This Week in Science

Science  05 May 2000:
Vol. 288, Issue 5467, pp. 769
  1. Stable Transients

    Carbenes are reactive species containing a neutral, formally divalent carbon atom. They are transient intermediates in many organic reactions of synthetic importance, but they have been difficult to isolate and characterize in detail. Buron et al. (p. 834) have succeeded in preparing stable carbenes by modifying the substituents so that the electroneutrality of the carbene center is preserved. The compounds are stable for weeks in solution at −30°C.

  2. Stamping Your (Micro) Mark

    The patterning of electrodes with micrometer resolution is achieved by a photolithographic process followed by an etching step. The time associated with this technique, however, introduces an additional cost to the overall fabrication process. Kim et al. (p. 831) introduce a cold-welding technique, whereby a metal in contact with another metal will weld together. With a patterned metal stamp, the contacted metal can then be removed by lift-off. The authors apply their technique to patterning an array of organic (soft polymer) light-emitting diodes.

  3. A Dumbbell Asteroid

    Asteroids of the M-class are thought to be rich in FeNi metal and may represent the partially exposed metallic cores of differentiated asteroids. Asteroid 216 Kleopatra has been studied intensely because of its large size and unusual light curves and occultation timings, which suggest a bifurcated structure or a binary system. Ostro et al. (p. 836; see Perspective by Hartmann) observed Kleopatra with the upgraded Arecibo radar system and propose a dumbbell shape for the asteroid. This model suggests that Kleopatra was once two separate pieces that came together during a low-velocity encounter and that much of its surface is covered by unconsolidated rock fragments.

  4. Metal Grain Formation

    Primitive meteorites (chondrites) are thought to contain the first particles formed in the solar nebula, the protoplanetary disk of swirling gas and dust around our nascent sun. Meibom et al. (p. 839) explain the origin of chemically zoned Fe,Ni metal grains in chondrites with a model for gas-solid condensation relying on large-scale evaporation of dust near the plane and center of the disk. The gas rises convectively above the disk plane to cooler regions, where the metal grains condense. These grains then remain on the cooler outer fringes of the disk, preserving their chemical zonation, until they are accreted into larger objects.

  5. Age of a Bilaterian Fossil

    Ediacaran faunas mark the rise of soft-bodied animals prior to the Cambrian. Their age and correlation have been uncertain. They occur at a time of major swing in the carbon isotope composition of the oceans, which also may be related to their diversity and the subsequent explosion of animal life in the Cambrian. Martin et al. (p. 841; see news story by Kerr) now show that one of the most diverse and best preserved Ediacaran faunas in Zimnie Gory, White Sea, Russia, is older than 555 million years ago. The new age for these fossils breaks any simple relation between the Ediacaran diversity and the carbon isotope composition of seawater.

  6. Penetrating Cloud Cover

    Sea surface temperature (SST) has a profound effect on weather and climate and provides important information about ocean circulation, storms, and biological productivity. Satellites have measured SST with infrared detectors, but these measurements are blocked by clouds, which cover about half of Earth's surface. Wentz et al. (p. 847) present global and regional measurements of SST made by satellite microwave radiometry, which is able to penetrate clouds. Their observations reveal a wealth of detail about winds, tropical instability waves, and upwelling.

  7. Growth of Crust

    The isotopes of potassium and argon can be used to trace the evolution and growth of Earth's crust because 40K is preferentially incorporated into melt, and 40Ar into the gas phase, when magma is generated in the mantle. Coltice et al. (p. 845) analyzed the K and Ar concentrations in the atmosphere, crust, and mantle. From their mass-balance calculations, they infer that less than 30% by mass of the modern continents were subducted over Earth's history and that continental crustal growth was mainly by accretion of oceanic plateaus.

  8. Community Diversity and Homogenization

    It is now well known that invasions by alien plants and animals are a major factor in the current biodiversity crisis. Levine (p. 852; see news story by Kaiser) presents data that address the factors that render species invasive or communities invadable. In Californian riparian vegetation, diversity does indeed reduce invasion at the local level, but biotic and abiotic factors correlated with diversity result in the more diverse communities having a greater number of invaders. In a study of fish faunas across the United States, Rahel (p. 854) examines the relative roles of species introductions and extirpations. Almost all states are more similar than they used to be; some states that originally shared no species now share two dozen or more. Even extinctions make states more similar, because extinctions generally have affected the more geographically restricted species. The main reason for the homogenization has been the east-to-west transport of gamefish and their associated prey species.

  9. The Benefits of Giving

    In a world motivated by self-interest, why do people cooperate, especially in situations where they are unlikely to benefit directly? Wedekind and Milinski (p. 850; see Perspective by Nowak and Sigmund) tested the idea that people help others because it enhances the way they are viewed by their community. A group of students played a “game” in which they donated and received money, although reciprocal transactions were not allowed. Individuals with a history of donating generously were more frequently the beneficiaries of largess from other members of the group.

  10. A Steadying Influence

    In order to control eye movements properly, the brain must integrate information about muscle activity from all over the body, the position of the head in space, and movement intentions. This is achieved by a distributed neuronal circuit, the oculomotor neural integrator. Nakamagoe et al. (p. 857) identify in the pontine region of the brainstem a group of cells that project to the flocculus of the cerebellum and describe their role in the conversion of velocity to position.

  11. Light Regulation of Transcription

    Plants respond to changes in light with physiological or developmental changes, processes mediated by light-responsive genes. Martinez-Garcia et al. (p. 859; see the Perspective by Nagatani) now clarify the molecular interactions that connect light to changes in gene transcription by showing that the activated form of phytochrome binds to a transcription factor that, in turn, binds to an element found in some light-responsive gene promoters.

  12. Setting Boundaries

    Telomerase is a ribonucleoprotein (RNP) enzyme that replenishes the ends of eukaryotic chromosomes by using a small portion of its RNA component as a template for DNA synthesis. Working with the yeast enzyme, Tzfati et al. (p. 863) show that the template boundaries (the sites at which DNA synthesis begins and ends) are determined not by the telomerase protein component but by a phylogenetically conserved secondary structure in the nontemplate region of the RNA.

  13. Gene Expression at Its Core

    The discovery that the TATA-binding protein (TBP)-related factor 1 (TRF1) is expressed in a tissue-restricted fashion during Drosophila embryogenesis raised the possibility that the core RNA polymerase II machinery might display tissue-specific properties. Now, Holmes and Tjian (p. 867) have found a TRF1-responsive promoter in the Drosophila gene tudor. The tudor gene contains two tandem start sites; transcription from start site 1 is stimulated by TBP, while transcription from start site 2 is stimulated by TRF1. This arrangement of tandem promoters could allow TRF1 to substitute for TBP in regulating specific subsets of genes.

  14. Self-Destructing Inhibitors

    Mammalian cells contain proteins that function as endogenous inhibitors of apoptosis (IAPs) and that act, at least in part, by inhibiting caspases (proteases that mediate signals leading to cell death). In thymocytes, apoptosis can be blocked by inhibitors of the proteasome (a protein complex that mediates proteolysis of ubiquitin-tagged proteins). Yang et al. (p. 874) find that the IAPs have ubiquitin ligase activity and that they appear to catalyze auto-ubiquitination. This activity increases in response to apoptotic stimuli, indicating that regulated proteolysis of IAPs contibutes to control of cell death in thymocytes.

  15. Packing the Herpesvirus Capsid

    Human herpesviruses cause a variety of ailments ranging from cold sores to cancers. Now Zhou et al. (p. 877) have determined the three-dimensional structure of the herpesvirus capsid—a 0.2-billion-dalton capsid shell of four unrelated proteins—at 8.5 angstrom resolution by electron cryomicroscopy. The structure reveals how the four proteins are arranged in the hexon, penton, and triplex subunits; α helices can be identified at domains involved in capsid assembly and DNA packaging.

  16. The Future of Coherent Chemistry

    The control of quantum phenomena in atoms—for instance, the two-photon excitation of Na—and molecules—for instance, maximizing the selective fragmentation at a particular bond—has undergone rapid growth with the development of femtosecond pulse lasers. Rabitz et al. (p. 824) review the current state of theory and practice and look ahead to deeper exploration of the nature of interatomic forces and quantum computation.

  17. Two Paths to Death

    The c-Jun NH2-terminal protein kinase (JNK) is activated in response to cellular stress and is required for apoptosis of hippocampal neurons induced by the excitatory neurotransmitter glutamate. Tournier et al. (p. 870) have used fibroblasts from mice that lack all functional Jnk genes to show that Jnk is also required for apoptosis induced by ultraviolet radiation. Unlike the apoptotic effects of JNK in neurons, however, which depend on control of the AP-1 transcription factor complex, the effect of JNK in fibroblasts is mediated by a mitochondrial death signaling pathway.

  18. Extraordinary Evidence or Extraordinary Conclusion?

    Gould et al. (Reports, 15 Oct., p. 548) found evidence that new neurons are added to neocortical association areas of adult monkeys and suggested that such neurons “may play a role in the functions of association neocortex.” Nowakowski and Hayes take issue with several key points in the Gould et al. analysis. They note, for example, that the use of bromodeoxyuridine (BrdU) labeling and immunohistochemical detection could label cells undergoing DNA repair rather than mitosis, and that the interpretation of BrdU-labeled cells as neurons migrating from the subventricular zone to neocortex implies an unusually fast neuronal migration rate. They also argue that calculations of neuron growth based on the work of Gould et al. “imply three specific, experimentally verifiable predictions, none of which have yet been supported.” In their response, Gould and Gross present additional data to support the notion that BrdU is indeed labeling cells that are preparing for division, argue that the supposedly excessive speed of neuronal migration cited by Nowakowski and Hayes lies “within the range of previously reported neuronal migrations,” and suggest that some of the neuron-growth calculations of Nowakowski and Hayes may rest on a misreading of the data presented by Gould et al.The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/288/5467/771a

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