This Week in Science

Science  21 Jul 2006:
Vol. 313, Issue 5785, pp. 269
  1. Ground Truth for Wind Direction

    CREDIT: DAVID LOOPE AND JON MASON

    Among the few proxies that recorded wind direction in the era before observation data are sand dunes, which also record episodes of drought. Sridhar et al. (p. 345) analyzed grass-stabilized dunes in the central Great Plains of North America to show that a prolonged drought occurred there between 1000 and 800 years ago (the interval commonly called the Medieval Warm Period). The drought was a consequence of the prevailing wind direction, which was from the dry southwest then, rather than from the south and the Gulf of Mexico, as they are today during the spring-summer wet season.

  2. Just Enough Reduction with Gold

    Many amine compounds, including aromatic amines that are derivatives of aniline, are often prepared by reducing the corresponding nitro compound, but many of the routes that can reduce the nitro group without hydrogenating olefinic double bonds either work in low yield, produce unwanted by-products, or require specific catalyst preparation and control of co-catalysts. Corma and Serna (p. 332; see the Perspective by Blaser) report that gold supported on TiO2 or Fe2O3 can selectively reduce nitro groups. For example, 3-nitrostyrene can be converted to 3-aminostyrene with selectivities >95% without the detrimental accumulation of hydroxylamines.

  3. Deliver More Faster

    Batteries and fuel cells can be used to store or generate electric current, but there are applications where capacitors could store and release current more effectively (one example would be hybrid vehicles). In designing the required dielectric materials, there is often a trade-off between the amount of charge that can be stored and how rapidly it can be stored or delivered. Dielectric polymers are promising materials, but they achieve high energy densities at the expense of a high breakdown field, which leads to a low charge density. Chu et al. (p. 334) show that by combining reversible nonpolar and polar molecular structure changes to realize high electric displacement, and by tuning dielectric constants to avoid early polarization saturation, a very high energy density with high discharge speed and low dielectric loss can be obtained in defect modified poly(vinylidene fluoride) polymers.

  4. Unfrozen Fluctuations

    CREDIT: KUMADA ET AL.

    In conventional ferromagnets or antiferromagnets, strong crystal fields tend to pin the magnetization to a particular crystallographic direction. In contrast, quantum Hall systems based on high-mobility two-dimensional electron systems (2DESs) are free from such unwanted perturbations and offer an ideal opportunity for experimentally investigating low-energy physics in two dimensions. Kumada et al. (p. 329) used a bilayer system of two closely separated 2DESs to look at the spontaneous ordering of the spins. Low-frequency electron-spin fluctuations did not freeze out even in the limit of zero temperature, a hallmark of canted antiferromagnetic order in two dimensions.

  5. Coherence in a Quantum Dot Ensemble

    The long relaxation time of the spin of an electron on a quantum dot makes it a good candidate as a basic element in quantum computing, a qubit. However, in an ensemble of quantum dots, the unavoidable inhomogeneities of the spin properties typically cause fast dephasing on time scales of a few nanoseconds, which is much shorter than the coherence time of the single spin. Greilich et al. (p. 341) overcame this problem of rapid dephasing by generating spin coherence in an ensemble of dots with a periodic train of laser pulses, which extended the coherence time to several microseconds.

  6. Producing the Powerhouses

    Mitochondria are one of the defining features of eukaryotic cells. Dolezal et al. (p. 314) review recent work from the fields of evolution, molecular biology, and cell biology that show how the mitochondrial protein import pathway was established, including the nature of the set of molecular machines responsible for driving this pathway of protein transport. The evolution and comparative aspects of the function of these mitochondrial machines provides a blue-print for understanding the evolution of cellular machinery in general.

  7. A Third Way to Silence RNA

    Two well-characterized RNA silencing pathways use small RNAs. Small interfering (si)RNAs act as targeting molecules in RNA interference (RNAi), and micro-(mi)RNAs are encoded in the genome as tiny noncoding RNA genes. Although distinct, these pathways share a number of components, such as the endonuclease Dicer, which produces RNAs with a characteristic length of ∼22 nucleotides (nt). Vagin et al. (p. 320, published online 29 June), and Lau et al. (p. 363, published online 15 June) report the initial characterization of a third putative RNA silencing pathway in animals, characterized by ∼30-nt small RNAs in the germline—so-called repeat associated (ra) siRNAs in Drosophila and Piwi-interacting (pi) RNAs in mammals (see the Perspective by Carthew). In both cases, these RNAs map specifically either to the sense or antisense strand, but rarely to both, which suggests that, in contrast to siRNAs and miRNAs, they do not arise from double-stranded precursors. The rasi- and piRNAs purify with Piwi proteins, homologs of the Ago proteins found in RNAi and miRNA pathways. Dicer enzymes do not appear to be involved in the generation of the rasiRNAs and, intriguingly, a weak slicing activity is associated with the piRNA complex.

  8. Transport Difficulties and Synuclein Toxicity

    Parkinson's disease (PD) can be associated with aberrant expression of alpha-synuclein (αSyn), and in multiple systems, including fly, worm, mouse, and rat, as well as in human neurons, increased αSyn expression is toxic. Cooper et al. (p. 324, published online 22 June) combined cell biological and genetic techniques to elucidate the mechanism of αSyn toxicity. Increased αSyn expression inhibited an essential endoplasmic reticulum (ER)-Golgi vesicle-trafficking step. Moreover, a genome-wide screen identified a class of highly conserved ER-Golgi trafficking components that could suppress this toxicity. This blockade in ER-Golgi vesicle-mediated transport may explain why dopamine-producing neurons are preferentially affected in PD and other synucleinopathies.

  9. More Ways to Modulate a Membrane

    CREDIT: YEUNG ET AL.

    Cellular lipid remodeling is important in a variety of situations, such as during apoptosis, and often specifically involves changes in anionic lipids. Yeung et al. (p. 347) describe the occurrence of localized changes in the surface potential of the inner aspect of the plasma membrane during receptor phagocytosis in macrophages. Localized alterations were observed in surface charge at the base of the phagocytic cup, while the rest of the plasma membrane remained unaffected. The surface potential change was caused by lipid remodeling during phagocytosis and involved phosphoinositide hydrolysis and phosphatidylserine redistribution. Furthermore, when the surface charge was diminished locally, several signal transduction proteins dissociated from the membrane.

  10. Bees in Decline

    Declines in pollinators and pollinator services, both in natural and agricultural landscapes, have attracted considerable interest both from the public at large and from policy-makers. Biesmeijer et al. (p. 351; see the cover and the news story by Stokstad), using historical and contemporary data, found links between the loss of pollinator diversity and the decline of outcrossing plant species in the United Kingdom and the Netherlands. Substantial shifts were seen in the functional mix of pollinators as well as declines in bee diversity and in plant species that rely on declining pollinator groups for their pollination.

  11. Ring Around the Viral RNA

    In negative-stranded RNA viruses, the viral genomic RNA is bound in a nucleoprotein complex that serves as the template for transcription and replication. Albertini et al. (p. 360, published online 15 June) and Green et al. (p. 357, published online 15 June) have determined the structures of ribonucleoprotein rings formed by recombinant nucleoprotein from rabies virus and vesicular stomatitis virus, respectively. In both cases, the nucleoproteins oligomerize to form a ring structure in which the RNA is sequestered between the two domains of the nucleoprotein, and presumably is thereby protected from attack by the host-cell innate immune system. The RNA must still be made available for processing, and examination of the structures reveals how protein motion could facilitate RNA exposure while maintaining the ribonucleoprotein complex integrity.

  12. Less Stress for Encapsulated Cells

    Encapsulation of single-celled organisms for use in sensors or bioreactors can be challenging. For example, the drying step for surfactant-templated silica networks often creates surface stresses that can lyse cells, and more advanced fabrication methods create networks that must be kept in buffers or under high humidity to maintain cell viability. Baca et al. (p. 337) show that live cells of yeast as well as bacteria (Escherichia coli and Bacillus subtilis) can be used as the templating agents along with amphiphilic phospholipids to form more stable networks. The cell surfaces redirect the templating process to minimize stress. More than 50% of yeast cells were viable 4 weeks after synthesis under ambient conditions. The lipid interface is sufficiently fluid to allow for further cell surface modification in situ.

  13. Magnesium Transporter Structure

    To maintain an appropriate intracellular concentration, the transport of divalent metal cations across the cell membrane is highly regulated. Eshaghi et al. (p. 354) provide insight into magnesium transport in bacteria and archeae with a 2.9 angstrom resolution structure of the Mg2+ transporter CorA from Thermotoga maritima. A long hydrophobic pore is bounded by a ring of aspartates at the cytoplasmic entrance and a funnel of carbonyls at the periplasmic side. These structures may serve as selectivity filters. In addition, metal binding at two sites is likely to regulate transport.

  14. Azole Avoidance by Aneuploidy

    Candida albicans is a common fungal pathogen of human beings that is showing increasing drug resistance in response to the widespread use of azole compounds. Candida does not have a complete sexual cycle but appears to compensate for a potential loss of genetic diversity by having a plastic genome tolerant of differing chromosome copy number (aneuploidy). Selmecki et al. (p. 367) discovered that varying degrees of azole resistance in this fungus were associated with differing and reversible degrees of aneuploidy, especially of the left arm of chromosome 5, which bears genes expressing enzymes targeted by the azole drugs. When drug pressure was removed from resistant clinical isolates, aneuploidy was reduced, as was drug resistance.

  15. Paddy Field Methanogen Genomics

    Rice fields are a global source of the greenhouse gas methane, which is produced by methanogenic archaea. Rice Cluster I (RC-I) methanogens in the rice rhizosphere are highly metabolically active and play the key role in methane production from plant-derived carbon. Erkel et al. (p. 370) have now obtained a complete genome sequence of a RC-I methanogen from a mixed-culture enrichment. The genome data offers insights into how RC-I archaea are able to outcompete other methanogenic groups in rice fields. For example, the rice paddy methanogens have pathways for assimilatory sulfate reduction, and mechanisms for dealing with oxygen toxicity that are not found in other methanogenic archaea.