This Week in Science

Science  15 Oct 1999:
Vol. 286, Issue 5439, pp. 373
  1. What's at Fault Under New Zealand?

    The South Island of New Zealand is being pulled apart by the large strike-slip Alpine fault that separates the northwestward moving Australian plate from the southeastward moving Pacific plate. Does the faulting of the upper crust continue into the underlying mantle lithosphere, or do these lower regions shear in a ductile manner? Molnar et al. (p. 516) measured seismic anisotropy and delays of seismic P waves (compression waves) on the South Island, which measure strain and lateral inhomogeneity, respectively, in the upper mantle. Their results indicate that the mantle beneath the island is continuous and is being sheared by the crustal deformation. Thus, the crustal fault does not cut down through the mantle.

  2. Expanding Networks

    Networks consist of a number of vertices that are connected to each other by edges in some manner. Barabási and Albert (p. 509) analyzed the structure of some very large networks, including a sample of pages on the World Wide Web (>300,000 vertices), the network of actor collaborations (>200,000 vertices), and the power grid system in western United States (>5000 vertices). They found that the topology of the networks, that is, the probability P that a vertex is connected to k other vertices, can all be described by power laws P(k) = k−γ, where γ may range from 2.1 to 4. Conventional network analyses predict an exponential relation of the topology rather than a power law, which suggests that vertices with a large number of connections should be absent in very large networks. The model provided here allows the network to grow by adding vertices and prefers to connect new vertices to ones that are already well connected. This model reflects aspects of real-world behavior—for example, people tend to link their new Web page to ones that are already popular.

  3. Barrier Control in Magnetic Tunnel Junctions

    A tunnel junction consists of two electrodes closely separated by an insulating barrier. If the two electrodes are ferromagnetic, the spin of tunneling electrons can be polarized and the tunneling rate should depend mainly on the direction of the magnetic moment in each of the ferromagnetic layers, not the choice of the insulator. De Teresa et al. (p. 507) show that this simple picture is not correct. They find varying results for different insulator materials (even reversals in the spin of the electrons that preferentially tunnel) and show that, counter-intuitively, it is the electronic density of states at the metal-oxide interface that determines how the tunnel junction operates. These results provide a new route for optimizing magnetic tunnel junctions.

  4. Plating Out Gold

    A giant submarine gold deposit exists off the coast of Lihir Island, Papua New Guinea. McInnes et al. (p. 512) dredged mantle xenoliths from this region to determine the provenance of the gold enrichment. They measured the osmium and oxygen isotopic concentrations and determined that most of the trace elements, including the gold, were derived from the mantle. The trace elements are concentrated near Lihir Island because the subduction of the Pacific plate under the Australian plate has stalled in this area, which allows a flux of fluids and melt from the mantle to remove trace elements and concentrate these elements in the oceanic crust.

  5. Back and Forth Over Time

    The North Atlantic Oscillation (NAO) is a large-scale seesaw in sea-level atmospheric pressure that is thought to be a major influence on ocean convection and air-sea forcing in that region. It is of great interest to paleoclimatologists to find proxies for the NAO so that climate may better be linked to past patterns of atmospheric circulation. Keigwin and Pickart (p. 520) may have found such a proxy in two sediment cores from south of Newfoundland. These cores provide a record of sea surface temperature changes caused by variability in the position of a minor bifurcation of the Gulf Stream that moves north and south in concert with reversals of the NAO. This type of information should help to clarify the patterns of ocean-atmosphere circulation during periods of warm, interglacial climate.

  6. Writing Between the Lines

    The lithographic methods used to print magazines in color or to fabricate integrated circuits return several times to the original substrate and print additional features in registry with those already on the surface. Hong et al. (p. 523; see the news story by Service) show that a recently demonstrated scanning probe method for writing nanoscale lines or dots on a surface, “dip pen” nanolithography, can now write on a surface several times in different “colors.” Registration marks on the surface allow a line of organic molecules to be placed between existing lines of a different chemical composition without disturbing the original pattern. New lines can be placed within 5 nanometers of existing lines

  7. Holding Off Bacterial Invasions

    One of the first lines of defense against microbial invaders are the defensins. These small peptides, which are produced by both invertebrates and vertebrates and provide an initial barrier that is bactericidal or bacteristatic (see the Perspective by Ganz), are the subject of a research article and a report. Tang et al. (p. 498) identified an unusual defensin in primates, rhesus theta defensin-1, that is a cyclic peptide. Genetic analysis revealed that the cyclic peptide originates from two separate loci. Two propeptides are produced that ultimately are trimmed and stitched together to form the active cyclic defensin. This unusual form for an animal peptide suggests that enzymes may exist that catalyze the cyclization reaction and, if so, these may not be their only substrates. Yang et al. (p. 525) report that human β-defensins can attract immature dendritic cells and T cells through the use of the CCR6 chemokine receptor, whose only other known ligand is LARC (also called MIP-3α). Thus, these peptides not only provide initial antimicrobial protection, but they attract the cells of adaptive immunity to provide a more efficient immune response.

  8. Death of a Synapse

    Neurotransmitter receptors form dense clusters at neuromuscular junctions—that is, until signal transmission across that synapse ceases. As Akaaboune et al. show (p. 507; see the Perspective by Salpeter), an apparent lack of signal on the postsynaptic side results in the acetylcholine receptors first migrating away from the junction and then being degraded. The resulting changes can be apparent in a matter of hours. Thus, neuronal activity regulates the density of receptors at the synapse.

  9. Out of Asia, Too

    What was the geographical origin of the anthropoid primates, and from which primitive primate group were they derived? These questions have been the subject of much debate among primate paleontologists for decades. Jaeger et al. (p. 528) describe newly found fossils from a primate that lived in Myanmar during the middle Eocene (about 20 million years ago). This finding strengthens the case for Asian as well as African anthropoid clades and suggests an affinity with the tarsiids rather than other primitive primate groups. Their results also suggest that the anthropoid clade diverged from other primates very early, possibly before the Eocene.

  10. Tail Wagging the Channel

    Improper function of the chloride channel known as the cystic fibrosis transmembrane regulator (CFTR) is associated with debilitating diseases in humans. Conductance of the channel is regulated in a complex manner. One mechanism of regulation requires phosphorylation of the channel in the so-called regulatory or R domain, which relieves an inhibitory effect of this domain on channel function. Naren et al. (p. 544; see the news story by Hagmann) now provide evidence that another region, the amino-terminal tail of the channel, also has critical regulatory effects. The amino-terminal tail appears to promote phosphorylation-induced conductance of the channel through interaction with the R domain. The results present a new potential target for drug development aimed at modification of CFTR channel activity.

  11. Identifying Tumors Through Gene Expression

    The ability to classify tumors into different categories has critical ramifications for making decisions about therapies. Golub et al. (p. 531) provide a proof-of-principle study to show that gene expression patterns can be used to classify cancer classes. A class-prediction procedure was able to use bone marrow from patients and distinguish acute myeloid leukemia or acute lymphoblastic leukemia. The procedure could have been used to identify these classes even if they had not been known previously to exist.

  12. Competition and Community

    The stability of an ecological community is thought to depend on the number of species it contains and the strengths of the interactions between them. Using a combination of theoretical approaches, Ives et al. (p. 542) show how the effects of competitive interactions and species number on community resilience depend on parameters such as the intrinsic rate of population growth and cross-correlations between exogenous factors that affect different species. They find that the number of species and interspecific competition have little influence on variance in community-level attributes such as total biomass; rather, total community biomass depends only on how the constituent species respond to environmental fluctuations.

  13. Plastic Brains

    Emerging evidence suggests that neurons in certain areas of the adult primate brain, such as the hippocampus and olfactory bulb, are capable of dividing. In a histochemical study, Gould et al. (p. 548) have found new neurons in the prefrontal cortex and inferior and posterior temporal cortex (regions connected with neural plasticity related to learning and memory) in Old World monkeys. They suggest that these new neurons originate in the subventricular zone and migrate through the white matter to the neocortex, where they differentiate into mature neurons and extend their axons to become integrated into the local circuitry

  14. Resealing DNA

    Topoisomerase I (topo I) is a critical enzyme in DNA replication that works by breaking the DNA backbone, allowing the DNA to untwist, and then resealing the break. During this process, a covalent intermediate is formed between topo I and its DNA substrate. If this bond is not eventually broken, the cell will die. Pouliot et al. (p. 552) have cloned the gene for an enzyme that hydrolyzes this bond in yeast and they show that it is conserved in higher eukaryotes. This enzyme, tyrosine-DNA phosphodiesterase, may have implications for improving the efficacy of certain cancer chemotherapeutic drugs, such as camptothecin, which act by stabilizing the covalent topo I-DNA complex.

  15. Sugar Sequences

    New roles continue to be found for complex polysaccharides present at cell surfaces, but these molecules are not as readily sequenced as polynucleotides and proteins, in part because the number of potential modification sites on each sugar ring. Venkataraman et al. present a method for sequencing an important class of polysaccharides, the heparin-like glycosaminoglycans (HLGAGs). Matrix-assisted laser desorption ionization mass spectrometry in conjunction with enzymatic degradation allowed the rapid sequencing of several HLGAGs from minute quantities (picomoles) of samples.