This Week in Science

Science  21 Jan 2005:
Vol. 307, Issue 5708, pp. 313
  1. Brittle Boundaries


    The addition of sulfur to many metals and alloys causes them to become brittle, but the reason for this weakening is not well understood. Yamaguchi et al. (p. 393, published online 6 January 2005) modeled the embrittlement of nickel by progressively adding sulfur atoms to a grain boundary. First-principles calculations reveal that the weakening of the boundary is caused by the aggregation of sulfur atoms at the boundary, which repel each other. The sulfur atoms are forced into non-ideal bonding because the nickel-sulfur bonds are stronger than the sulfur-sulfur bonds.

  2. Semiconducting Aerogels

    Aerogels are porous, very low density materials that have the appearance of frozen smoke. They are typically made from oxides and are thus insulators. Mohanan et al. (p. 397) have made ana-logous aerogels from metal chalcogenides (sulfides, selen-ides, and tellurides), which are materials commonly used for making semiconductor quantum dots. As a result, the aerogels retain semiconducting properties such as photoluminescence, and yet have a porous network structure with pores in the 2- to 50-nanometer-size range.

  3. Combing the Ultraviolet

    The use of ultrashort, broadband laser pulses, or optical combs, was recently extended from being a reference standard for continuous wave lasers to being a way to probe the energy levels of atoms. The advantage of using the combs is that they combine the high temporal resolution needed to study dynamics with precise frequency measurement. Witte et al. (p. 400; see the Perspective by Udem) have now extended this method to the short-wavelength, deep ultraviolet region of the spectrum by creating a train of the pulses with the fourth harmonic of an optical laser. The authors measured a high-energy transition frequency in Kr atoms with an order of magnitude reduction in uncertainty from prior studies.

  4. Producing Orders Pockets of Spin

    The sensitivity of magnetic resonance force microscopy (MRFM) is reaching the point where single spins can be detected. Making measurements on a small ensemble of localized spins created by microwave irradiation of silicon, Budakian et al. (p. 408) show that that MRFM cannot only detect spin fluctuations but can also be used to manipulate them. Pockets of ordered spin can be formed from a background bath of thermally fluctuating spins in the vicinity of the cantilever tip, and these pockets of ordered spin can be stored and read out. The technique itself should prove useful as a probe of the dynamics of nanoscale magnets, and the ability to create, store, and read out small pockets of ordered spin should prove useful in quantum computing.

  5. Slips in Slip Rates


    The Karakorum fault is a major strike-slip fault trending northwest just north of the western Himalayan Mountain Range. The rate of slip on the fault is difficult to estimate, but these rates are needed to understand the tectonics of the region and the strength of the crust. Chevalier et al. (p. 411) estimated a rate of slip of about 11 milli-meters per year over about 20,000 to 140,000 years on one branch of the Karakorum based on offset moraines, which is consistent with the extrusion of western Tibet owing to the collision of India with Eurasia. This rate is higher than some geodetic estimates of recent slip over shorter time periods and suggests that slip rates on the fault have varied over time.

  6. Sudden Changes in Lions' Ranges

    Population dynamics of social species can be highly complex because of the interplay of group-level factors and population-level factors. Packer et al. (p. 390; see the Perspective by Ranta and Kaitala) present long-term data from the Serengeti plains of East Africa which show how herbivore populations (wildebeest, buffalo, zebra, and gazelle) influence lion populations directly and indirectly through the herbivores' impact on vegetation. The herbivore population changes are smooth and gradual, but the lion populations show sudden shifts between alternative equilibria. A model that constrained the upper and lower limits of pride size gave rise to the observed patterns of sudden shifts. Thus, population trends cannot necessarily be understood solely on the basis of individual survival and reproduction.

  7. Separation and Speciation

    Ring species, which are isolated species connected by intergraded populations, have long been thought to exemplify the occurrence of speciation in the presence of gene flow. However, some taxonomic and molecular evidence have cast doubt on this classic model. Irwin et al. (p. 414) conducted a genome-wide survey for the greenish warbler, whose territory encircles the Tibetan plateau. Two genetically distinct and reproductively isolated forms of the warble are indeed connected by a chain of populations through which genetic patterns change gradually.

  8. Big Bacteria Promote Phosphorite Formation

    Thiomargarita namibiensis is a colossus among bacteria (almost 1 millimeter in diameter) found off the Namibian coast. Schulz and Schulz (p. 416) show it accumulates intracellular polyphosphates under aerobic conditions and releases phosphate under anoxic conditions, thereby creating pore water supersaturated in phosphate that precipitates as phosphorite. Energy gained by breakdown of polyphosphate under anoxic conditions is used for intracellular accumulation of sulfide and acetate or other organic carbon. The sulfide is oxidized to elemental sulfur by using nitrate as an electron acceptor. The release of phosphate by these organisms could be sufficient to explain the large accumulations of phosphorite observed in many parts of the world's oceans.

  9. Exercise, Oxygen Metabolism, and Health

    Human epidemiological studies have suggested that low aerobic capacity is a strong predictor of mortality. Wisløff et al. (p. 418; see the news story by Marx) compared two lines of rats produced by 11 generations of genetic selection for high or low scores in endurance running. Rats with low aerobic capacity had many of the risk factors that define metabolic syndrome, including high blood pressure, elevated levels of plasma triglycerides, and impaired glucose tolerance. Preliminary expression data were consistent with a decline in mitochondrial function in the unfit rats.

  10. Motility in a Mollicute


    Mollicutes (Mycoplasma, Achole- plasma, and Spiroplasma) are small prokaryotic cells that have distinct morphologies and that are motile despite their lack of cell walls or appendages such as flagella. Recent studies have identified a fibril protein that forms a cytoskeletal ribbon likely involved in promoting motility. Kürner et al. (p. 436) have used cryo-electron tomography to visualize the three-dimensional structure of the whole cell for the spiral-shaped mollicute Spiroplasma melliferum. The cytoskeletal structure consists of two outer ribbons, comprising five thick filaments each, joined by an inner ribbon comprising nine thin filaments. The thick filaments are polymers of fibril protein and the thin filaments are polymers of the actin-like protein MreB. Cell motility could be promoted by coordinated length changes of the cytoskeletal ribbons.

  11. An Insulin Mimic Secreted by Visceral Fat

    Excessive amounts of abdominal visceral fat, sometimes referred to as “bad fat,” significantly increase an individual's risk of developing insulin resistance and other metabolic disorders. These adverse health effects may be mediated in part by fat-derived cytokines that circulate in the blood. Fukuhara et al. (p. 426, published online 16 December 2004; see the Perspective by Hug and Lodish) characterized “visfatin,” a cytokine that is highly expressed in visceral fat and whose blood levels correlate with obesity. Surprisingly, functional analyses in mice revealed that visfatin has beneficial, insulin-like activity, causing a lowering of blood glucose levels. Even more surprisingly, visfatin was shown to bind to the insulin receptor and activate the insulin signal transduction pathway. While the precise physiological role of visfatin remains to be established, the discovery of this natural insulin mimetic could open exciting new avenues in diabetes research and therapy.

  12. Transcription Factors and Helper T Cell Lineage Determination

    In helper T (Th) cells, cell fate is primarily determined by the transcription factors GATA3, which directs Th2 type cells and T-bet, which regulates Th1 lineage choice. Hwang et al. (p. 430) found that during the early stages of a T helper precursor's decision to become a Th1 cell, T-bet has an unusual means of repressing the Th2-promoting effects of GATA3. After T cell stimulation and under the right polarizing conditions for Th1 cells, T-bet becomes phosphorylated by the tyrosine kinase, ITK, which allows it to bind GATA3. This process prevents it from interacting with its Th2 cytokine target genes. This study reveals a further means by which transcription factors may directly cross-regulate one another in specifying cell lineage fate.

  13. Charging and Gold Cluster Reactivity

    One explanation for the enhanced catalytic activity of gold supported on early transition metal oxides such as TiO2 is that oxygen vacancies donate negative charge to small gold clusters. Yoon et al. (p. 403) present model reactivity studies and density functional calculations for the CO oxidation reaction. They soft-landed gold octamers on magnesium oxide surfaces that were either near-perfect or that had a high fraction of oxygen vacancies. When CO and O2 were coadsorbed on these clusters, the defect-site surface showed enhanced reactivity and the vibrational stretching frequency of absorbed CO molecules was red-shifted. The calculations can account for CO bond weakening via the classical model of back-donation into antibonding orbitals.

  14. Modeling Planar Cell Polarity

    Epithelial cells of many organisms show planar cell polarity. In the Drosophila wing, hexagonally packed cells accumulate various planar cell polarity signaling components in localized areas (proximally or distally), and an actin-rich hair develops from the distal vertex and points distally. Although several molecular components for this patterning have been elucidated, the complexity in number of factors and their various interactions, including the effects of neighboring cells on each other, has hindered a full understanding of planar cell polarity. Amonlirdviman et al. (p. 423) now combine biology and mathematical modeling to address the complex phenotypes produced by genetic manipulations. The model incorporates trans-cellular interactions between membrane proteins and is based on reaction-diffusion and partial differential equations. This mathematical model shows how contact dependent signaling can account for the effect a mutant cell can have on the phenotype of a neighboring cell.

  15. Bookmark for Stress

    Genomes are tightly compacted during mitosis. However, some gene promoters do not display this high degree of compaction, and this phenomenon is termed bookmarking. The mechanism maintaining this open chromatin state is unknown. The heat shock gene hsp70i shows bookmarking, and the accessible promoter is thought to allow cells to activate hsp70i expression rapidly upon stress induction. Xing et al. (p. 421) now provide mechanistic insight for bookmarking at the hsp70i gene. The heat-shock protein HSF2 binds to the hsp70i promoter in mitotic cells and recruits protein phosphatase 2A to dephosphorylate and inactivate condensin, preventing local chromatin compaction. When bookmarking is disabled, expression of the hsp70i gene is prevented and cells die in G1 if subjected to stress. Thus, HSF2-mediated bookmarking can play a role in cell survival.

  16. Limiting Light Damage

    During photosynthesis, high light exposure can lead to excess energy in the singlet excited states of chlorophyll (1Chl*) that can in turn generate toxic intermediates harmful to the plants. Plants have developed a strategy known as feedback de-excitation quenching that dissipate excess energy via carotenoids. Holt et al. (p. 433) performed femtosecond transient absorption measurements on intact, isolated thylakoid membranes and found strong evidence that quenching occurs through exciton coupling of Chl* to a zeaxanthin to form a carotenoid radical. This information should facilitate efforts in biotechnology in the engineering of plants with enhanced stress tolerance and in the design of artificial photosynthetic systems.

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