This Week in Science

Science  23 Feb 2007:
Vol. 315, Issue 5815, pp. 1049
  1. Smelling Their Way to an Early Grave


    When animals are reared on a near-starvation diet, they live much longer than those that eat freely. Even the fruit fly Drosophila has this reaction to a low glucose diet, and lives considerably longer on a 5% than on a 15% sugar-yeast diet. This effect of dietary restriction is easily reversed when flies consume more food. Libert et al. (p. 1133, published online 1 February, see the 2 February news story by Leslie) report a less expected effect: Just the smell of the flies' food (yeast) can inhibit some of the effects of dietary restriction and shorten the flies' life span by 6 to 18%. Flies lacking an essential part of their odor receptors, which results in their having greatly impaired senses of smell, live longer than flies with intact odor sensation.

  2. The Shape of Sheets to Come

    When the sides of a flat piece of paper lying on a desk are slowly pushed together, the sheet will eventually buckle with a periodicity that depends on its thickness or stiffness. The same procedure applied to a piece of aluminum foil may also lead to buckling, but will also lead to crumpling or wrinkling on a local scale. If either material has been previously deformed, the change in shape and curvature will likely be affected by defect sites. Klein et al. (p. 1116) propose a general description for shrinking of thin sheets that depends only on the resting shape of the material and an accounting of the stresses placed on the material. Ideally, the thin sheet will deform in a competition between bending and stretching energies that maintains a shape near the resting shape. The authors demonstrate their concepts using a series of polymer films with nonlinear concentration gradients of monomers that swell to different extents across their surface. In a Perspective, Kamien shows how this approach can be applied to everyday objects, like potato chips, and how more complex shapes can be designed.

  3. Outlook for Bright Neutron Source

    X-ray and photon sources for spectroscopic and diffraction methods have increased in brightness by several orders of magnitude during the past 20 years, but the brightness of neutron sources for structural studies have increased by only an order of magnitude during the past 40 years. With conventional neutron source technologies (spallation and fission reactors) reaching a plateau in their output, Taylor et al. (p. 1092) propose that inertial fusion energy facilities and high-power lasers could run in tandem to provide neutron sources that would be three orders of magnitude brighter than those available at present.

  4. Classical Quasicrystalline Tilings


    Islamic architecture beginning in the 10th century is marked by distinctive star and line patterns called girih. These figures were thought to be interwoven continuous lines, created by simple tools such as straightedge and compass. Lu and Steinhardt (p. 1106; see the news story by Bohannon) find that by the beginning of the 13th century, artisans began to create these patterns from tilings of decorated polygons. The tilings became increasingly complex, and by the 15th century the patterns had evolved into quasicrystalline designs, well before the mathematical description of these space-filling patterns that possess rotational symmetry yet lack translational symmetry.

  5. Supernova Smoke Rings

    Supernova 1987A, the first supernova that could be seen with the naked eye since Kepler's supernova in the 17th century, is of an unusual type, and its characteristics have suggested that its explosion resulted tens of thousands of years after the merger of two massive stars. However, a detailed model has eluded astronomers. Morris and Podsiadlowski (p. 1103; see the Perspective by Soker) have performed three-dimensional computer simulations of such a stellar merger that can explain many of these features. After the two stars merge, detailed modeling of the angular momentum and accretion of gas from the two stars predicts two explosions, one that causes an “hourglass” ejection of material and the second that produces a smaller puff of gas. Together, these events account for the three rings of gas around the remnant of the exploded stars—two from the projected hourglass explosion, and an inner one from the second explosion.

  6. Empowering Powder

    A central challenge in the structural elucidation of zeolites and related porous solids is the tendency of these materials to adopt highly polycrystalline morphologies that fail to yield clean x-ray diffraction data. The structures are nonetheless highly sought after in light of the catalytic significance of such materials. Baerlocher et al. (p. 1113; see the Perspective by Gies) present an algorithmic advance that facilitates combined analysis of powder diffraction and electron microscopy data to solve particularly intractable zeolite structures. Using this method, they resolve the markedly complex 10-ring channel system forming the IM-5 zeolite, an active catalyst for hydrocarbon cracking and related reactions.

  7. Beyond the Diffraction Limit

    Most imaging techniques are diffraction limited, so that subwavelength information about an object is lost through the rapidly decaying evanescent waves. A number of techniques have been proposed, and some demonstrated, with the ability to form patterns beyond this diffraction limit, but these tend to operate in the near field. Lerosey et al. (p. 1120) use a time-reversal mirror—an antennae array that first detects the signal from a point source placed at the intended focus position, and then sends the signal back (time-reversal reflection)—to demonstrate the ability to focus microwaves on a receiver array to better than 1/30th of their wavelength. The technique was used to increase transmission rates for telecommunications by a factor of 3.

  8. Plant Defense Coordination


    Plants respond to pathogen attack by recognizing molecular signals from the invaders and instigating their own cellular responses to limit damage. Shen et al. (p. 1098, published online 21 December; see the Perspective by Dangl) now show that two of the plant's defense systems themselves interact at the level of gene transcription. Studying barley's mildew A proteins, part of one of the plant pathogen response pathways, the authors show how these proteins are localized to the nucleus, where they alter transcription of factors that regulate the other plant pathogen response pathway. The plant's response to pathogenic attack is thus coordinated and tuned to be appropriate to the challenge.

  9. New Clovis Culture Dates

    The first well-established culture in the New World has been long thought to be that of Clovis, characterized by a distinctive shape of their hunting points. Waters and Stafford (p. 1122; see the cover and the news story by Mann) present a series of new radiocarbon dates on several Clovis sites and reassess previous more scattered dates. Together, these imply that Clovis persisted for only a few hundred years and occurred somewhat later than was previously thought. The dates are similar to dates for other cultures such as Folsom and Goshen and may imply that the Clovis culture may have emerged in the New World after a previous colonization.

  10. Customizing MicroRNAs

    MicroRNAs (miRNAs) are ubiquitous small noncoding RNAs that regulate gene expression in eukaryotes. Because of the double-stranded nature of intermediates in their synthesis, miRNAs are potential targets for RNA editing. Kawahara et al. (p. 1137) now show that members of a mammalian miRNA cluster are edited in their seed regions, the region which determines their target specificity. This editing of adenosine bases to inosine, which is tissue specific, changes the potential target range of the miRNAs. The edited miRNA, but not the unedited version, acts to repress an enzyme involved in purine metabolism, and mice expressing this miRNA have elevated levels of uric acid in the cortex of the brain.

  11. Reversal of Rett Syndrome in Mice

    Rett syndrome is a rare genetic disease caused by a mutation in the X-linked gene MECP2, which causes mental retardation and autism-like symptoms in young girls. Guy et al. (p. 1143, published online 8 February; see the 9 February news story by Miller) engineered mice with an inserted sequence in the MECP2 gene that blocks its expression and show that the mice exhibit many of the symptoms of Rett syndrome. Reactivation of MECP2 in these mice before symptoms appear prevents disease. Reactivation in animals exhibiting Rett syndrome eliminated disease in both young adult males and mature females. Although such genetic manipulations are not possible in human patients, the apparent reversible nature of the disease suggests that therapy may be feasible.

  12. Switching Off the Extra X

    Methylation of DNA sequences typically results in the repression of gene expression. In dosage compensation, the expression of the two X chromosomes in a female cell are reduced to that of the single X in a male cell, often by inactivating the expression from one of the female X chromosomes (Xi), which has been thought to involve methylation at CpG islands (regulatory regions). Hellman and Chess (p. 1141) used a genome-wide analysis of DNA methylation to show, surprisingly, that the transcribed regions of genes on the active human X chromosome (Xa) are hypermethylated, both in females and males. Equivalent regions on the Xi are hypomethylated. Prior to X inactivation, both X chromosomes are methylated, which suggests that methylation is lost from the Xi.

  13. Assessing Microbial Communities

    The assessment of variation and evolution within environmental communities of microbes has proved difficult. Von Mering et al. (p. 1126; published online 1 February) describe a method for quantitatively assessing the phylogenetic composition of complex environmental communities. Their findings challenge the notion that “everything is everywhere.” Instead, microbial taxa have habitat preferences that can be traced far back in time. Furthermore, the speed of evolution varies in different habitats so that, for example, species in surface water evolve considerably faster than those in soil.

  14. Necrotizing Pneumonia Threat

    Methicillin-resistant Staphylococcus aureus is escaping hospital wards and causing severe disease among otherwise healthy people in the community. Increasingly, this bacterium is now traveling with a distinct virulence phenotype called Panton-Valentine leukocidin toxin, which alone can cause fatal pneumonia, but Labandeira-Rey et al. (p. 1130, published online 18 January; see the Perspective by Kahl and Peters) have found that the disease picture is even more complicated. By characterizing the pathology in a mouse model, they have discovered that the insertion of the two genes that express the toxin components also causes down-regulation of a repressor, which regulates the expression of an inflammatory mediator, and other cell wall-anchored proteins. The combination of these effects exacerbates the risk of dangerous pneumonia.

  15. Portable Proton Probe

    High resolution in nuclear magnetic resonance spectroscopy requires extremely homogeneous magnetic fields, and electromagnetic shimming coils are often used to correct small inhomogeneities that persist in carefully constructed symmetric probes. For the study of large samples outside the laboratory, open-sided probes are desirable, but in such a configuration, the shim currents required to homogenize the magnetic field are prohibitively high. Perlo et al. (p. 1110, published online 11 January) surmount this problem by using an array of small permanent magnets in place of the coils. They achieve sufficient resolution with a portable open probe to resolve the relatively similar proton spectra of acetic acid and toluene, as well as the components of a mixture of water and crude oil.