This Week in Science

Science  18 Nov 2005:
Vol. 310, Issue 5751, pp. 1085
  1. Sauropod and Early Mammalian Grazing


    The origin of grasses has been uncertain. Photosynthesis in grasses is distinct from that used in most other plants, and grasses contain specific silica structures within their cell walls (phytoliths) that can be preserved in the fossil record. Prasad et al. (p. 1177, see the Perspective by Piperno and Sues) have found grass phytoliths in Late Cretaceous sauropod coprolites (fossilized dung). The diversity of phytoliths is consistent with evolution of all of the crown-group grasses by this time, and much earlier than had been thought. Although grasses do not seem to be the primary food of sauropods (they form a minor component of the food sample), they might have been used by certain early mammals with enigmatic teeth.

  2. Incommensurate Quantum Solids

    Recent experiments which showed that solid helium-4 has a nonclassical moment of inertia were interpreted in terms of the existence of a supersolid phase that can “flow” like a superfluid. Anderson et al. (p. 1164, published online 3 November) present a thermodynamic description of an “incommensurate” quantum solid, in which there is a mismatch between the number of lattice sites and the number of atoms and look at the role of interstitials and vacancies on its subsequent temperature-dependent structural and specific-heat properties. The consistency of their model with the existing experimental data prompts the authors to suggest that the ground state of solid 4He may be an incommensurate quantum solid.

  3. Graphite's Porous Relations

    Numerous examples now exist of microporous materials formed from organic ligand that are linked by metal centers. Côté et al. (p. 1166) now report the formation of organic frameworks based on condensation reactions of diboronic acid that yields stacks of from graphite-like planar networks. Condensation of diboronic acid alone forms hexagonal pores 15 angstroms in diameter in which the layers are staggered, whereas condensation of this molecule with hexahydroxytriphenylene leads to larger pores (27 angstroms) with the layers stacked in an eclipsed configuration. These materials are stable to between 500° and 600°C and have surface areas of ~700 and ~1600 square meters per gram, respectively.

  4. Cycles Underlying Growth


    Cultures of the budding yeast Saccharomyces cerevisiae that are grown in limited nutrients, rather than in rich media more common in the lab, will show 4- to 5-hour cycles of respiration, as measured by O2 consumption. Tu et al. (p. 1152, published online 27 October) found that under these conditions, more than half of the yeast genes are transcribed cyclically with a period of 300 minutes. Three large clusters of genes with related functions cycled together. The “oxidative cluster,” which peaked when respiration was greatest, contains genes with roles related to protein synthesis, perhaps to make use of the high levels of adenosine triphosphate readily available at that time. The second supercluster, the “reductive/building” phase, contained many components of DNA replication and cell division, and the third “reductive/charging” group of genes contributed to nonrespiratory metabolism and protein degradation.

  5. Infrared-Radiating Carbon Nanotubes

    In light-emitting diodes (LEDs), oppositely charged carriers (electrons and holes) are injected into an active region where they can recombine and release energy as photons. Chen et al. (p. 1171) show that in suspended carbon nanotubes, the local acceleration of a single type of carrier (electrons or holes) creates excitons. Under these conditions of one-dimensional confinement, excitons recombine and release radiation in the infrared. This process is 100 to 1000 times more efficient than that of electron-hole recombination in LEDs.

  6. Trapped Below

    The atmospheres of Earth and Mars have much less xenon than expected from the present concentration of other rare gases, primordial abundances, and losses and production of rare gases from outgassing and radioactive decay. Sanloup et al. (p. 1174; see the Perspective by McMillan) present experiments which show that large amounts of xenon can substitute for silica in quartz at high pressures and temperatures, including conditions consistent in the deeper continental crust where quartz is abundant. Xenon is released rapidly upon decompression, which makes analysis of exposed deep crustal rocks problematic.

  7. Structural View of Cytokine Interactions

    Interleukin-2 (IL-2), a cytokine produced by activated T cells, promotes the proliferation, differentiation, and survival of mature T and B cells. Its actions are primarily mediated through a quaternary signaling complex that consists of IL-2, the α and β receptors (IL-2R α, IL-2R β), and the γc chain. Now Wang et al. (p. 1159) present the extracellular structure of the quaternary complex at 2.3 angstrom resolution. Besides providing insight into IL-2 interactions that might facilitate design of IL-2 agonists and inhibitors, the structure provides a view of the γc receptor. This receptor is shared for IL-2, -4, -7, -9, -15, and -21 and is mutated in X-linked severe combined immunodeficiency diseases. Several mutations associated with X-SCID map to residues in the γc binding sites.

  8. Circumventing Plant Pathogen Defenses

    Certain plant pathogens can only initiate disease within certain plant species. What makes host species susceptible, or, conversely, what makes nonhost species resistant to infection? Studying fungi, including the one that causes potato blight, Lipka et al. (p. 1180) found a way to make Arabidopsis, which is normally resistant to infection, susceptible. The catalytic activity of a glycosyl hydrolase is required to keep the pathogen out, and a fail-safe program of regulated cell death further shores up defenses. The redundancy of this pathogen-defense system may help to explain its robustness.

  9. Crucial Genes in Craniofacial Development

    In humans, Williams-Beuren syndrome (WBS) results from a chromosomal deletion that usually removes 28 genes. The mutation affects craniofacial development and some aspects of cognitive and social development. Patients with WBS may be characterized by over-friendliness as well as by deficient numerical abilities. Tassabehji et al. (p. 1184, published online 3 November) have now analyzed the chromosomal disruption responsible for WBS in one patient. The results, which are supported by parallel analyses in mice, identify the gene GTF2IRD1 in the WBS region as critical for the craniofacial defects.

  10. A Pathway to Schizophrenia?

    Schizophrenia and related mood disorders are thought to arise from a combination of genetic and environmental factors, but the identification of specific causative genes has been challenging. The disrupted in schizophrenia 1 (DISC1) gene is on a short list of promising candidate-susceptibility factors, but the function of its encoded protein has been unclear. Millar et al. (p. 1187; see the Perspective by Sawa and Snyder) now present evidence suggesting that the DISC1 protein modulates cellular cyclic AMP (cAMP) signaling through its physical interaction with the enzyme phosphodiesterase 4B, and that disruption of this interaction may play a mechanistic role in the development of schizophrenia. Notably, cAMP signaling has previously been implicated in learning, memory, and mood in other experimental systems.

  11. Reshaping the Synapse


    The immune synapse forms at the interface between a T cell and an antigen-presenting cell (APC) and is composed of discrete domains of stimulatory molecules and receptors critical for T cell activation. Mossman et al. (p. 1191) have imposed physical constraints on the synapse domains using a hybrid junction between a live cell and an anchored lipid bilayer (representing the APC surface). The authors directly tested the effects of membrane reorganization on the signals delivered by the synapse. Constraint of T-cell receptor ligand pairs to the periphery—rather than the center of the synapse where they normally coalesce—sustained (rather than diminished) synapse signaling, establishing a relation between the duration of T-cell receptor signals and their position in the synapse.

  12. Golgi Inheritance in Trypanosomes

    Centrins are highly conserved components of centrosomes that have long been implicated in the duplication and segregation of organelles ranging from chromosomes to mitochondria. He et al. (p. 1196, published online 27 October) have identified a new cellular structure in trypanosomes, defined by Centrin2, which is involved in the duplication of the Golgi complex. This structure has two lobes: one associated with the old Golgi, the other marking the site where the new one appears.

  13. Kinases Involved in Promoting Longevity in Yeast

    In many organisms, nutrient-sensing and caloric intake regulate aging and longevity, and in the budding yeast Saccharomyces cerevisiae, calorie restriction can increase replicative life span. Kaeberlein et al. (1193; see the Perspective by Rine) analyzed 564 single-gene yeast deletion strains and identified 10 gene deletions that significantly increase replicative life span. Six of these encoded components of the highly conserved, nutrient-responsive TOR and Sch9 pathways. Calorie restriction of cells lacking TOR1 or Sch9 failed to increase life span further. Thus, it appears that TOR and Sch9 kinases are involved in a pathway through which excess caloric intake limits life span in yeast and, perhaps, higher eukaryotes.