatic intrusions allows estimation of the cooling rate of the crust, and their systematic increase in age away from the center also indicates that slow and even spreading occurs at least at the location sampled.

  7. Stiffening Before Melting

    A general everyday experience is to heat something up and observe the material become softer, as injecting energy into matter weakens the bonds holding things together. However, certain transient molecules, termed exciplexes, have stronger bonds in an electronically excited state than in the ground state. Theory predicts that a highly excited electronic state of metallic gold should exhibit a similar bond, stiffening event before the burst of injected energy equilibrates throughout the lattice and causes melting. Using electron diffraction with subpicosecond temporal resolution to measure the effect and high-energy laser pulses to melt the gold, Ernstorfer et al. (p. 1033, published online 22 January) observed a delay in the onset of melting and concluded gold exciplexes had been formed.

  8. Digits and Development

    During embryonic development, cell populations achieve their correct identities, expand by controlled proliferation, and organize into functional tissues and organs. Although many associated signaling pathways are understood, it is less clear how they interact during organ and tissue morphogenesis in vertebrates. Bénazet et al. (p. 1050) have uncovered feedback loops interlinking three signaling pathways that impact differentially on the bone morphogenetic protein antagonist Gremlin1, which has emerged as the major node in the system. This self-regulatory signaling system may have contributed to stabilization of digit numbers and identities during evolution.

  9. Differential Aging

    On one plant, leaves may age at different rates. Kim et al. (p. 1053) have analyzed the molecular signals that govern aging of leaves in Arabidopsis and found that mutations in a handful of genes slowed the normal process by which the leaves aged. One gene encodes a transcription factor previously known for its response to saline conditions. The transcript of this gene is a target for microRNA: miR164. As the amount of miR164 decreases with age (a decline that is regulated by yet another gene captured in this screen), the gene encoding the transcription factor activates cell death processes, which ultimately results in the death of the entire leaf.

  10. Marine Millefeuille

    Given the scale and energy of fluid flow in the ocean, it is perhaps surprising that many planktonic microorganisms can aggregate within a few hours below the surface to form sheets that range from millimeters to meters in thickness and can extend for kilometers. Durham et al. (p. 1067; see the Perspective by Grünbaum) explain how thin layers form when the upward swimming of photosynthetic algae is interrupted by a discontinuity, or shear, in water flow that flips the organisms into a tumbling nondirectional movement. This makes them halt and accumulate at the flow interface. Depending on the predominating species, the thin layers of phytoplankton will be a focus for grazing, predation, sex, and death.

  11. RIGing the CARDs

    RIG-1 is a multidomain cytoplasmic protein that recognizes double-stranded (ds) regions and 5′-triphosphates on viral RNA and triggers an immune response. It consists of two caspase activation and recruitment domains (CARDs) that participate in downstream signaling; an adenosine triphosphatase domain, whose function to date is unclear; and a regulatory domain that inhibits activity in the absence of viral RNA. Myong et al. (p. 1070, published online 1 January) used a single-molecule fluorescence assay to show that adenosine triphosphate fuels RIG-1 translocation on dsRNA, but this activity is suppressed by the CARD domains unless the RNA also carries a 5′-triphosphate. The translocation activity integrates the distinctive molecular patterns of the two viral RNA components and subsequently leads to a conformational change, with the CARD domains exposed to downstream activators that then cascade into an innate immune response.

  12. Old Brains for New

    In the adult brain, transient circuit activity in mature neurons can exert long-lasting effects on adult neurogenesis, and new neurons are continually generated in certain regions. Ma et al. (p. 1074, published online 1 January) found that an activity-inducible gene called Gadd45b in mature neurons enhanced the expression of diffusible factors, including neurotrophic and growth factors, to promote adult neurogenesis. Gadd45b appeared to act via epigenetic DNA modification to elicit long-lasting cellular changes. Thus, transient but behaviorally relevant neuronal activity can be coupled with the local generation of new neurons and stimulate long-term structural plasticity for the brain.

  13. Mating Music

    Mosquitoes are common vectors for a multiplicity of disease scourges, including malaria and dengue fever. Cator et al. (p. 1077, published online 8 January) found that males and females participated in a mating duet in which they alter their wing beat frequencies to match the higher harmonics of their partners. In the case of Aedes aegypti, this occurs at roughly 1300 Hz, which lies beyond what was previously supposed to be the hearing range of males (females were widely assumed to be deaf). This discovery may become a valuable adjunct to transgenic or sterile insect approaches for vectorborne disease control.

  14. Sapphire Wafers Yield Block Copolymers

    Block copolymers are used as lithographic templates because the two chemically dissimilar polymeric components can be made to phase-separate into patterned structures. A challenge is to avoid defects, but pre-patterning techniques that have been used to break the symmetry of the underlying substrate are slow or chemically complex. Park et al. (p. 1030) used commercially available sapphire wafers, which have a sawtooth topography over their entire surface that guides the self-assembly of various block copolymers in a way that maintains the order of the microdomains across the whole wafer.

  15. DNA Kills

    DNA in the cytoplasm of mammalian cells indicates an infection or gross DNA damage, and, on detecting it, cells respond suicidally to stop the spread of infection. Roberts et al. (p. 1057, published online 8 January) have discovered that the activation of the cell-death enzymes caspase 1 and 3 in macrophages is regulated by members of a family of double-stranded DNA binding proteins. One of these, p202, dampened the caspase response, whereas another, AIM2, escalated inflammasome-dependent caspase activation. The balance of p202 and AIM2 thus provides a regulatory mechanism for DNA-dependent caspase activation in the cell and the switch for life or death.

  16. Caloric Intake and Stress Relief?

    What biochemical mechanisms link an organism's caloric intake with the mechanisms that help protect cellular proteins during stress? The nicotinamide adenine dinucleotide-dependent deacetylase SIRT1 contributes to the life span-extending effects of low caloric intake by altering gene transcription. The transcription factor heat shock factor 1 (HSF1) also has a role in this process, acting to enhance transcription of genes encoding protein-protective chaperone proteins. Westerheide et al. (p. 1063; see the Perspective by Saunders and Verdin) showed that SIRT1 may acetylate HSF1 directly, thus enhancing its activation of transcription of target genes. Furthermore, overexpression of SIRT1 in cultured cells helped the cells to survive exposure to heat stress.