This Week in Science

Science  12 Mar 2004:
Vol. 303, Issue 5664, pp. 1577
  1. Forming Ferrihydrite Fibers


    Insight into how biological organisms can produce complex inorganic structures has been provided by Chan et al. (p. 1656; see the Perspective by Fortin), who studied bacteria from a flooded mine. They found that pseudo-single crystals of akaganeite (FeOOH) with incredibly high-aspect ratios had formed on polysaccharide templates extruded by bacteria. The crystalline akaganeite fibers appear to direct the aggregation and ripening of amorphous FeOOH to ferrihydrite, which precipitated as outer coatings on the akaganeite. These mineralized filaments could also be synthesized in the lab by immersing synthetic polysaccharides in a polynuclear iron solution similar to that found in nature.

  2. A New Spin on the Hall Effect

    The microscopic mechanism underlying the anomalous Hall effect found in ferromagnetic materials has long been controversial. Lee et al. (p. 1647) report Hall effect measurements on a metallic ferromagnetic compound systematically doped with impurities. At low temperatures, where impurity scattering dominates the carrier transport, the Hall current was independent of the impurity content. The result provides evidence for a dissipationless spin current in ferromagnets, in support of recent theoretical work.

  3. Stretched Magnesium Networks

    In a normal silicate glass, the network structure is made up by the fourfold-coordinated Si atoms modified by the inclusion of twofold- and threefold-coordinated atoms, such as oxygen and magnesium (Mg). As the Si content is lowered, the SiO4 tetrahedra or Si2O7 dimers become isolated and the glass structure becomes inverted. Studying these invert glasses is challenging because they typically have very complex compositions. Kohara et al. (p. 1649) used a containerless melting method to form bulk particles of fosterite (Mg2SiO4) glass. When forced to form a glass, the Mg creates the network structure by taking on four-, five-, and sixfold coordinations and engaging in highly extended bonding.

  4. Genome Invasions

    Mammalian genomes contain a huge amount of so-called junk or noncoding DNA. Much of this DNA is the result of the accumulation of retrotransposons within the genome. Kazazian (p. 1626) reviews how retrotransposons have accumulated within the genome and how this process has been important during evolution.

  5. Elastomeric Organic Transistors


    The electronic properties of some of the materials currently being developed for organic electronics can be sensitive to the conditions met during typical processing steps. Sundar et al. (p. 1644) present a new method to fabricate organic transistors whereby the delicate materials need not be exposed to the processing environment, thus ensuring that dependable measurement of intrinsic transport properties of the material are made. They used an elastomeric stamp to pattern the transistor terminals. Soft contact could then be made to the organic crystal, and good transistor function was demonstrated.

  6. Brains Ticking in Synchrony

    In the early days of software development, the acronym wysiwyg (what you see is what you get) referred to a program that could display what would actually be printed out on the personal computer screen. Hasson et al. (p. 1634; see the Perspective by Pessoa) have addressed the question of whether what you see is the same as what everyone else sees, and they conclude that, to a remarkable degree, it is. They imaged brain activity in five subjects while they watched a movie. Not only were the moments of global arousal synchronous, but functionally selective areas, such as those sensitive to faces and to objects, were active at the same time, even though the scenes being watched contained numerous stimuli of various sorts. Furthermore, scenes featuring critical finger-based actions triggered neural responses in a somatosensory region that encodes manual movements.

  7. Keeping Friends at Bay

    The commensal bacteria of the intestine share some molecular signatures with pathogenic bacteria but do not normally stimulate intestinal inflammation. Macpherson and Uhr (p. 1662; see the Perspective by Kraehenbuhl and Corbett) show that commensal strains of bacteria selectively stimulate intestinal B cells to produce immunoglobulin A (IgA) antibodies. Commensal bacteria invade and survive within dendritic cells, which travel to inductive sites of the mucosal lymphoid system. This IgA-mediated inhibition of mucosal penetration by commensal bacteria may help the intestine avoid unwanted inflammatory responses.

  8. Remodeling the Visual Cortex

    The portions of the mammalian brain's cortex responsible for interpreting visual input undergo continued refinement after birth. Indeed, a critical period of unusual plasticity exists during which visual experience can alter development. Two studies have used benzodiazepines, which disrupt GABAergic signaling, to dissect the mechanisms involved (see the Perspective by Ferster). Hensch and Stryker (p. 1687) show that the physical architecture of columns in the neocortex typical of a brain experiencing appropriately balanced vision is altered in response to benzodiazepine treatment. Fagiolini et al. (p. 1681) show that benzodiazepines can affect the timing of the critical period via affects on a subset of GABA receptors containing the α1 subunit. Thus, GABAergic signaling contributes both to the formation of columnar architecture and to the timing of the critical period.

  9. The Whys of Winter Wheat

    Winter wheat can be planted in the fall and, after a period of winter exposure, or vernalization, will begin rapid growth in spring. Spring wheat does not require the exposure to cold to promote flowering. Yan et al. (p. 1640; see the news story by Marx) have cloned from wheat the gene responsible for controlling vernalization. The predicted protein, VRN2, has homologs in other cereals, and has features of a zinc finger-containing transcription factor. VRN2 plays a central role in the repression of flowering in temperate cereals, and its down-regulation by vernalization is critical for the initiation of the reproductive phase.

  10. A Tale from the Crypts


    Proper patterning of the gastrointestinal tract requires signaling interactions between the gut epithelium and the adjacent mesoderm. In a study of transgenic mice, Haramis et al. (p. 1684) identified the expression of bone morphogenetic protein (BMP)-4 in the mesenchyme as a critical factor in this cross-talk, with a particular role in maintaining the intestinal crypt-villus axis. When BMP signaling was inhibited, numerous ectopic crypts formed at right angles to the normal axis. These intestinal abnormalities resemble those seen in the cancer predisposition disorder juvenile polyposis and thus underscore the important role of mesenchymal-epithelial communication in cancer development.

  11. Designer Stem Cells

    The therapeutic application of stem cells will require ways to direct their differentiation and avoid immune rejection of the transplanted cells. The use of cells from the treated patient for somatic cell nuclear transplant technology will reduce the likelihood of rejection. Hwang et al. (p. 1669; see the cover and 13 February news story by Vogel) report the generation of a pluripotent stem cell line from a human blastocyst that was generated by nuclear transfer of an adult somatic cell into an enucleated oocyte from the same individual. Although a parthenogenetic origin of the stem cell line has not been completely ruled out, experiments indicate that the stem cell line was derived from a somatic nuclear transfer embryo.

  12. Revealing Protein Folding Trajectories

    Understanding the mechanism of protein folding remains a major challenge in biology. Fernandez and Li (p. 1674) tracked the complete folding trajectories of single poly-ubiquitin chains using force-clamp atomic force microscopy. After unfolding the polyprotein at high force, the stretching force was quenched, and protein folding was monitored. Ubiquitin folding did not consist of transitions between well-defined discrete states, but rather occurred through a series of continuous stages.

  13. Alumina Comes to Order

    Aluminum oxide films grown on metal surfaces find various uses, from electron tunneling barriers to supports for metal particles as model catalysts, and could act as heat- or corrosion-resistant barriers. Highly crystalline alumina layers are often grown by oxidizing the (110) surface of nickel-aluminum alloy, and Stierle et al. (p. 1652) now present a structural determination of this overlayer from a surface x-ray diffraction study. A distorted hexagonal oxygen ion layer plays host to an equal distribution of tetrahedral and octahedral sites for aluminum cations. Such subsurface features are difficult to resolve with scanning probe methods.

  14. Big Little Particles

    Chemical analyses have been able to identify only a small fraction of the components present in atmospheric organic aerosols. Polymerization processes might explain the discrepancy between the amount of organic aerosols known to exist, on the basis of measurements of their mass, and the much lower mass that can be accounted for on a species-by-species basis. Kalberer et al. (p. 1659) present chemical evidence that products of polymerization reactions dominate the composition of these aerosols. By inducing the photooxidation of aromatic compounds in a reaction chamber, they show that a large fraction of the organic aerosol mass is composed of polymers with molecular masses up to 1000 daltons. The identification of these polymers could affect how other aerosol properties, such as their optical effects and their potential as cloud condensation nuclei, are understood.

  15. Evolution of SARS

    Tracing the evolution of the severe acute respiratory syndrome (SARS) coronavirus outbreak in southern China reveals some interesting patterns. Several independent sources of infection emerged from the recently established exotic animal markets in the region. Using comprehensive molecular analysis of viruses isolated from patients, The Chinese SARS Molecular Epidemiology Consortium (p. 1666) discovered that the initial phase of the epidemic was characterized by rapid mutation at specific hotspots. These mutational hotspots caused amino acid changes in the virus spike protein that is required for host cell adhesion. The “super-spreader events” in Hong Kong were characteristic of the middle phase of virus evolution, during which the amino acid substitutions rate slowed. Finally, the late phase of the epidemic was marked by strong purifying selection. This insight into the genetic dynamics of the SARS epidemic has implications for preemptive control strategies and for the development of therapeutics and vaccines.