This Week in Science

Science  25 May 2001:
Vol. 292, Issue 5521, pp. 1445
  1. Spin Injection Goes Flat-Out

    The success implementation of spintronics, in which the spin properties of charge carriers are used to carry information, will require getting polarized charges into semiconductors. LaBella et al. (p. 1518) studied the spin-injection efficiency into a semiconductor from a ferromagnetic tip. Spin injection can be as high as 92% for current tunneling into the flat terraces of the GaAs, but it is almost decimated when current transport takes place near or at the terrace step.

  2. Seeing Solar System-Sized Disks

    Disks of gas and dust around stars are considered to be the most likely places for planet formation. Such disks have been observed around young, low-mass stars, but similar observations for high-mass stars have been more difficult, and have left astronomers pondering how common the conditions may be for planet formation in the universe. Shepherd et al. (p. 1513) have now observed a very massive disk (similar to the mass of the solar system) around a high-mass young star, G192.16-3.82. They used the National Radio Astronomy Observatory's Very Large Array radio interferometer coupled with the Very Long Baseline antenna in Pie Town, New Mexico, to improve their resolution by an order of magnitude over previous observations.

  3. Silent Earthquake

    The Juan de Fuca plate is being subducted under the North American plate along the coast of southern British Columbia and of Washington and Oregon. A network of about 14 Global Positioning System (GPS) sites show motion to the northeast consistent with that of the Cascadia subduction zone. Dragert et al. (p. 1525; see the Perspective by Thatcher) observed seven of these sites reverse their motion to the southwest over a period of tens of days in 1999. This shift was modeled as an aseismic event with about 2 centimeters of upward-propagating slip over a large area along the deeper part of the subduction zone. Such a silent slip could add stress or possibly trigger an earthquake on the shallower, seismogenic part of the subduction zone.

  4. Colossal Magnetoresistance to Order

    The wealth of magnetic, electronic, and temperature-dependent properties displayed by the doped bilayered manganites make them potentially very useful not just for novel devices but also for further understanding the complex magnetic and electronic phase transitions observed in other correlated-electron systems. Chuang et al. (p. 1509; see the Perspective by Keimer) used high-resolution angle-resolved photoemission spectroscopy to determine the transport parameters of the manganite and found that the measured conductivity is an order of magnitude lower than that expected from the transport data. The discrepancy may be caused by the formation of a pseudogap (through the nanoscale phase separation of charge and spin) that removes a considerable fraction of carriers for conduction.

  5. A Salty Satellite

    Ganymede, the largest of the Jupiter's satellites, has a surface dominated by water ice, and it has been hypothesized that Ganymede, like Europa, has an ocean below the ice. McCord et al. (p. 1523) analyzed reflectance spectra of Ganymede taken by the Galileo spacecraft's near-infrared mapping spectrometer during a relatively close flyby of the satellite. The ice contains hydrated minerals, such as hydrated magnesium sulfate, that may be derived from a briny layer of water lying beneath the ice.

  6. Superconducting MgB2 Thin Films

    The recent observation of superconductivity in magnesium diboride with a relatively high transition temperature (Tc) of 39 K has generated a flurry of activity. Studies are now aimed at establishing the mechanism, teasing out possible routes for boosting the Tc, and developing more practical preparation techniques. Using precursor boron films, Kang et al. (p. 1521) report that thin films of MgB2 prepared by a pulsed laser deposition and sintering technique exhibit a sharp Tc at 39 K and can sustain large current densities.

  7. Assembly Rules for Coral Reefs

    Protection of the rich faunas of coral reefs will require a greater understanding of their complexity and diversity. Bellwood and Hughes (p. 1532; see the Perspective by Knowlton) have compiled a data set for all Indo-Pacific coral reefs and show that the taxonomic composition of corals and reef fishes is very conservative and that only a narrow range of many possible configurations occurs at the family level. More than half of the observed variation in species diversity is explained by area of available habitat, and there is little relation between latitude and diversity. These results suggest a need for the protection of habitat over regional-scale areas, with international coordination of conservation programs.

  8. Counting Sheep

    A challenge in animal population dynamics is to tease apart the deterministic factors that affect population size from noise. Coulson et al. (p. 1528; see the Perspective by Gaillard et al.) used a combination of empirical analyses of survival and fecundity and age-structured modeling to analyze long-term population data for Soay sheep in the isolated archipelago of St. Kilda off the northwest coast of Scotland. Much of the “noise” is actually deterministic in origin, because animals of different age and sex respond in contrasting ways to population density and to weather patterns. This approach allows much improved predictions of future changes in population size.

  9. A Handle on Histones Marks Chromatin

    The protein RCC1 can be considered as a marker for chromatin, and as such plays an important role in the transport of molecular cargoes between the nucleus and cytoplasm, spindle formation during cell division, and formation of the nuclear envelope. These functions are mediated through RCC1's interaction with the small guanosine triphosphatase Ran. However, little is known about how RCC1 associates with chromatin. Nemergut et al. (p. 1540) show that RCC1 interacts not with DNA, but with the histones that package DNA into chromatin, and specifically with histones H2A or H2B, and does not require higher order chromatin structure for binding. Binding of chromatin promotes RCC1's ability to stimulate guanine nucleotide exchange on Ran.

  10. Early Mammalian Traits

    Two characteristics of living mammals are enlarged brains and the separation of the middle ear bones from the jawbone. Luo et al. (p. 1535; see the cover and see the Perspective by Wyss) present a fossil find from the early Jurassic, named Hadrocodium, that extends the “first appearance” of these advanced anatomical features by more than 40 million years, or about one-quarter of the period of mammalian existence. It suggests that mammalian features were in place long before the major diversifications of mammals.

  11. Metabolism, Excitability, and Seizures

    Potassium channels that are sensitive to adenosine triphosphate (ATP) couple the intracellular metabolic state to electrical activity at the plasma membrane. Yamada et al. (p. 1543) show in a knockout study that these channels exert a protective effect in the brain during hypoxia. Mice lacking ATP-sensitive K+-channels were more sensitive to generalized seizures. In slice experiments, neurons from knockout animals in the substantia nigra pars reticulata, an important brain area for the control of seizures, began to depolarize upon a hypoxic challenge rather than hyperpolarize like the neurons from normal control animals.

  12. Fatal Indigestion

    During the later stages of mammalian embryogenesis, erythrocytes (red blood cells) are formed in the liver and bone marrow from erythroid precursor cells that have expelled their nucleus. The molecular events involved in erythrocyte enucleation are not well understood. Using genetically modified mice, Kawane et al. (p. 1546) show that mice lacking the enzyme deoxyribonuclease II are severely anemic and die shortly before birth. The critical cellular source of the enzyme appears to be macrophages, which are present at the site of erythropoiesis in the fetal liver and probably digest the nuclear DNA expelled from the erythroid precursor cells.

  13. Alzheimer's Balancing Act

    One of the initial steps in the course of Alzheimer's disease (AD) is the deposition of amyloid β peptide (Aβ) in the brain. The enzyme neprilysin, which can degrade Aβ, may be an important player in the pathophysiology of AD. Iwata et al. (p. 1550; see the news story by Marx) compared the metabolism of Aβ in homozygous and heterozygous neprilysin knockout mice with that in wild-type animals. They observed a reduced catabolism of exogenously labeled Aβ in the gene-deficient animals, and endogenous Aβ was elevated in a gene dose-dependent way in the knockout mice. Even a slight imbalance between production and removal of Aβ due to a reduced rate of enzymatic catabolism could increase the risk of developing AD.

  14. Protein Aggregation and Degradation

    Many neurodegenerative disorders are characterized by the prevalence of protein aggregates in the brain. These aggregates are often composed of proteins that have been modified by the addition of ubiquitin—a marker that should lead to degradation of the offending protein by a cytosolic proteolytic complex known as the proteasome—but are the aggregates a cause or a consequence of the disease process. Bence et al. (p. 1552; see the news story by Helmuth) now show that protein aggregation per se appears to inhibit the proteasomal protein degradation system.