This Week in Science

Science  13 Jul 2007:
Vol. 317, Issue 5835, pp. 165
  1. Deep Gas, Destructive Eruptions


    The explosive activity of Italian volcano Stromboli makes it a dangerous neighbor for local populations, tourists, and volcanologists. The explosions are thought to be caused by gas slugs that rise faster than the surrounding magma and generate seismic activity near the surface of the volcano. However, the source of the gas is unclear. During quiescent and explosive periods, Burton et al. (p. 227) have measured spectroscopic variations in the gas composition emanating from Stromboli that indicates the gas has a deep origin. Using a gas-solubility model, they show that gas slugs form 3 kilometers beneath the summit craters, at the base of the volcanic pile and away from seismological processes at the surface.

  2. Graphene Transport Up Close

    Defects may play a critical role in the transport properties of graphene (single sheets of graphite supported on a substrate), especially in possible applications in electronic devices. Rutter et al. (p. 219) used scanning tunneling spectroscopy to probe the local electronic properties of graphene bilayers grown epitaxially on a silicon carbide substrate. They show that the transport properties are critically influenced by the microscopic properties of the sample, particularly electronically active defects in graphene that can scatter the electrons and cause interference and localization.

  3. Cleaving Alkyl Mercury

    Bacteria have evolved a group of enzymes that can deactivate highly toxic alkyl mercury contaminants, but many of the molecular details underlying their mode of action remain unclear. The organomercurial lyase MerB specifically accomplishes scission of Hg-C bonds. Melnick and Parkin (p. 225; see the Perspective by Omichinski) report that a ligand bearing three coordinating sulfur groups, analogous to active-site cysteines in the enzyme, efficiently induces reaction of a mercury methyl, ethyl, or cyanomethyl center with a thiol to liberate the alkane or nitrile. Characterization of the Hg methyl and ethyl complexes in the solid state and in solution reveals that although an overall two-coordinate geometry is favored, the metal interacts rapidly with the additional sulfur groups in the ligand, which appear to promote reactivity lacking in other molecular Hg compounds.

  4. Oxygen Reservoir


    Oxygen isotopic anomalies have been found for planets, asteroids, and comets, but their origin remains an outstanding problem in cosmochemistry. In the early solar system, two isotopically distinct nebular reservoirs, one rich in 16O and the other rich in 17O and 18O, appear to have been mixed together. However, measuring their original ratios in secondary minerals is difficult because aqueous environments allow isotope exchange between water and rocks. Sakamoto et al. (p. 231, published online 14 June; see the Perspective by Young) have found a distinctive material in the matrix of a primitive carbonaceous chondrite meteorite (Acfer 094) that is highly enriched in 17O and 18O relative to the Earth. It formed by oxidation of iron-nickel metal and sulfides by water in the protoplanetary disk. This meteorite is the most extreme 16O-depleted material that is not demonstrably a presolar grain, and may be sampling a 17,18O-rich reservoir in the early solar system.

  5. More Rain Likely

    Climate models and observations both suggest that global precipitation and the amount of water in the atmosphere will rise with temperature, but models predict that rainfall will increase only half as fast as humidity. Wentz et al. (p. 233, published online 31 May; see the cover) analyzed satellite data on precipitation and found that precipitation and total atmospheric water content have actually increased at approximately the same rate during the past 20 years. The reason why models predict a difference between the rise of atmospheric water content and rainfall is unclear, but these results suggest that the potential for global warming to cause drought may be less than was feared.

  6. Suppressing Emotional Memories


    Can people suppress emotional memories and, if so, how do they do it? By examining activity in brain regions that support memory processing, Depue et al. (p. 215) provide evidence that an active memory suppression mechanism really exists. First, one portion of prefrontal cortex suppresses regions involved in the sensory aspects of memory. Second, a different part of prefrontal cortex suppresses brain regions that support memory processes as well as those brain areas that support emotional associations with memory. The results may help to explain the lack of control exhibited in a variety of psychiatric disorders, over emotional memories and thoughts, and extend our understanding of brain mechanisms that control their formation.

  7. Chemical Consequences

    Global regulators of commercial chemicals apply a scientific paradigm that relates the biomagnification potential of the chemical in food webs to the chemical's hydrophobicity. However, Kelly et al. (p. 236; see the news story by Kaiser) show that current methods fail to recognize the food web biomagnification potential of certain chemicals. Certain chemicals do not biomagnify in most aquatic food chains, but biomagnify to a high degree in air-breathing animals, including humans, because of low respiratory elimination. Thus, additional criteria for evaluating biomagnification and toxicity in chemicals that biomagnify are required.

  8. Crystallized Kinase Regulation


    Many human cancers involve gain-of-function mutations in the phosphoinositide 3-kinase PI3Kα. The kinase is a heterodimer of a catalytic subunit (p110α) and a regulatory subunit (p85α), with both subunits comprising multiple domains. Miled et al. (p. 239; see the Perspective by Lee et al.) have determined the crystal structure of the adaptor-binding domain of p110α bound to the inter-SH2 domain of p85α at 2.4 angstrom resolution, and have performed functional studies to investigate the effect of oncogenic mutations in the helical domain of p110α on its interaction with the N-terminal SH2 domain of p85α. The studies suggest how these two classes of mutations cause the up-regulation of PI3Kα that can lead to cancer.

  9. Cohesin Does the Business

    To ensure the sorting of a complete complement of chromosomes to both daughter cells in cell division, sister chromatids are bound together by a ring-shaped molecular complex called cohesin. The accurate repair of double-stranded lesions in DNA also relies on cohesion between homologous regions of sister chromatids. Both these processes are often misregulated in cancer. Cohesion has been thought to require ongoing DNA replication (see the Perspective by Watrin and Peters). Ünal et al. (p. 245) and Ström et al. (p. 242) now show in yeast that double-stranded breaks can induce cohesion in the absence of replication, and that the deposition of cohesin is not limited to the region of the break but extends across the entire genome, and thus may play a role in maintaining genome stability.

  10. Growth Hormone and Development

    During development, genes are often transcribed in a temporally and spatially regulated manner. The murine growth hormone gene is differentially expressed in the developing pituitary gland. Lunyak et al. (p. 248) now examine the region surrounding the growth hormone gene and show that a repeated DNA sequence (short interspersed nuclear element B2) in the growth hormone locus functions as an insulator to produce a boundary for chromatin domains and limit the action of regulatory factors such as enhancers and silencers.

  11. Solution Routes to Tandem Solar Cells

    Tandem solar cells can take advantage of more of the available radiant energy by linking two cells in series and combining their voltage output via a transparent conducting layer. One cell has a larger band gap and absorbs at shorter wavelengths, and the other with a narrower band gap uses the longer wavelengths. Although such cells have been realized, their multilayer architecture often leads to high fabrication costs, as at least one layer requires vapor-deposition methods. Kim et al. (p. 222) report on an all-solution-processed tandem semiconducting polymer cell in which all of the layers, including the transparent TiOxconnecting electrode that enables a number of important carrier transport functions, are processed from solution. The efficiency exceeds 6% even at high light intensities.

  12. From Protein Interactions to Complex Physiology

    Docking or adaptor proteins may provide the building blocks from which complex cellular biological regulatory systems could be built during the course of evolution. Hardy et al. (p. 251) examined the docking protein ShcA, which has a protein tyrosine binding (PTB) domain and a Src-homology 2 (SH2) domain, each of which can bind to phosphorylated receptors and other proteins. Shc proteins also contain tyrosine residues that become phosphorylated and serve as binding sites for the adaptor protein Grb2 (growth factor receptor-bound protein 2). In mice that expressed mutant proteins with one or more domains altered, the PTB domain was essential for development of the heart but the SH2 domain and Grb2-binding domains were not. However, successful development of skeletal muscle required that the PTB, SH2, and Grb2-binding domains be present within the same Shc molecule. Thus, acquisition of new protein-protein interaction motifs within proteins and their distinct use in various cell types or organisms may support complex physiological functions.

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