This Week in Science

Science  12 Sep 2003:
Vol. 301, Issue 5639, pp. 1439
  1. Functionalization Follows Fermi Levels

    Single-walled carbon nanotubes can form as either metallic or semiconducting entities. Ionic doping can create new metallic bands in semiconducting tubes. Two groups now show how covalent chemistry can convert the metallic tubes to semiconductors and may lead to efficient separation processes. Kamaras et al. (p. 1501) show that reactions with dichlorocarbene rapidly opens a gap near the Fermi level of the metallic nanotubes. Strano et al. (p. 1519) show that diazonium reagents are particularly selective for reactivity with metallic tubes and display an autocatalytic effect that functionalizes the entire tube. The derivatization reaction can be reversed thermally.

  2. Homologous Galactic Halos

    Clues into the origin of the Milky Way Galaxy and other large spiral galaxies can be found in their surrounding hot halos of old, metal-poor stars. Minniti et al. (p. 1508), using the Very Large Telescope (VLT), have detected such a halo around the Milky Way's smaller neighbor, the Large Magellanic Cloud. The presence of hot halos around galaxies of different masses suggests that all galaxies form in a similar manner. In particular, this finding limits the formation mechanisms of large galaxies to two possibilities, either a hierarchical merging of small satellites with old stars, or a monolithic collapse and old-star ejection into a spherical halo before the disk collapses.

  3. Disturbing Results for Metapopulations

    Metapopulation biology concerns the dynamics of subdivided populations of species inhabiting a landscape broken up into discrete patches. Levins' classic patch-dynamics model has provided the main framework for understanding the colonization and extinction of populations in a landscape of habitat islands. By a simple adjustment of parameters, Hastings (p. 1525; see the Perspective by Dobson) extends the classic model to allow patch quality to vary with patch age (as would be the case, for example, in the succession of species after a disturbance such as fire). The author shows how disturbances that reduce mean patch age could cause a metapopulation to go extinct.

  4. Creating Colder Condensates

    The ability to create colder molecular and atomic Bose-Einstein condensates (BECs) is the focus of two reports. Although several techniques have provided evidence for the formation of molecules within an atomic condensate, the molecules have not been separated from their atomic reactants. Herbig et al. (p. 1510) take advantage of a Feshbach resonance created by an external magnetic field to produce cold cesium (Cs) molecules within a BEC of Cs atoms. They then exploited the different magnetic signatures of molecules and atoms and used magnetic levitation to separate out the molecules. Leanhardt et al. (p. 1513) have developed a trapping technique in which gravitational and magnetic forces are used to confine and cool a sodium atom BEC. Using an adiabatic decompression procedure, they cooled atoms to temperatures below 500 picokelvin, compared to a previous low of 3 nanokelvin. By entering this lower temperature regime, it may be possible to probe and tease out previously unseen quantum interactions.

  5. Going with Where It Flowed

    Even at low concentrations, dilute solutions of polymers can exhibit a number of unusual and nonlinear flow effects because of the long relaxation times and large number of chain conformations associated with the polymer chains. About 30 years ago, it was proposed that these polymer chains will undergo a sharp transition from being coiled up to fully stretched. Through a set of experiments on single DNA chains, Schroeder et al. (p. 1515) show that the flow history, or chain hysteresis, plays a critical role in determining the critical values for this flow transformation.

  6. Reading the Patterns

    An animal's ability to cope with a given pathogen depends largely on the genetic program of the immune response. In some diseases, such as leprosy, this response dictates the divergent clinical forms that the disease eventually takes. Using microarray analysis of samples taken from leprosy lesions, Bleharski et al. (p. 1527) identify distinct patterns of gene expression between the two main forms of the disease, the relatively mild tuberculoid form and the severe lepromatous form. A general increase in expression of leukocyte immunoglobulin-like receptor (LIR) family genes in lepromatous patients included LIR-7. Activation of this receptor resulted in elevated interleukin-10 expression and inhibition of Toll-like receptor function in vitro that corresponded with significantly impaired cell-mediated immune responses in these patients.

  7. A Lifetime of Gene Expression in the Malaria Parasite

    Oligonucleotide-based microarrays have been constructed by Le Roch et al. (p. 1503; see the Perspective by Waters) in order to generate a genome-wide picture of gene expression in the causative agent of malaria, Plasmodium falciparum. Transcripts were detected for nearly 90% of the predicted genes. Of these expressed genes, 43% varied during the life cycle of the parasite. Cluster analysis provided predictions by analogy about the potential cellular role of thousands of newly identified but uncharacterized genes.

  8. Coated Platelets Welcome Cold Storage

    Platelet transfusions prevent life-threatening blood loss in surgery and trauma patients. However, refrigerated platelets are rapidly cleared from the patient's circulation, and room-temperature storage reduces platelet shelf life and leads to constant shortages for transfusion. Hoffmeister et al. (p. 1531; see the cover and the news story by Couzin) show that the integrin receptor of liver macrophages binds to exposed sugar residues of the clustered GPIbα subunit of von Willebrand factor on the surface of chilled platelets. This binding results in phagocytosis of transfused, chilled platelets and their rapid clearance from the circulation. This binding could be blocked by masking the exposed sugar residues of GPIbα, and increased survival of treated, chilled murine platelets after transfusion into mice was observed. These findings offer a potential strategy for cold storage of platelets.

  9. Fighting TB in the Presence of HIV

    Antiretroviral (ARV) drugs have the potential to halt the rapid growth of human immunodeficiency virus (HIV)-related tuberculosis (TB), which is threatening to undo the worldwide progress in TB control. However, Williams and Dye (p. 1535) have used mathematical modeling to suggest that the immediate role for ARV in TB control will be to improve the quality of treatment for patients and to provide an entry point for ARV therapy, rather than to prevent new cases.

  10. Self-Inflicted DNA Wounds

    Organisms that produce antibiotics have evolved mechanisms such as drug elimination and drug modification to evade their own chemical arsenals. Enediynes are potent, naturally occurring antibiotics that act by cleaving DNA. Biggins et al. (p. 1537) show that resistance to the enediyne calicheamicin produced by Micromonospora involves a self-sacrifice mechanism. The active enediyne is quenched when it specifically proteolyses the resistant protein. Such a mechanism, where detoxification is accomplished at the expense of both the metabolite and the resistant protein, may be particularly effective for extremely potent metabolites.

  11. Transferring RNA Transfer

    RNA interference, the RNA-mediated posttranscriptional repression of gene expression, can spread from the site where it is initiated to other cells at distant locations in plants and in nematode worms. Previous results have shown that the gene SID-1, a putative membrane protein, is involved in this systemic effect. Feinberg and Hunter (p. 1545) extend this work by showing that expression of Caenorhabditis elegans SID-1—which is indeed a multipass transmembrane protein—can confer the exogenous uptake of RNAs (without transfection) on normally refractory Drosophila tissue-culture cells. RNA uptake mediated by SID-1 is a passive process that does not require direct energy expenditure.

  12. How Niches Impose Asymmetry on Stem Cell Divisions

    The Drosophila male germline serves as a model system for investigating how stem cells are regulated in the context of their normal microenvironment, or niche. Yamashita et al. (p. 1547; see the Perspective by Wallenfang and Matunis) used this system to investigate the intracellular mechanisms that lead to the reliably asymmetric outcome of stem cell divisions to produce a stem cell and a cell that is ready to differentiate further (in this case, a gonialblast). The mitotic spindle in dividing germline stem cells orients with respect to the support-cell niche throughout their cell cycle. This process requires centrosome function and homologs of the human tumor suppressor gene Adenomatous Polyposis Coli (APC).

  13. Heavy Nitrogen in Comets

    Comets are primordial balls of ice that can provide estimates of the early chemistry of the solar system before planetary bodies and structure were fully evolved. Arpigny et al. (p. 1522) measured the carbon and nitrogen isotopic ratios in the CN molecule from comet C/2000 LINEAR WM1. The comet is enriched in 15N relative to the terrestrial value and the measured nitrogen ratio for HCN in comet Hale-Bopp. HCN was thought to be the parent molecule for the CN, but the heavy nitrogen in WM1 suggests that more complex macromolecules, such as polycyclic aromatic hydrocarbons, may be the parents for the CN. Thus, the early solar system may have been more heterogeneous and more chemically complex than previously assumed.

  14. A Light-Sensing Protein Motif

    Phototropins are plant proteins that respond to blue light and regulate processes such as phototropism and leaf opening. On exposure to light, a per-ARNT-Sim (PAS) domain forms a covalent bond with a flavin chromophore, and this process somehow activates a kinase domain. Harper et al. (p. 1541) used nuclear magnetic resonance spectroscopy to show that an α-helix located outside of the PAS domain core associates with the domain in the dark, but the interaction is disrupted in the illuminated state. This conformational change likely couples light-dependent bond formation to kinase activation.

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