This Week in Science

Science  08 May 1998:
Vol. 280, Issue 5365, pp. 797
  1. Speaking more freely about risky behaviors

    Speaking more freely about risky behaviors Surveys of risky behavior [such as sexual relations or intravenous (iv) drug use], particularly in adolescents, have often been compromised by the hesitancy of subjects to reveal illegal or socially unaccepted activities and by problems in administering tests to illiterate or poorly educated subjects. Turner et al. (p. 867; see the Policy commentary by Bloom, p. 847) apply a new approach, using a voice and written questionnaire on a computer, to assess adolescent sexual behavior and iv drug use in 1690 males in the United States, and compared results with a traditional interview and written survey. Reported incidences of many behaviors increased by a factor of 3 or more: 5.5 percent of respondents had engaged in male-male sex, 5.2 percent had used iv drugs, and 12 percent had carried a gun.

  2. Living longer

    Death rates for many organisms—worms, yeast, and humans—decelerate with increasing age, and in recent years human mortality at older ages has decreased substantially. Vaupel et al. (p. 855) review longevity research and focus on insights from biodemographic data. Although genes exist that explicitly control longevity, nongenetic changes account for increased human life-spans.

  3. Dust to Earth

    Every year about 107 kilograms of interplanetary dust particles (IDPs) fall to Earth from the tenuous cloud of dust (called the zodiacal cloud) orbiting the sun in the inner solar system. Kortenkamp and Dermott (p. 874; see the news story by Kerr, (p. 828) have modeled the flux of IDPs to Earth during the last 1.2 million years; they assumed that the bulk of this material is derived from the Eos, Themis, and Koronis asteroid families. A maximum flux of IDPs occurred every 100,000 years, consistent with control by variations in Earth's eccentricity and the idea that the 100,000-year peak in helium-3 concentrations in sea-floor sediments are controlled by the IDP flux. They also expect that large fluxes may occur every 107 years because of collisions between asteroids in the asteroid belt.

  4. Magnets in bacteria

    Many magnetotactic bacteria produce an intercellular iron sulfide mineral, greigite. Understanding the mineralogy and origin of the greigite and other minerals is important for resolving the origin of these bacteria and their behavior and biomagnetism in general. Pósfai et al. (p. 880) used transmission electron microscope to show that the greigite, which is magnetic, actually forms in magnetotactic bacteria over a few days to weeks from another iron sulfide mineral, mackinawite, which is nonmagnetic.

  5. Xenon and Earth's atmosphere

    Measurements of the xenon isotopes provide information on the time for accretion, mantle degassing, and the evolution of Earth's atmosphere. However, because of the small isotopic enrichments and the complications that some xenon isotopes result from multiple decay schemes, resolution of the anomalies has been difficult. Kunz et al. (p. 877); see the commentary by Kaneoka, (p. 851) have now succeeded in isolating the effects of production of xenon from plutonium and uranium fission (xenon is also produced from decay of iodine) in measurements of mantle gases trapped in mid-ocean ridge basalts. The data imply that 32 percent of the xenon-136 in Earth's atmosphere is from plutonium fission (half-life of 82 million years) and that Earth's mantle began to retain gases within 50 to 70 million years after formation of the solar system, soon after a giant impact is thought to have hit the Earth and formed the moon.

  6. Smooth and thin

    Thin, smooth films with uniform properties are important in many applications ranging from bioadsorption studies to microelectronics. However, they are often difficult to fabricate. Many of the methodologies currently under investigation focus on self-assembly processes. Esker et al. (p. 892) pursued a different route. They used the well-known Langmuir-Blodgett technique for making multilayer assemblies through layer-by-layer transfer of polyelectrolyte monolayers formed at an air-water interface; posttransfer modification formed cross-linked multilayers with high regularity, smoothness, and stability. The method should facilitate chemical derivatization of the films and the assembly of composite structures that differ by as little as a monolayer.

  7. Highly flexible

    Certain materials contract when heated, and this counterintuitive behavior has been rationalized in terms of the high flexibility of the molecular framework in these materials. Theoretical studies have suggested that such materials may become amorphous under pressure and that the two properties may have a common origin. Perottoni and da Jornada (p. 886) investigated the structural behavior of ZrW2O8, a material with a particularly large temperature range over which contraction occurs, under increasing pressure. Amorphization occurred gradually at relatively low pressures.

  8. Tripping the dark fantastic

    In nonlinear optical materials, intense laser beams can become “self-trapped,” that is, the light changes the index of refraction so as to produce a lens effect, which concentrates and traps the light energy as it propagates. Trapping of dark beams—light beams with holes in the intensity profile—has also been observed. Chen et al. (p. 889) report that this dark trapping can occur with incoherent light through an unexpected mechanism. The results raise the possibility that high-power laser light can be controlled with low-power optical sources such as light-emitting diodes.

  9. To be or not to be

    In response to the hormone progesterone, oocytes of the frog Xenopus laevis undergo maturation into unfertilized eggs. In this process, a variable stimulus—the concentration of progesterone—is converted to an all-or-none response of the oocyte. Ferrell and Machleder (p. 895; see the commentary by Koshland, p. 852) reveal the nature of the biochemical switch that accounts for this phenomenon. Maturation is caused by activation of a series of protein kinases known as the mitogen-activated protein kinase (MAPK) cascade. Activation of MAPK through this mechanism in vitro shows a highly cooperative response, and MAPK activation in oocytes showed similar ultrasensitivity. The cooperative nature of MAPK activation, along with positive feedback from MAPK to promote accumulation of one of the enzymes that leads to its activation, appear to account for the switchlike behavior of the maturation process.

  10. Cell shape effects

    Changes in cell shape occur in critical biological processes that include formation of normal tissues during development and invasive behavior of tumor cells. Such changes in cell morphology are coupled to changes in gene expression. Kheradmand et al. (p. 898) explored a signaling pathway through which expression of the collagenase-1 gene is regulated in rabbit synovial fibroblasts. Treatment of these cells in culture with antibodies to a protein that mediates cell adhesion—the integrin α5β1—caused reduced adhesion of the cells to the culture plate, cell rounding, and increased expression of the collagenase-1 gene. The altered gene expression resulted from activation of the small guanine nucleotide binding protein Rac1, generation of reactive oxygen species, and consequent activation of the transcription factor NF-κB.

  11. Spoken and written language

    Aspects of the modular organization of language have been uncovered by Baynes et al. (p. 902; see the news story by Strauss, p. 827), who examined writing and speaking abilities in a left-handed female who had undergone surgical resection of the corpus callosum. Prior to surgery, she was able to write with her left hand and speak normally. After surgery, she could speak words shown to her right visual field (left visual cortex) but not write them with her left hand (right motor cortex), and she could write words shown to her left visual field but not speak them. This dissociation suggests a hemispheric lateralization of orthographic and phonographic aspects of language in sinistrals.

  12. Pre-TCR processes

    Lymphocytes reorganize their genomes to create heterodimeric antigen receptors. Rearrangement is an ordered process—the first locus must be successfully rearranged before rearrangement of the other locus begins—that is coordinated with other phenotypic changes. T cells control this developmental checkpoint with a “precursor” T cell antigen receptor, the pre-TCR, comprising a rearranged TCRβ chain and the pre-Tα, which will later be replaced by a rearranged α chain. Irving et al. (p. 905) examined if binding to an extracellular ligand was necessary for the pre-TCR to send the cell developmental signals. They used a truncated pre-TCR that expressed no extracellular immunoglobulin domains and showed that these domains were not needed for the pre-TCR to transduce a transient signal or for an immature T cell to pass the checkpoint. Stable expression of the truncated pre-TCR was sufficient to do both.

  13. Making our way

    Way-finding or navigation requires that we integrate the relations of parts of the environment to each other (allocentric) and the directions of body movements toward the target (egocentric). Maguire et al. (p. 921) provide evidence for a network of brain regions on the basis of the results of a functional imaging study in which humans traveled through a virtual reality town. Activation in the right hippocampus (coding for allocentric representation of space) and the right inferior parietal lobe (coding for egocentric positioning) brain regions correlated with the accuracy of navigation, while activation in the right caudate was correlated with speed of travel.

  14. Triple-bond chemistry in plants

    The biochemistry behind the carbon-carbon triple bond found naturally in some fatty acids has resisted insight. Now, searching for fatty acid-modifying enzymes first in the moss genome, and confirming their results in Arabidopsis, Lee et al. (p. 915) have found genes encoding an acetylenase and an epoxygenase. Although these two catalytic reactions have rather different effects on their fatty acid substrates, the DNA sequences encoding the enzymes show some intriguing similarities. Fatty acids and their vast array of derivatives find commercial utility as plasticizers, lubricants, varnishes, and many other products.

  15. Potassium in plants

    Potassium channels, better known for their functions in neurons, are also found in plants. Hirsch et al. (p. 918; see the cover) elucidated the actions of a plant Shaker-like K+ channel encoded by the AKT1 gene in Arabidopsis. The properties of the channel are such that, at negative membrane potentials and in the presence of low concentrations of external K+, it may allow passive uptake of K+ ions into plant root cells.

  16. Braking the cell cycle

    To ensure that chromosomal DNA is fully replicated before the cell divides, cells have a signaling mechanism known as a checkpoint that prevents cell division in the presence of unreplicated DNA. Lack of such control can contribute to cancer or other abnormal cell function. Boddy et al. outline new biochemical steps in the replication checkpoint pathway in fission yeast that leads to inactivation of Cdc2 (the protein kinase that serves as a master regulator of mitosis). They provide evidence that the protein kinase Cds1 phosphorylates another kinase, called Wee1, which in turn phosphorylates and inactivates Cdc2. The abundance of the protein kinase Mik1 was also increased in a Cds1-dependent manner in cells with unreplicated DNA. Mik1, in turn, can inhibit mitosis by direct phosphorylation of Cdc2. Cds1 and Mik1 thus appear to join Chk1 as essential kinases that contribute to cell cycle arrest in response to the replication checkpoint.

  17. Clammy temperatures

    The clam Calyptogena magnifica spends its life attached to the sea floor near hydrothermal vents along mid-ocean ridges. Hart and Blusztajn measured the variation of the strontium-calcium ratio in layers of the clam shells that developed as the clams grew along the East Pacific Rise. They then correlated high strontium-calcium ratios with high temperatures of the vent fluids and determined that two particular high-temperature events could be associated with observed vent eruptions in 1991 and 1992. Thus, it is possible that clams can be used as a temperature proxy for eruption episodes at vents over time periods of several decades, given that the chemical tracers can be measured with high spatial resolution, the age of the clams can be determined, and other factors, such as nutrient concentrations and water chemistry, do not change significantly.

  18. Strain developments

    A theoretical framework that can be used to understand the evolution of strains of infectious agents has been developed by Gupta et al. Depending on the strength of the immune response to the particular organism, discrete strains can evolve, there can be cyclic or chaotic fluctuations in the prevalent strain, or there can be no apparent strain structure. This type of modeling is of importance in evaluating the results of vaccine trials.

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