This Week in Science

Science  02 Apr 1999:
Vol. 284, Issue 5411, pp. 9
  1. Human Beginnings

    The genus Homo, which includes modern humans, seems to have arisen about 2 million years ago, although its origins and the number of species (and which species) included in Homo have been debated. Wood and Collard (p. 65) review the status of Homo and conclude that two species previously assigned to the genus, Homo habilis and Homo rudolfensis, do not belong.

  2. A Superradiant Collision

    Gamma-ray bursts quickly release an enormous amount of energy, yet the source of this energy remains elusive. Van Putten (p. 115) offers a model involving the coalescence of a neutron star with a black hole that provides such energetic bursts. As the neutron star approaches the massive black hole, its orbit may be circularized and the tidally disrupted star may form a torus of debris with a remnant magnetic field around the black hole. Then the newly formed magnetosphere and the horizon of the black hole can work together to create a resonant waveguide of the plasma waves leading to the formation of a bubble in this cavity that bursts when it becomes unstable. Hence, the superradiance of the plasma waves initiated by the turbulent torus provides a viable explanation for gamma-ray bursts.

  3. Coral Reef Peril

    What effect will increasing atmospheric CO2 concentrations have on marine ecosystems? Kleypas et al. (p. 118) predict a severe negative impact on coral reefs: An increase in dissolved CO2 concentration in seawater will enhance the solubilization of calcium carbonate, decreasing the saturation state of aragonate and therefore reducing calcification. The authors predict that between preindustrial times and the middle of the next century there will have been a drop in reef calcification of 14 to 30%.

  4. Shifting Boundaries

    Seismic data reveal a complicated boundary between Earth's core and mantle. Some parts of the boundary layer may be molten; the nature of the boundary varies laterally. Most studies have examined either compressional or shear waves. Wysession et al. (p. 120) analyzed both wave types to infer that part of the boundary layer beneath the mid-Pacific Ocean is anisotropic. These observations imply that the layer is flowing there or may be recording a relic fabric.

  5. Smoothing the Way

    The growth of thick, atomically flat single-crystal films on a single-crystal substrate usually requires ultrahigh vacuum conditions. Electrochemical deposition, although far simpler, tends to form rough or polycrystalline films or is limited to thicknesses of ∼10 monolayers. Sieradski et al. (p. 138) show that flat, thick epitaxial films (∼250 monolayers) of silver can be deposited on gold if a mediator such as lead or copper is periodically co-deposited and stripped from the surface. The mediator helps nucleate the growth of two-dimensional islands of silver that promote smooth layer-by-layer growth.

  6. Transporting Rock

    Subduction during continental collision can bury rocks to great depths, but it can also bring deeply buried rocks to the surface. Erosion and faulting have brought rocks in several mountains to the surface from depths of 100 kilometers or more. Bozhilov et al. (p. 128; see the news story by Kerr) describe the presence of clinoenstatite in material from the Alpe Arami peridotite in the Swiss Alps, rock that, previously and controversially, has been proposed to have been exposed from depths of several hundred kilometers. The authors suggest that the clinoenstatite exsolved from another mineral, diopside, at depths greater than 250 kilometers.

  7. Mesoscopic Magnetic Molecular Clusters

    When a magnetic particle is reduced to the extent where its quantum wave function is comparable to its physical dimensions, interference effects for the spin system can be observed similar to those found for electronic charges in quantum dots. Wernsdorfer and Sessoli (p. 133) used highly sensitive detectors to record transitions in the magnetization of single crystals of magnetic clusters that could occur between two stable states only through quantum-mechanical tunneling. The two tunneling paths that the spin states can take interfere, thus providing direct observation for the quantum spin phase in a magnetic system.

  8. Spectra of a Molecular Scramble

    Protonated methane, CH5+, has been known to form under certain plasma conditions, but it has been difficult to study both experimentally and theoretically. Calculations indicate that several structures have nearly the same ground-state energy, and thus the molecule can be viewed as having well-defined C-H bond distances but no well-defined angles between the bonds. Despite this highly fluxional character, White et al. (p. 135; see the Perspective by Marx and Parrinello) show that CH5+ has a well-defined C-H stretching band. Tuning of the preparation conditions and elimination of related hydrocarbon species lines were needed to identify CH5+, and further theoretical work will be needed before an actual assignment of lines can be made.

  9. Localizing Light

    Materials are generally weakly absorbing to light at frequencies less than their electronic band gap, and making a material porous should make it even more transparent at such frequencies. Schuurmans et al. (p. 141) report that macroporous gallium phosphide actually becomes less transparent and scatters more light as it is made more porous. This increased scattering is analogous to that found in electronic systems where charged carriers are scattered from defects and are said to be “Anderson localized” when the density of defects is sufficiently high. Under conditions of extreme disorder, the scattering of the light increases until the propagation ceases to be diffusive and instead adopts a tunneling mode of transport. However, the optical systems are much simpler, in that, unlike charged electrons, photons do not interact with each other. These results give theorists an ideal system in which to model their predictions of light scattering in disordered materials.

  10. New Tricks from an Old Enzyme

    Aminoacyl-transfer RNA (tRNA) synthetases link amino acids to the appropriate tRNAs during protein synthesis and play a fundamental role in translational fidelity. Wakasugi and Schimmel (p. 147; see the Perspective by Weiner and Maizels) report that human tyrosyl-tRNA synthetase (TyrRS) harbors two cytokine activities that are unmasked when the native enzyme is proteolytically cleaved. Recombinant forms of a carboxyl-terminal TyrRS fragment induced chemotaxis of leukocytes and monocytes, and an amino-terminal fragment had properties in common with interleukin-8. Native TyrRS was secreted by apoptotic cells in culture and processed into fragments similar in size to the biologically active fragments. The authors hypothesize that TyrRS cleavage may be important in apoptosis, leading both to arrest of protein synthesis and recruitment of cells that engulf apoptotic cell debris.

  11. Turning Out a New Leaf

    Although maize may not much resemble a snapdragon, it seems that the molecules that initiate and direct the pattern of their leaves and floral organs are quite similar. Timmermans et al. (p. 151) and Tsiantis et al. (p. 154) have both cloned the rough sheath2 (rs2) gene from maize and show that it resembles another gene, phan, known from snapdragon. The two genes are similar in sequence and contain features typical of transcription factors; mutations in both generate disturbances in leaf development. The comparison supports intriguing insights into the cascade of regulatory controls, suggesting that the rs2 gene controls, perhaps epigenetically, the knox-type genes that also direct leaf formation.

  12. Tumor Control

    To prevent cancer, an organism will induce the death of cells that are going awry. To determine how p53 is involved in the death of tumor cells, Soengas et al. (p. 156) used a system in which cells express the oncogene Myc and die in response to conditions that simulate those in tumors (hypoxia, lack of growth factors, or growth in suspension). This p53-dependent cell death was meditated by the apoptosis regulator Apaf-1 and the apoptosis protease caspase-9. Tumors that did not express one or the other of these apoptosis effectors had enhanced tumorigenicity. Thus, interference with the effectors of apoptosis can contribute to transformation and tumor development.

  13. Bone Marrow Stem Cells

    Stem cells by definition will self-perpetuate but also differentiate along specific lineages to generate fully mature cells. In adults, hematopoietic stem cells are found in greatest number in the bone marrow. Pittenger et al. (p. 143) now report that a mesenchymal stem cell can be isolated from human bone marrow that differentiates into adipocytes, chondrocytes, or osteocytes, depending on the in vitro culture conditions. Thus, bone marrow may be a cornucopia of stem cells for many lineages.

  14. Transcriptional Infidelity

    In acting as a template for the synthesis of messenger RNA (mRNA), it is important that the DNA code be transcribed faithfully. Chemicals and ionizing radiation can damage DNA by deaminating the DNA base cytosine to produce uracil. During transcription, RNA polymerase would place guanine opposite cytosine, but when the DNA template contains a deaminated cytosine (uracil), an adenine would be inserted. Viswanathan et al. (p. 159; see the Perspective by Bridges) now show that such transcriptional mutagenesis occurs in vivo in nondividing cells, with translation of a mutated mRNA yielding a protein of altered function. Due to the accumulation of DNA damage over time, transcriptional mutagenesis may be involved in diminishing cellular function with increasing age.

  15. Imagine That

    Functional brain imaging has made it possible to observe which areas of the brain become active when a subject receives sensory input, executes a motor behavior, or thinks. For example, previous work revealed activation in the primary visual cortex during imagery even in the absence of any visual input. What has been unclear is whether activity in this first stage of visual processing was required for imagery or was merely a reflection of imagery-producing activity at later stages of the processing pathway. Kosslyn et al. (p. 167; see the news story by Barinaga) used transcranial magnetic stimulation to scramble the processing within visual cortex and find that imagery is impaired, arguing in favor of a requirement.

  16. The Rise and Fall of a Lake

    The first major civilization arose in northwest India about 4500 years ago during what has been thought to be a wet period characterized by a heightened Indian monsoon. Enzel et al. (p. 125) present a detailed climate record from a dry lake core in northwestern India. The data and dating imply that moist climates ended earlier than previously suggested and that the civilization arose instead during the semiarid period.

  17. Phosphorylation and Plasticity

    CamKII is an important protein kinase in the sequence of events involved in synaptic plasticity. How does the nerve cell manage to recruit this enzyme to its site of action? Shen and Meyer (p. 162) show that the dissociation of the β-CamKII isomer from actin into the cytosol is induced by autophosphorylation or by binding of calmodulin. In a second step, with different kinetics, calmodulin-bound α-or β-CamKII is translocated to the postsynaptic density, where it phosphorylates its target proteins. This process is triggered by activation of NMDA receptors and subsequent Ca2+ influx into the neurons. Thus, autophosphorylation and calmodulin-binding not only control the activity of this enzyme, but they also determine its movement from one cellular site to another.

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