This Week in Science

Science  10 Nov 2006:
Vol. 314, Issue 5801, pp. 889
  1. Silicate Slide Show


    How molten magma and other geophysical fluids move through rocks is a basic question in geology. Schiano et al. (p. 970; see the Perspective by Holness) show that when driven by a temperature gradient, a silicate melt can move through the lattice structure of olivine but gas-rich fluid inclusions do not. The melt migrates within the mineral matrix instead of along crystal edges in a series of progressive dissolution and recrystallization steps. This process is controlled by interface kinetics and not chemical diffusion. In contrast, bubbles of gas-rich fluids remain stuck and do not migrate. Thus, transcrystalline migration is faster than intergranular porous flow at all melt fractions less than 0.1% and allows for grain-scale percolation and segregation of early mantle melts that have a low degree of melting.

  2. Core Conundrum

    The Earth's inner core grows slowly from the solidification of the outer core, and the heat released helps drive convection in the outer core and fuels the magnetic dynamo. Wen (p. 967, published online 28 September) measured the inner core's growth directly in one spot using seismic compression waves reflected off the inner core boundary. Similar waves received at seismic stations in Russia and Kyrgyzstan after a pair of earthquakes in 1993 and 2003 were tens of milliseconds earlier in the later earthquake, which indicates that this part of the inner-core boundary had grown by about 1 kilometer in a decade. This speed is much greater than predicted by the thermal history model of the core. Such a rapid change may indicate either differential rotation of an irregular inner core boundary or nonuniform growth of the inner core.

  3. Well-Rounded Diet

    The diets of early human ancestors have been difficult to determine. Our closest common ancestor, the chimpanzee, forages primarily on fruits and nuts that come from plants that use the C3 photosynthetic pathway. Later hominins, of the genus Homo clearly had a diverse diet that included animals feeding on grasses (which use the C4 pathway). It has been thought that the development of tools by Homo allowed this diversification in diet. Sponheimer et al. (p. 980; see the Perspective by Ambrose) studied the carbon isotope signature of enamel layers from teeth of Paranthropus robustus. This early hominin switched from a diet rich in C3 plants to one with a C4 source seasonally. Thus, the extinction of P. robustus, who apparently did not use tools, cannot be explained by a restricted diet.

  4. Etching a Route to Nanotube Electronics

    Carbon nanotube preparation processes generally produce mixtures of semiconducting and metallic nanotubes, which has hindered their development for large-scale electronics. Zhang et al. (p. 974) describe how a methane plasma and annealing treatment can selectively remove the metallic nanotubes. Combined with controlling the diameter of nanotubes during growth, pure semiconducting devices can be reliably obtained, as demonstrated by fabrication of high-current transistors.

  5. Microwave Cloaking Realized

    The ability to tune the electromagnetic response of materials recently provided theorists with the cue to propose the construction of a “cloak,” a space that not only excludes electromagnetic radiation but also steers that radiation around itself as if it was not there. Schurig et al. (p. 977, published online 19 October; see the 20 October news story by Cho) present experimental results demonstrating that such a cloak can be constructed in the microwave regime from a metamaterial consisting of split-ring resonators. Although this system suffers from losses and only works in two dimensions, the results demonstrate the principle of cloaking.

  6. Magnetic Separation with Single Domains


    Mesoporous materials such as zeolites have high sorption capacities for separations, but mass transport through these materials can be a limiting factor. Nanoparticles offer potentially high surface areas and rapid contact with the sample, but as their size decreases, their separation from solution becomes more difficult. Magnetic separation routes, either batchwise or continuously from solution, that are useful with larger particles would appear to need prohibitively large-field gradients for submicrometer-sized particles. Yavuz et al. (p. 964) now show that this supposed limitation does not apply to single-domain magnetite particles about 10 nanometers in diameter; at relatively modest fields, these particles aggregated, apparently because of their much higher surface field strength compared to multidomain particles. Particles of different sizes could be separated, and sorption onto the magnetite surfaces was used to capture and remove arsenic impurities from water.

  7. The Bald Truth About Lipids

    Hair loss can be traumatic, whether it is linked to illness or simply part of the natural aging process, and there is considerable interest in dissecting its underlying mechanisms. Kazantseva et al. (p. 982) identify a culprit gene in a group of Russian families who show an inherited deficiency in hair growth but are otherwise healthy. The mutant gene, LIPH, encodes lipase H, a phospholipase thought to regulate the production of lipid-signaling molecules. This discovery will likely stimulate investigations aimed at understanding the precise role of lipase H in hair follicle biology and whether the LIPH gene also contributes to the more common forms of baldness in the general population.

  8. I'm a Pathogen, Let Me Out of Here


    Certain microbial pathogens replicate within host cells, and virulence requires the dissemination of bacteria from cell to cell within the host. Yoshida et al. (p. 985; see the Perspective by Gorvel) now show that intracellular Shigella secretes VirA, a cysteine protease-like effector, into the host cell, which destroys microtubules and promotes its own intracellular motility. This process helps Shigella spread intracellularly and subsequently disseminate into adjacent epithelial cells.

  9. Toward Defeating Blindness in the Elderly

    Age-related macular degeneration (AMD) is a common cause of blindness in the elderly and is characterized by a breakdown of light-sensitive cells in the retina that results in progressive loss of central vision. The neovascular, or “wet,” form of AMD is especially devastating for patients because vision loss is rapid. Studying a Chinese population, DeWan et al. (p. 989, published online 19 October) identified a single nucleotide polymorphism (SNP) in the HTRA1 gene that confers a greatly increased risk of developing wet AMD. The HTRA1 gene, located on chromosome 10q26, encodes a heat shock serine protease, and the SNP resides within the gene's promoter region. Yang et al. (p. 992, published online 19 October) find that the same SNP also increases AMD risk in a Caucasian population and is associated with higher expression levels of HTRA1 messenger RNA and protein. Identification of this gene may ultimately lead to improved diagnosis and treatment of AMD (see the 20 October news story by Marx).

  10. Spotting Invaders

    The cell's ability to distinguish invading RNA or DNA from the plethora of its own nucleic acid sequences plays a critical role in protecting the genome from potentially harmful damage, and a number of systems have evolved to sniff out unwanted alien genes and trigger cellular responses (see the Perspective by Fujita). Retinoic acid-inducible protein I (RIG-I), part of the cellular alarm system in the cytoplasm, specifically recognizes a number of RNA viruses, but what is RIG-I actually sensing? Hornung et al. (p. 994, published online 12 October) and Pichlmair et al. (p. 997, published online 12 October) show that RIG-I detects and binds to an unusual feature of the 5′ end of the viral RNA, specifically, a 5′-phosphate group.

  11. Choosing Channel Selectivity

    Ion channel proteins form pores in the membranes of cells and are regulated by voltage or small messenger molecules to control information flow to and from cells. The K+ channel, important for the excitability of nerve cells, conducts only K+ ions, while completely excluding a smaller ion, Na+. Valiyaveetil et al. (p. 1004) show that this selectivity is accomplished in two ways: In the presence of K+, the pore remains open and conductive, but collapses when K+ concentrations are low, excluding Na+. In addition, in the conductive state, the pore is lined with multiple binding sites that are specific for K+.

  12. Viewing the Landscape

    Understanding how proteins and nucleic acids fold is key to understanding their function. Free-energy landscapes provide the conceptual framework for describing folding, but only limited aspects of folding landscapes have been characterized experimentally. Woodside et al. (p. 1001) have determined the complete shapes of the free-energy landscapes for reversible folding of DNA hairpins with various designed sequences. The experimental energy landscapes, determined by deconvolution from high-resolution, single-molecule folding trajectories, agree well with calculated energy profiles.

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