This Week in Science

Science  01 Oct 1999:
Vol. 286, Issue 5437, pp. 9
  1. Climatic Wrinkles on Venus

    Most of the surface of Venus is covered by ridged plains that were formed by major volcanic eruptions and then deformed by tectonic processes. The wrinkling of some of these ridged plains has been suggested to have occurred on a short time scale (less than 100 million years) soon after the plains were emplaced. In a modeling study, Solomon et al. (p. 87) found that an increase in the temperature of the atmosphere, which would have resulted from a major volcanic eruption releasing greenhouse gases such as water and sulfur dioxide, could have thermally stressed the subsurface sufficiently to wrinkle the ridges

  2. Older Martian Alteration

    The martian meteorite ALH84001, which has been suggested to carry relics of biogenic organisms, has been dated by isotopic methods. These previous studies put the age of the primary minerals at about 4.5 billion years old (Ga); however, the age of the secondary minerals (the carbonates formed by aqueous alteration of the primary rock) was only about 1.3 Ga. Borg et al. (p. 90) dated the carbonates extracted with a leachate technique and derived much older ages of about 3.9 Ga by lead-lead dating and about 4.0 Ga by rubidium-strontium dating. These older ages suggest that the carbonate minerals formed when there was water flowing on the martian surface and possibly during the late heavy bombardment period on Mars, when impacts occurred at a much higher rate.

  3. Sulfur Cycles on Europa

    Galileo's near-infrared imaging spectrometer detected hydrated species in spectra of Europa's icy surface. Carlson et al. (p. 97) have derived laboratory spectra of hydrated sulfuric acid (H2SO4·8H2O) that match the europan spectra. The presence of hydrated sulfuric acid would suggest that cycling occurs on the surface (between polymerized sulfur, sulfur dioxide, and sulfuric acid) that is driven by irradiation from the jovian plasma and possibly by extrusion of sulfur-rich liquids from below. Such a sulfur cycle may explain the patterns of dark and bright terrains on Europa.

  4. More than Methane Inside?

    Uranus and Neptune are thought to contain middle layers of ice rich in methane, water, and ammonia at high pressures and temperatures. Benedetti et al. (p. 100; see the news story by Kerr) laser-heated methane in a diamond anvil pressure cell to comparable conditions and found that methane partially dissociated into diamond, amorphous carbon, and hydrocarbons. These experiments suggest that models of the dynamics of the interiors of Uranus and Neptune should consider the effects of carbon phases and hydrocarbons rather than methane, which may be unstable.

  5. Surveying Martian Gravity

    Doppler tracking of the Mars Global Surveyor spacecraft has allowed Smith et al. (p. 94) to measure gravitational anomalies. A global map of the gravity field reveals large, distinct positive gravity anomalies associated with the Tharsis, Olympus Mons, and Alba Patera volcanoes. These results suggest that each volcano had separate sources of magma. A large negative gravity anomaly associated with the 11-kilometer-deep Valles Marineris canyon extends into the Chyrse basin, which suggests that the basin formed by water erosion of material from the canyon.

  6. Arrested Cracks in Layered Ceramics

    Metals and other ductile materials can deform plastically and heal the formation of cracks and voids, but ceramics usually fail catastrophically because there is no internal mechanism to hinder or blunt crack growth. Rao et al. (p. 102) fabricated a material containing alternating thick layers of alumina (0.6 millimeters) and thin layers of alumina and mullite (0.04 millimeters) that exhibited crack arrest at or near a layer interface for tensile stresses up to 480 megapascals. They present a theoretical analysis to account for the role of thin compressive layers in resisting crack extension.

  7. Eyesight for Beginners

    Experiments in animals have shown that visual deprivation can arrest visual acuity at or near newborn values, and the onset of visual input can induce a rapid development of acuity toward normal levels. Maurer et al. (p. 108; see the Perspective by Sireteanu) use a human disease state—children with congenital cataracts—to study this problem. Children who had cataracts removed 1 week to 9 months after birth showed significant improvements in visual acuity, in some cases in 1 hour after the onset of focused vision. Patterned visual experience is thus necessary for acuity to develop.

  8. A Little LTD

    The minimal localization of plasticity on a neuronal dendrite is still not known. Dodt et al. (p. 110) approached this question by using ultraviolet lasers to uncage glutamate in neurons under spatially controlled conditions. They recorded postsynaptic excitatory currents and induced long-term depression (LTD) caused by presenting a train of light flashes at 5 hertz onto a 1-micrometer spot on a dendrite. The LTD induced was dependent of N-methyl-D-aspartate receptors and on calcium and could completely occlude stimulation-induced synaptic LTD. The spread of LTD along the dendrite was less than 10 micrometers, which shows that LTD can be evoked and expressed purely postsynaptically and can be localized on a single synapse.

  9. Matrilysin and Innate Immunity

    Antimicrobial peptides are found on mucosal surfaces, such as in the lungs or in the intestines, and are thought to be one of the body's first lines of defense against bacterial invasions. However, what their true role is has been difficult to assess, because they are a large family of cleaved proteins that come from multiple genes. Wilson et al. (p. 113) report that mice that lack the matrix metalloproteinase matrilysin, which is produced by the same cells in the gut that produce the a-defensin antimicrobial peptides, also lack the cleaved forms of the peptides and were more susceptible to infection. Matrilysin was able to cleave the defensins in vitro. Thus, matrilysin may be responsible for regulation of defensin activity in vivo and may be key to innate immunity.

  10. Tethering Telomerase

    The proteins Est1 and Cdc13 bind to telomeres (the ends of chromosomes) and, together with the enzyme telomerase, control telomere replication in the yeast Saccharomyces cerevisiae. By constructing a series of fusion proteins that in effect tether telomerase to the telomere, Evans and Lundblad (p. 117) show that Cdc13 may mediate accessibility of telomeres to the telomerase complex and that Est1 assists in that function. These results suggest that telomere length homeostasis is maintained at least in part by restricting access of telomerase to chromosome ends

  11. In Small Packages

    Protamines neatly package DNA into chromosomes in sperm cells. Brewer et al. (p. 120) used “laser tweezers” and fluorescence microscopy to examine this packaging process. A single DNA molecule was grabbed by the laser, and protamines were applied to and then removed from the DNA molecule to observe the condensation and decondensation processes, respectively. The technique used here to perform and repeat biophysical measurements on a single DNA molecule may be applied to many other protein-DNA analyses

  12. Base Motifs

    One of the key capabilities of protein-based enzymes is general base (or acid) catalysis of a reaction. Because intracellular pH must be maintained within a narrow window, it is not possible to increase the concentration of hydroxide (or hydrogen) ion in order to increase the rate of reaction. Enzymes rely on amino acid side chains for abstraction (or donation) of a proton and can, for example, increase the negative character of an attacking nucleophile. Ribozymes (RNA-based enzymes) have a more limited side-chain repertoire and often use metal ions to generate hydroxide ions at neutral pH. Perrotta et al. (p. 123; see the Perspective by Westhof) now show that a cytosine residue in the self-cleaving ribozyme of hepatitis delta virus can perform general base catalysis.

  13. Mom's Favorite

    In birds, offspring sired by more highly ornamented males often have enhanced survival. Is this because these males have greater genetic quality (the so-called “good genes” model of sexual selection) or because the females invest differentially in the eggs fertilized by these males? Gil et al. (p. 126; see the news story by Vogel) show that female zebra finches mated to experimentally ornamented males deposit more testosterone in eggs than when mated to unornamented males. Additional testosterone has effects on the growth rate and later dominance of the chicks within the brood. Thus, it seems that post-mating maternal effects may play an important role in the differential development of offspring.

  14. Neandertal Cannibalism

    The possibility that Neandertals practiced cannibalism has been a recurrent theme in paleoanthropology, but evidence has remained scant. New excavations of 100,000-year-old cave deposits in France, reported by Defleur et al. (p. 128; see the news story by Culotta), have revealed extensive indications of butchery in the skeletal remains of several neandertal individuals and deer at the site. Cut marks on the bone provide evidence of disarticulation and defleshing, and damage consistent with a hammer-and-anvil technique indicates that marrow was extracted. Cannibalism thus appears to have deep prehistorical roots.

  15. Antipsychotic Drug Action

    The dopamine hypothesis of schizophrenia has been challenged by the introduction of the atypical antipsychotic drug clozapine. In addition to its lower affinity for dopamine D2receptors, clozapine has been shown to also interact with α2adrenoceptors. Hertel et al. used selective antagonists for D2 receptors and for α2 adrenoceptors to study dopamine release in the prefrontal cortex and conditioned avoidance response in rats. These measurements are thought to correlate with the therapeutic efficacy of antipsychotic drugs. The α2 antagonist, although having no effect by itself, potentiated the selective dopamine receptor blocker. However, catalepsy (loss of voluntary movements) induced by a D2 receptor antagonist was unaffected by the α2 antagonist. These results will further our understanding of the mechanism of action of atypical antipsychotic drugs and will also be useful for the development of future effective treatments with minimal side effects.

  16. Knotted Polymers and Bond Breaking

    M. Grandbois et al. (Reports, 12 Mar., p. 1727) covalently attached polysaccharide molecules to surfaces and then used an atomic force microscope to pull the molecules off. From an analysis of the rupture events, they concluded that changes in force with extension corresponded to progressive rupture of surface attachments, which allowed a determination of Si-C bond strengths.

    A. Stasiak et al. comment that the authors' interpretation is unlikely and suggest instead that “the observed peaks preceding the final breakage could be the ‘signatures’ of progressive tightening of complex knots in the polysaccharide chain.”In response, although H. Gaub et al. “agree that knotted polymers will break at lower forces than unentangled ones,” they present a reanalysis of their data which indicates that only about 5% of the events they observed might reflect tightening of knots. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/286/5437/11a

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