This Week in Science

Science  09 Nov 2007:
Vol. 318, Issue 5852, pp. 883
  1. Speedier Yellowstone Uplift

    CREDIT: ROBERT B. SMITH/UNIVERSITY OF UTAH

    The Yellowstone caldera is the remnant of three giant eruptions 640,000 years ago, as well as numerous smaller eruptions prior to 70,000 years ago. The region is still very active and experiences earthquakes, heat flow, and ground deformation, as well as hydrothermal activity. Chang et al. (p. 952) present satellite radar and global positioning satellite measurements which show that the caldera underwent a period of accelerated uplift between 2004 and 2006. The highest rate of about 7 centimeters per year is more than three times faster than uplift rates measured since the 1920s. The uplift may reflect ongoing magma recharge and fluid redistribution.

  2. Energizing Cosmic Rays

    Cosmic rays are energetic particles that are accelerated by magnetic fields in space. A very small fraction of the cosmic rays that penetrate Earth's atmosphere have tremendous energies, exceeding tens of EeV (1 EeV is 1018 electron volts). Their presence is puzzling because cosmic rays should lose energy very quickly as they travel through space, and so these highest-energy particles would not be expected to survive the journey. The Pierre Auger Collaboration (p. 938; see the cover and the news story by Cho) detected 80 of the highest-energy cosmic rays and located their directions in the sky by combining two detection techniques. The most energetic cosmic rays originate statistically from areas of the sky that are populated by nearby active galactic nuclei, which themselves trace galaxy-rich regions that include the supergalactic plane. Thus, the cosmic rays' huge energies might be explained if they were accelerated around giant galactic black holes lying within 75 megaparsecs of the Milky Way.

  3. Morning Methane Mist

    When the Huygens probe landed on Titan, it descended through clouds of methane and landed on a damp surface wetted by methane mist. Larger fluvial channels suggest that Titan's weather may become stormier and has prompted the need for understanding of its atmospheric methane cycles. Ádámkovics et al. (p. 962, published online 11 October) used terrestrial telescopes to map areas of high-infrared methane opacity in Titan's lower troposphere. These studies revealed methane clouds and droplets below 15 kilometers altitude and methane drizzle below 5 kilometers. These phenomena are seen preferentially on the morning side of the leading hemisphere, which suggests that diurnal temperature gradients may contribute to variations in methane relative humidity, as well as to winds and topography. Thus, morning drizzle is widespread and may be the dominant mechanism for returning from the atmospheric methane to the surface.

  4. Thrown for a Dislocation Loop

    CREDIT: ARAKAWA ET AL.

    The migration behavior of vacancies and self-interstitial atoms are of importance in understanding radiation damage effects in materials and mechanical responses to applied stresses (see the Perspective by Wirth). Arakawa et al. (p. 956) show that for α-iron, defect loops can migrate even under a zero-stress condition. Motion arises through the formation of double kinks, and fluctuations in their number drives the motion of loop defects; their diffusion rate depends on the loop size. These observations help directly confirm many of the effects associated with the migration of radiation-induced defects and dislocations loops. Prior simulation studies have indicated that small vacancy loops would not undergo one-dimensional (1D) migration in face-centered cubic metals. Matsukawa and Zinkle (p. 959) use in situ transmission electron microscopy observations of gold foil specimens that contained a large number of vacancies and observed vacancy loops showing 1D oscillatory motion. A prismatic, highly mobile vacancy-type dislocation loop can spontaneously transform into a stacking fault tetrahedron.

  5. Straining to Understand Prion Diseases

    Mammalian prion diseases are infectious lethal neurodegenerative disorders. The pathogenic agent of prion diseases is thought to lack nucleic acids, but instead relies on the propagation of a pathological conformation of the prion protein, an endogenous glycoprotein. Collinge and Clarke (p. 930) review several aspects of prion diseases, including how prion strains can be generated and maintained and suggest a model for how prion propagation involves the generation of toxic intermediates.

  6. Life and Death Decisions

    How do cells determine whether to live or die after exposure to stressors that cause misfolding of intracellular proteins? Lin et al. (p. 944) find that the IRE1 signaling branch of the unfolded protein response plays a key role in promoting cell survival after protein misfolding in the endoplasmic reticulum (ER). Human cells shut off IRE1 signaling after prolonged ER protein misfolding in order to enhance cell survival.

  7. Hybrid Vigor?

    Hybridization between species is generally considered maladaptive because the hybrid offspring are often less fit. However, Pfennig (p. 965) challenges this idea by showing that female spadefoot toads are more likely to respond to and mate with males of another species in certain habitats where such matings may result in higher offspring fitness. Furthermore, these females demonstrate increasing preference for mates that are of the same or different species on the basis of cues that predict whether hybridization is beneficial.

  8. Doing Double Duty

    During intracellular vesicular trafficking, delivery of Rab guanosine triphosphatases to sites of activation involves their release from a guanine nucleotide dissociation inhibitor (GDI) by a so-called GDI displacement factor (GDF). The identification of GDFs has been difficult. Now Machner and Isberg (p. 974, published online 18 October) provide evidence for the existence of such an activity, surprisingly in a bifunctional protein—a guanine nucleotide exchange protein associated with the formation of a replication vacuole by the intracellular pathogen, Legionella pneumophila.

  9. Toward Functional Diversity

    CREDIT: R. CAMERON COATES

    Polyketides are structurally diverse secondary metabolites that are built by large modular polyketide synthases PKSs). Initiation of synthesis usually involves S-acyltransferase and ecarboxylase domains, but several pathways lack these domains and instead contain a domain with homology to the GCN5-related N acetyltransferases (GNAT) superfamily. Gu et al. (p. 970) used structural, biochemical, and molecular biology techniques to characterize the GNAT domain from a marine PKS. The domain displays functional diversification to act as an S-acyltransferase and gain-of-function to catalyze decarboxylation, which significantly broadens the chemical reaction inventory of the GNAT enzyme superfamily.

  10. Human Brain Stem Cells in Living Color

    The adult mammalian brain can to generate new neurons from neural stem and progenitor cells that reside in the hippocampus and the subventricular zone. Manganas et al. (p. 980; see the news story by Miller) now describe a metabolic biomarker that allows the detection and quantification of neural progenitor or stem cells in the human brain in vivo, which should enable the study of neurogenesis during normal human physiology and disease pathology.

  11. Termite Determinism

    Termites are social insects with sterile worker and reproductive castes of both sexes. Previously, caste determination was believed to be determined solely by to the environment in which the offspring was raised. However, Hayashi et al. (p. 985; see the news story by Whitfield) show that caste determination has a genetic component that is linked to the X chromosome. Thus, a blend of genetics and environment governs phenotypic plasticity in termites.

  12. Conflict on the Brain

    What exactly is the function of the anterior cingulate cortex (ACC) and the dorsolateral prefrontal cortex (DLPFC) in conflict detection and conflict resolution? Mansouri et al. (p. 987, published online 25 October) performed complementary neuronal recording and lesion-behavioral studies to address this question. Monkeys performed tasks in which conflict level was manipulated. The animals exhibited behavioral effects after experiencing conflict just like those previously seen in humans. The behavioral modulations persisted after lesions in the ACC but were impaired after lesions in the DLPFC. Neurons in the DLPFC represented current and previously experienced conflict levels, and this representation was correlated with the monkeys' behavior.

  13. Minimal Interference

    The wave-particle duality of light is easily visualized in the well-known double-slit setup. Akoury et al. (p. 949) show in an ion imaging experiment that a simple H2 molecule actually suffices as a minimally complex system to visualize the boundary between classical and quantum behavior. Ejection of both electrons by a high-energy photon leads to an angular distribution that shows strong interference when the imaged electron carries most of the momentum. If the two electrons share the excess energy more equitably, however, the decoherence induced by their interaction transforms the interference pattern into a more classical, isotropic distribution.

  14. Bypass to Repair

    A class of so called “bypass” DNA polymerases has evolved that, unlike the “high-fidelity” DNA polymerases that replicate the genome in preparation for cell division, are able to copy past and “repair” lesions in the DNA. DNA polymerase η (Pol η) can replicate past two adjacent damaged bases, such as 1,2-d(GpG) cisplatin-induced adducts (Pt-GGs) formed during anticancer therapy. To understand how Pol η negotiates such “road blocks,” Alt et al. (p. 967) have determined the structure of a portion of yeast Pol η bound to DNAs containing a Pt-GG. It seems that when a deoxynucleotide triphosphate (dNTP) binds the enzyme and base-pairs effectively with the 3′ lesion base, relatively efficient elongation across the 3′G of the Pt-GG is allowed. Movement of the 5′G into the active site creates substantial template misalignment that greatly hinders elongation and reduces the dNTP-substrate interaction to a single hydrogen bond that can only form with dATP and dCTP. This interaction accounts for the promiscuity of Pol η for these bases during the second bypass step.

  15. Regulating the Regulators

    Growth factors and nutrients, particularly amino acids, regulate cell growth and proliferation in part through the serine-threonine protein kinase mTOR (mammalian target of rapamycin). Activity of mTOR is regulated through the small guanosine triphosphatase Rheb (Ras homolog enriched in brain). Bai et al. (p. 977; see the Perspective by Proud) reveal a mechanism by which Rheb could control activity of mTOR. The protein FKBP38 binds to mTOR and inhibits its kinase activity. Active GTP-bound Rheb can interact with FKBP38 and thereby free mTOR from inhibition. This process may represent a critical control mechanism whose altered function is associated with human diseases, including cancer.

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