This Week in Science

Science  01 Feb 2008:
Vol. 319, Issue 5863, pp. 545
  1. Assessing Earth's Inorganic Hydrocarbons


    A long-standing question, important not just for petroleum resources but possibly in the origin of life, is the degree that a series of inorganic reactions that lengthen carbon chains (known as Fischer-Tropsch type reactions) might yield hydrocarbons from mantle methane. Although several examples of such hydrocarbons have been inferred, it has been difficult to demonstrate a purely mantle, abiogenic origin in the face of abundant biogenic hydrocarbons. Proskurowski et al. (p. 604) now show that the abundance of hydrocarbons in the Lost City vent field, an off-axis system in the Atlantic Ocean, decreases systematically with chain length in a manner predicted by Fischer-Tropsch type reactions. Analysis of carbon isotopes further support an inorganic origin. Because this system is likely representative of many similar systems in the oceans, an abundant source of mantle-derived hydrocarbons may be present on Earth, as well as during Earth's early history.

  2. Water as Glass and Liquids

    When molecular liquids form glassy phases, the energetic change is often of the same magnitude as when they form crystals—in both cases, large amounts of translational and rotational energy must be lost. In that regard, the glass transition for pure water that occurs at between 120 and 160 kelvin is puzzling in that it occurs with a very modest change in heat capacity. Angell (p. 582) reviews the many studies of water's glass transition, including those of aqueous solutions and of water confined to nanoscopic environments. He concludes that ∼225 kelvin, a temperature often associated with water's “second critical point,” an order-disorder transition occurs that accounts for most of the energetic changes. Hence, liquid water appears to exist in two forms—a “fragile” liquid (a poor glass-former) above this temperature, and a “strong” liquid (a good glass-former) below.

  3. More Relaxed on the Surface

    The glassy state of materials, in which a liquid-like structure is frozen in place below a specific temperature, may manifest differently in the bulk of the materials versus the surface region. Fakhraai and Forrest (p. 600; see the Perspective by Dutcher and Ediger) probed the glass transition in an amorphous polymer by embedding gold nanoparticles onto the surface of a polystyrene film and allowing them to sink into the film, where they make small indentations. They then removed the gold, using mercury, and were able to watch the surface relaxation at various temperatures. They observed enhanced surface relaxation, that is, greater mobility of the polymer chains, at the surface relative to the bulk.

  4. Cuprate Liquid Crystal


    Recent experimental work has revealed exotic electronically ordered phases in correlated electron systems akin to those seen in conventional liquid crystals. These effects have manifested themselves as anisotropic transport properties, in which conductivity depends upon direction within the sample. Hinkov et al. (p. 597, published online 10 January) used neutron scattering to investigate the role of spin fluctuations in the macroscopic, nematic liquid-crystalline electronic behavior of the high-temperature superconductor YBa2Cu3O6.45. They find that an anisotropic ordering of the spins begins at 150 kelvin, well above the temperature where static magnetism occurs, and appears to develop in parallel to the previous reported transport properties. They argue that these fluctuating spins are at the core of the electronic liquid-crystalline behavior in correlated electron systems.

  5. Food for Thought

    One of the most potentially harmful effects of climate change may be its impact on agriculture in food-insecure regions. Lobell et al. (p. 607; see the Perspective by Brown and Funk) analyze the climate change-related risk for agriculture in 12 regions worldwide that collectively represent a population of nearly 1 billion people in order to identify which general approaches to adaptation will be most effective in different areas. They find that South Asia and Southern Africa are two regions particularly at risk from negative impacts on several crops, and that uncertainties vary widely by crop. Also, because the reasons underlying a region's vulnerability differ, the adaptation priorities that ultimately need to be followed will depend on how investment institutions perceive uncertainty and risk.

  6. Getting the Balance

    Right Balancing selection, the maintenance of multiple alleles within a population, is a means by which genetic diversity may be maintained within a species. Seidel et al. (p. 589, published online 10 January) have discovered a globally distributed genetic incompatibility that causes embryonic death among natural isolates of the nematode worm, Caenorhabditis elegans. The incompatibility persists despite its negative consequences for fitness, which contradicts the prediction that natural selection should eliminate genetic incompatibilities from interbreeding populations.

  7. Regulating Ovulation

    In mammals, the ability of a female to remain fertile for an extended period depends on the continuous awakening of primordial follicles from their dormant state in the ovary. Menopause, or the natural end of female reproductive life, occurs when the pool of primordial follicles has been depleted. The mechanisms controlling follicular activation have remained a mystery. Reddy et al. (p. 611; see the news story by Marx) now reveal that follicle activation is controlled by the oocyte PTEN (phosphatase and tensin homolog deleted in chromosome 10)-phosphatidylinositol 3-kinase pathway. In a mouse model where Pten is deleted specifically in oocytes, the entire pool of primordial follicles is prematurely activated and subsequently depleted in early adulthood, which results in premature ovarian failure.

  8. Maternal Influences

    Fertilization is a dynamic process of the transition from two highly specialized cells—the oocyte and spermatozoon—into the totipotent zygote. Maternal and paternal contributions to the zygote are not equal. In addition to nuclear DNA, oocytes and spermatozoa are equipped with complementary arsenals of structures such as mitochondria and centrioles for the creation of developmentally competent embryos. By using the microsurgical manipulation of mammalian oocyte nucleolus, Ogushi et al. (p. 613) demonstrate that the nucleolus, a subnuclear organelle important in ribosome assembly, is exclusively of maternal origin. The oocyte nucleolus is essential for nucleolus assembly in zygotes, and is thus also essential for normal embryonic development.

  9. Growth and Survival, the Complete Toolkit


    As tumors progress to a more aggressive state, they acquire multiple genetic alterations, some of which have little functional impact and others that are essential for the continued growth and survival of the tumor cells. Schlabach et al. (p. 620) and Silva et al. (p. 617) have developed a functional genomics strategy that will allow, at a genome-wide level, systematic identification of genes required for cell growth and survival. Cell lines derived from human mammary and colorectal cancers and normal mammary tissue showed a similar pattern of so-called “essential” genes, with many residing within functional pathways known to be critical for fundamental cellular processes such as cell cycle and translational control. Importantly, however, additional genes were identified as being essential for the growth of specific cell lines. This functional genomics strategy complements the cancer genome sequencing approaches that have shown recent success and could set the stage for high-throughput discovery of cancer drugs.

  10. Interfering with Inflammation

    The efficient and selective targeting of small interfering RNA (siRNA) molecules to cells could help to harness this technology for treating disease. Peer et al. (p. 627; see the Perspective by Szoka) combine nanoscale liposomal packaging of siRNAs with antibody targeting to immune cells. The targeted siRNA cargo was able to find and efficiently inactivate its target, a key cell-cycle regulating molecule called Cyclin D1. Furthermore, the systemic injection of the packaged siRNA particles reversed pathology in a mouse model of inflammatory bowel disease.

  11. T Cell Role for Cathepsin K

    Cathepsins are cysteine proteases that degrade proteins in the lysosome and some cathepsins assist with the processing of antigens for the immune system. Asagiri et al. (p. 624; see the Perspective by Krieg and Lipford) uncover a further but distinct immunological role for another cathepsin, cathepsin K, which is known to be involved in osteoclast function in the bone. Cathepsin K is expressed in immunological dendritic cells and is needed for the complete induction of the inflammatory T helper 17 T cells. In animal models for two autoimmune conditions, pathology was ameliorated by cathepsin K deficiency because of its unexpected involvement in signaling through the innate immune receptor TLR9.

  12. DNA-Assisted Molecular Delivery

    Many examples have been reported of assembly of atoms and small molecules into patterns on surfaces by pick up, transfer, and release with a scanning probe microscope tip under vacuum conditions. Kufer et al. (p. 594; see the news story by Service) assembled larger single molecules into patterns on surfaces in aqueous solution with an atomic force microscope (AFM) tip by taking advantage of differential forces acting on double-stranded DNA. “Depots” and “target” areas containing single-stranded (ss) 30-nucleotide (nt) DNAs were patterned onto glass slides, and then ssDNAs bearing the complementary sequence, a 20-nt “shear” sequence, and an organic dye molecule were attached to the depot area. An AFM tip bearing the complementary shear sequence could pick up DNAs from the depot area and then deposit them in the target area with a precision of ∼ 10 nanometers. Measurement of the rupture forces differentiates single from multiple or zero DNA transfers and ensures the fidelity of delivery, which could be repeated up to 5000 times.

  13. Molecular Switch Mechanism Revealed

    Riboswitches, messenger RNA elements that regulate gene expression, are comprised of two domains, an aptamer domain that binds ligands and an expression platform that controls gene expression. Regulation is achieved through ligand-dependent conformational changes in the aptamer domain. By observing folding and unfolding of individual molecules, Greenleaf et al. (p. 630, published online 3 January) reconstruct the energy landscape for folding the aptamer domain of the pbuE adenine riboswitch from Bacillus subtilis both with and without adenine. Folding is hierarchical, but tertiary contacts that preorganize the adenine binding pocket are more stable than the least stable helix. Adenine stabilization of this helix acts as the mechanical switch underlying regulation.

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