This Week in Science

Science  20 Jan 2006:
Vol. 311, Issue 5759, pp. 299
  1. Magnetism with a Twist


    In helical spin order, the spins in a crystallographic plane of a material tend to align, and this direction rotates by a constant angle between adjacent planes. Knowledge of, and the ability to control, the relative orientation of the magnetic moment between the planes could have important consequences for the flow of spin-controlled current through such a structure. Reciprocal-space imaging probes such as neutron scattering only provide an average view of the overall spin structure. Uchida et al. (p. 359; see the Perspective by Nori and Tonomura), using Lorentz microscopy, found that the real-space structure of helical spin ordering is much richer than that expected from the averaged techniques. They also visualized the real-time dynamics of magnetic defects in response to changes in temperature and magnetic field, which may yield important information for spintronic devices that would rely on this effect.

  2. Spotlight on Structural Genomics Centers

    Projects in structural genomics aim to expand our structural knowledge of biological macromolecules, while lowering the average costs of structure determination. Chandonia and Brenner (p. 347) quantitatively review the novelty, cost, and impact of structures solved by structural genomics centers, and contrast these results with traditional structural biology.

  3. Spinning Membranes from Phospholipids

    Electrospinning is a simple but powerful method for making very thin polymer fibers that can then be collected to create porous films. McKee et al. (p. 353) expand the range of this technique by making fibers from small molecules, namely phospholipids. The phospholipids form wormlike micelles in specific concentration ranges of mixed solvent systems, and under these conditions they behave like polymers for electrospinning. The membranes formed from phospholipids should exhibit high biocompatibility.

  4. Bridging Nanotube Contacts

    In molecular electronics, the contacts between metal electrode and molecule are often the weakest link, and it can be difficult at times to exclude changes in this electrode contact as the cause of switching behavior. Guo et al. (p. 356) show how small gaps (less than 10 nanometers) in single-walled carbon nanotubes (SWNTs) can be bridged covalently with short oligomeric molecules whose conjugation makes them conductive. After metal contacts were made on a SWNT, patterning allowed a gap to be cut between two contacts. This oxidative cutting left terminal carboxylic acid groups that were bridged by making amide linkages to molecules bearing amine groups at each end. The devices formed are robust, and molecules that bear basic nitrogen atoms in the chain changed conductance with pH.

  5. Restricted Motion

    The assignment of gas-phase spectra to specific atomic motions for molecules with even as few as five or six atoms can prove challenging. Such assignments are of particular interest in piecing together the interactions of molecules in deep space, for which spectroscopic signatures are the sole source of data. Lee et al. (p. 365) take advantage of the unusual properties of solid para-hydrogen (p-H2) to simplify, and thus interpret, the vibrational spectrum of methanol. By embedding methanol in a matrix of the quantum solid, they prevent overall rotational motion but still observe internal torsion of the methyl group about the C-O bond.

  6. Mist-Made Martian Glaciers

    Water ice glaciers flank mountains and volcanoes in the tropics and midlatitudes of Mars. Current conditions on Mars are cold and dry and restrict water ice to regions near the poles, so the origin of these young glaciers at lower latitudes is a puzzle. Forget et al. (p. 368) used climate simulations of the planet at high obliquity to explain the locations of the glaciers. A few million years ago, the rotation axis of Mars was tilted by up to 45°, which caused more water vapor to evaporate from the poles into the atmosphere. Circulating across the planet, this watery mist then precipitated to build up glaciers on the leeward side of volcanoes and in mountainous regions.

  7. Rho, Rho, Rho Your Vaccinia


    Viruses subvert a variety of host cell mechanisms during infection, replication, and dissemination. Valderrama et al. (p. 377) now describe how vaccinia virus promotes cellular motility by interfering with the activity of RhoA, a small guanosine triphosphase-binding protein involved in intracellular signaling, which particularly affects the actin cytoskeleton. A conserved vaccinia protein, F11L, directly interacts with RhoA, mimicking one of its endogenous substrates, ROCK, and inducing cellular motility. The induced motility is likely to facilitate the spread of the virus within tissues.

  8. Squares in the Sand

    Language is so intimately linked with our thoughts that it is hard to imagine thinking without it, but how language influences thought remains a lively topic of discussion. Dehaene et al. (p. 381; see the news story by Holden) bring new evidence to light from their studies with an Amazonian group, the Mundurukú. Both Mundurukú children and adults proved competent at grasping and using geometric concepts, such as parallel lines and right-angled triangles, even though they lack the words for such terms and concepts. Furthermore, the Mundurukú used relations diagrammed on paper to locate hidden objects, and again performed as well as American children, but not as well as adults. Thus, the Mundurukú possess a basic sense of geometry, in addition to their previously discovered sense of arithmetic.

  9. Ubiquitous Antibiotic Resistance


    A major source of antibiotic resistance genes is soil microorganisms that produce antimicrobial agents and develop a variety of resistance mechanisms as a way of selfdefense against their own toxic products. D'Costa et al. (p. 374; see the Perspective by Tomasz) show that soil microbiota also represent an enormous reservoir of antibiotic-resistant organisms, most of which do not produce antimicrobial agents themselves. The authors characterized strains of spore-forming bacteria and tested them against 21 antimicrobial agents—some in long use as well as compounds recently introduced into the antimicrobial armamentarium. Every strain was multidrug resistant and exhibited resistance to at least 7 to 8 antibiotics, and sometimes to as many as 20.

  10. Turning Cuttings Back into Whole Plants

    Plants regenerate much better than do animals—an entire plant can regenerate from a small snip of tissue, whereas the best that animals can do is the occasional amphibian regeneration of a limb or tail. Xu et al. (p. 385) now analyze subcellular dynamics in the root tip of Arabidopsis to understand how regeneration is directed in response to localized cell ablation. Surprisingly, as new tissues are built, establishment of unidirectional flow of the hormone auxin follows, rather than precedes, cell fate specification. A suite of transcription factors that respond early to changes in auxin distribution directs cell fate respecification.

  11. Got to Hitch a Ride

    During cell division, chromosomes must establish connections to the opposing spindle poles and become positioned at the spindle equator. Uncorrected errors in this biorientation inevitably lead to aneuploidy and are associated with cell transformation and cancers. How chromosomes attach properly to the mitotic apparatus is not understood. Kapoor et al. (p. 388; see the cover and the Perspective by Heald) used live-cell two-color fluorescence, correlative light and electron microscopy, as well as chemical biology, to demonstrate surprisingly that chromosomes can congress to the spindle equator before they become bioriented. During congression, the leading kinetochore glides alongside kinetochore fibers of other already bioriented chromosomes toward microtubule plus ends. The gliding is mediated by the kinetochore-associated motor protein. Thus, cells possess a mechanism for repositioning monooriented chromosomes from the periphery to central areas of the spindle where they can establish connections to the other spindle pole.

  12. Mammoth DNA Sequences

    The sequencing of ancient DNA is hoped to lend insight into evolutionary studies of a variety of species, including mammals. Poinar et al. (p. 392, published online 12 December 2005) used a roughly 28,000-year-old bone from a woolly mammoth that had been preserved in the Siberian permafrost to directly sequence ancient DNA without prior repair or amplification bias. A total of 137,000 reads (13 megabases) of mammoth DNA were generated, with only traces of human DNA contamination. Genomic comparisons were used to establish the rate of sequence divergence between extinct species and modern elephants. Examination of microbial and plant sequences isolated from the same source may also give clues about the mammoth's environment.

  13. Alloy-Based Aldehyde Synthesis

    The oxidation of alcohols to aldehydes and ketones is readily accomplished stoichiometrically with chromate or permangate anions, but the resulting wastes pose a toxicity hazard. A number of solution-phase and heterogeneous catalysts have been developed to avoid this problem and make use of air as an oxidant. However, the conversion of unactivated primary alcohols like 1-octanol to aldehydes has remained challenging. Enache et al. (p. 362) report that gold-palladium alloy nanocrystals supported on titanium dioxide are efficient catalysts for aldehyde synthesis from primary alcohols, including 1-octanol, using O2 or air as the oxidant. These reactions proceed about 25 times faster on the alloy catalysts versus either of the pure metals alone.

  14. Uppers and Downers

    Some types of microaerophilic marine bacteria contain magnetosomes, which are thought to help the organisms to avoid exposure to high concentrations of oxygen. In the Northern Hemisphere, these bacteria were assumed to seek geomagnetic north, so that in high latitudes they swim down away from oxygenated water, the converse was assumed to occur in the Southern Hemisphere. Simmons et al. (p. 371) now challenge this dogma and show that the Northern Hemisphere magnetotactic bacteria have mixed polarity with occasional blooms of south-seeking barbell-shaped bacteria. Rod-shaped greigite-producing bacteria tend to aggregate in more reducing conditions, whereas south-seeking magnetite-containing bacteria form cocci or short chains of cocci (barbells) under more oxidizing conditions.

  15. A DNA Methyltransferase Prefers RNA Instead

    DNA methylation in many eukaryotes plays a vital role in the epigenetic regulation of gene expression and in helping maintain the integrity of the genome. Characterization of the DNA methyltransferase (Dnmt) activity of the most highly conserved member, Dnmt2, by Goll m et al. (p. 395) reveals that it is actually an RNA methyltransferase. Dnmt2 from a wide range of species, including mice and Arabidopsis, adds a methyl group specifically to the invariant position 38 of Asp transfer RNA. The close similarity between the eukaryotic Dnmts indicates that they may have originally evolved from a Dnmt2-like RNA methyltransferase.