This Week in Science

Science  08 Aug 1997:
Vol. 277, Issue 5327, pp. 741
  1. Mixed dates for carbon and methane

    Radiocarbon measurements of organic carbon have been too crude to identify ages of different compounds (biomarkers), and thus the age may reflect a mixture of carbon from several sources. This difficulty is critical in assessing the burial of organic matter, a key term in the carbon budget, as noted in a Perspective by Freeman (p. 777). Eglinton et al. (p. 796) were able to analyze individual biomarkers in organic carbon from the Black Sea, a stratified basin, and the Arabian Sea, where upwelling is important. The results show that distinct biomarkers can have ages as much as 10,000 years apart. Zimov et al. (p. 800) also use radiocarbon dating to show that methane emitted from high-latitude Siberian lakes is predominantly fueled by carbon deposited during the last glaciation.

  2. Tuberous sclerosis gene

    Tuberous sclerosis complex (TSC) is an autosomal dominant disease in which multiple tumor-like growths develop in the nervous system and other organs. About half of the familial TSC cases are caused by mutations in TSC2, a gene on chromosome 16p13 that encodes a GTPase activating protein. Van Slegtenhorst et al. (p. 805) have identified the TSC1 gene on chromosome 9q34, which is responsible for most of the remaining familial cases. The TSC1 gene encodes a 130-kilodalton hydrophilic protein with a potential coiled-coil domain but has no strong homology to proteins of known function.

  3. Small but stable

    At small sizes, metal oxides often form phases that are not the thermodynamically stable bulk phases. Simulations have indicated that at these small sizes, there is actually a switch in thermodynamic stability, but experimental evidence has been lacking. McHale et al. (p. 788) performed an experimental study on the thermodynamic stability of different phases of alumina and show that the stable bulk phase, corundum, becomes thermodynamically less stable than γ-alumina at small sizes.

  4. Pressured exchange

    Fractionation of isotopes has generally been considered to depend on temperature but not pressure. However, Driesner (p. 791; see the Perspective by Sheppard, p. 775) shows that the fractionation of hydrogen and oxygen isotopes in water-mineral reactions does depend on pressure, and for fractionation of hydrogen isotopes, particularly at conditions close to the critical region of water, the effect is large. Calculation of the effect allows seemingly disparate experimental determinations of mineral-water fractionation curves for hydrogen isotopes to be rectified.

  5. Seamount dates

    Volcanic activity on the ocean floor forms seamounts, but only a few have been dated, so it has been difficult to infer the magmatic histories. Wessel (p. 802) presents an approach to obtain pseudo-ages for Pacific plate seamounts using gravity data, seamount shapes, and the age of the oceanic crust on which they formed. The data imply that smaller seamounts tend to form on younger crust and that many seamounts formed during the latest Jurassic and Cretaceous.

  6. Death decoy

    One way to activate a cell's intrinsic death pathway is for “death” receptors on the cell surface to receive an external signal. A family of ligands, including the protein TRAIL, bind to a family of related receptors. Pan et al. (p. 815) and Sheridan et al. (p. 818; see news story by Gura, p. 768) identified two more receptors that bind to the TRAIL ligand. The first is similar to the known receptor for TRAIL and seems to be an effective mediator of the death signal. But the other receptor lacks a transmembrane domain, may insert into the membrane through lipid linkages, and blocks cell death. Binding TRAIL without being able to send the standard death signals may cause the decoy receptor to act as a sink for the ligand.

  7. Why practice makes perfect

    As a motor task is practiced, the movements become automatic and are consolidated in memory. Shadmehr and Holcomb (p. 821) find that the brain regions active during task performance shift from the prefrontal cortex, which is responsible for short-term or working memory, to the premotor and parietal cortices and the cerebellum, which together support the preparation and execution of a learned sequence of movements.

  8. Rank and reproduction

    The stereotype of the noncompetitive female chimpanzee is dismissed by the long-term study by Pusey et al. (p. 828; see the Perspective by Wrangham, p. 774). More than 30 years of observations of the population at the Gombe National Park in Tanzania have revealed considerable variation on female reproductive success, much of it accounted for by differences in dominance rank. Higher rank means higher infant survival, faster maturing daughters, and more rapid production of young. This effect is likely affected by access to a better food source rather than to direct aggression between females.

  9. Aging assay

    The molecular basis for the limited life-span of somatic cells, a characteristic thought to underlie some of the physiological effects of aging, has been unclear. Brown et al. (p. 831) overcame these difficulties and used targeted homologous recombination to remove both copies of the gene encoding the cyclin-dependent kinase (Cdk) inhibitor p21 in normal (not immortalized) human fibroblasts in culture. Cells lacking p21 (which normally functions to inhibit cell division by inhibiting Cdks) have a prolonged cellular life-span.

  10. Flight plans

    Some fruit flies (Drosophila) travel large distances in their search for food (rovers), while others prefer to find their food closer to home (sitters). Osborne et al. (p. 834) report that this behavior is controlled by two alleles in the foraging gene, which codes for a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG). Sitter flies have less PKG messenger RNA and enzymatic activity than rover flies. This is a rare example of a single gene link to a behavior. An analogous example in humans may be a polymorphism in the serotonin transporter, which has been proposed to underlie variation in anxiety.

  11. Transmitted stress

    To what extent does mountain building due to, say, continental collisions at the margins of plates affect the interiors? Van der Pluijm et al. examined calcite twins in central North America to look at the distal effects of different orogenies in the Appalachians and Rocky Mountains. The data imply that stresses induced at the margins of the North American plate were transmitted over thousands of kilometers into the plate interior regardless of the cause or style of the deformation on the margin.

  12. A better fit

    Spectroscopy provides a probe of the dynamics of vibrations, and thus bonds, and electronic states in molecules. It has been difficult, though, to relate the spectra of large molecules and polymers to appropriate theoretical models, as needed for interpretations. In an article, Mukamel et al. overview one recent approach to this challenge, known as the collective-electronic oscillator representation, which relates the optical spectra to the dynamics of charges and bond orders, rather than the full calculations considering the dynamics of many electrons and excited molecular states.

  13. Light shed on fate

    What is the fate of a single cell in a multicellular organism? Cambridge et al. have devised a technique to examine the spatial localization, morphology, and proliferation of cells throughout development. In this method, light-activated gene expression is used to mark individual cells within different mitotic domains of the early Drosophila embryo in order to monitor cell fate. The study confirmed the hypothesis that mitotic domains are indicators of cell fate. Light-induced gene activation was also used in single cells of Xenopus. This technique is expected to have applicability in other invertebrates and vertebrate systems.

  14. Heightened effects

    Under certain conditions, repeated administration of an addictive drug causes the animal or person to become even more sensitive to its effects. This sensitization is accompanied by an increase in GluR1 in the part of the brain that mediates the addictive action of drugs. Carlezon et al. have artificially increased the expression of GluR1 in this area of the rat brain by injecting a vector containing the GluR1 sequence. As a consequence, the animals' response to morphine is sensitized, indicating that the GluR1 increase that accompanies sensitization is necessary and sufficient for the sensitization to occur.

  15. Polymerase delivery

    Although the small genomes of mitochondria and chloroplasts use RNA polymerase to transcribe their genes, the genes for these polymerases are not necessarily encoded in the relevant genome. Hedtke et al. identified two genes from the nuclear genome of the plant Arabidopsis that encode RNA polymerases similar to the polymerases found in bacteriophage. Specialized transit peptides direct one enzyme to the mitochondria, where it alone transcribes the mitochondrial genome, and the other enzyme to the chloroplast, where it shares transcription of the plastid genome with another RNA polymerase encoded within the chloroplast. A relatively recent duplication of a nuclear gene, followed by redirection of one of the genes, may have generated this system of multiple RNA polymerases.

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