This Week in Science

Science  05 Nov 1999:
Vol. 286, Issue 5442, pp. 1049
  1. Stress Across the Generations

    Although it has been clear that parents can transmit personality traits to their children, it has not been easy to determine the extent to which such transfer occurs through genetic inheritance or through nongenomic, behavioral mechanisms. Francis et al. (p. 1155) have looked at generations of mice that show genetic differences in fearfulness. By cross-fostering offspring, they have shown that the environment can trigger differences in behavior and in stress-related gene expression that are passed on to the next generation.

  2. A Dip in Deep Water?

    Deep ocean circulation is driven in part by cold water from polar regions sinking and flowing underneath and mixing with the warmer ocean. This deep-ocean circulation has played a major role in large, abrupt climate changes such as deglaciations. The last major reorganization of the deep ocean's thermohaline circulation is believed to have occurred at the end of the Younger Dryas, approximately 11,500 years ago. Since then, climate and oceanic circulation apparently have been remarkably stable, but this stability may be an illusion. Broecker et al. (p. 1132; see the news story by Kerr) compared recent measurements of the chlorofluorocarbon tracer CFC-11 in the deep Southern Ocean with data on the distribution of carbon-14 and phosphate. They conclude that the rate at which Southern Ocean deep water has been forming during the past century is only about one-third of that during the preceding 700 years. This result would imply that the period of unusually cold weather in Europe that lasted from about 1400 to 1900, called the Little Ice Age, may have been a mild analog of the Younger Dryas, and only one of a continuing series of such episodes that have been occurring roughly every 1500 years for the past 130,000 years.

  3. Assessing Antiaromaticity

    The unusual reactivity and stability of benzene compared to alkenes can be attributed in simple models to resonance interactions between its three double bonds. The stabilization of cyclic unsaturated molecules, or aromatic molecules, in which 4n + 2 electrons (n = 0, 1, 2,…) form π bonds, contrasts with the destabilization of antiaromatic molecules, cyclic species with 4n π-electrons. The most notable example is cyclobutadiene, which is destabilized to such an extent that its high reactivity makes measuring antiaromaticity difficult. Deniz et al. (p. 1119) have now used photoacoustic calorimetry to determine the enthalpy of formation of cyclobutadiene produced in a photochemical reaction. They estimated its antiaromaticity relative to a hypothetical strainless, conjugated diene reference and a reference with isolated double bonds.

  4. Biogenic Glass Formers

    Diatoms are able to produce highly organized nanostructured silica cell walls at ambient temperatures and at high rates. The initial formation of a silica sphere is regulated by biomolecules, but their characterization has remained difficult. Kröger et al. (p. 1129; see the news story by Amato) show that these biomolecules—called silaffins—are polycationic peptides that contain posttranslational modifications crucial for their activity at the pH conditions relevant for biosilicification in diatoms. When silaffins were added to solutions of silicic acid, silica nanospheres were generated in a few seconds.

  5. Room-Temperature Hydrogen Storage in Nanotubes

    The use of hydrogen as a clean-burning fuel is limited in part by difficulties in storing it. Carbon nanotubes of various types have shown promise as a hydrogen-storage medium but have required either cryogenic storage conditions or high-temperature desorption conditions. Liu et al. (p. 1127) show that wider single-wall nanotubes (mean diameters of 1.85 nanometers) are promising materials for this application. Such nanotubes, after cleaning with acid and thermally annealing, can store 4.2% by weight hydrogen at room temperature after exposure to hydrogen at 100 atmospheres.

  6. Caught in the Trap

    Isotopes with extremely low abundance present a difficult analytical challenge, but their measurement can prove useful. For example, krypton-81 could be used for dating ice and groundwater or in neutrino detection if more sensitive detection methods could be developed. Chen et al. (p. 1139) analyzed krypton gas samples that were excited into a metastable electronic state and introduced into a magneto-optical trap. Isotope analysis was performed by measuring laser-induced fluorescence. Single atoms of the rare isotopes could be detected, and abundances were determined by counting.

  7. Diversity and Productivity

    Satisfactory answers to major ecological questions frequently demand large-scale experiments. Hector et al. (p. 1123; see the Perspective by Tilman) examined the key question of the relation between biodiversity and ecosystem productivity in an experimental study at eight different locations across Europe. Different levels of grassland plant species diversity were simulated by synthesizing communities of different numbers of species. After harvesting, a consistent pattern of decreasing productivity was seen with decreasing plant diversity.

  8. Not Just Sitting Around

    Before a fibroblast cell migrates, it must modulate its adhesive contacts to an underlying substrate. These points of contact, called focal adhesions, are comprised of cell surface integrins that bind externally to the substrate and internally to the cell's actin cytoskeleton. Smilenov et al. (p. 1172; see the cover and the Perspective by Horwitz and Parsons) labeled focal adhesions in living cells with a fluorescent probe and observed that these contact points are motile in stationary cells. When a cell was stimulated to move, focal adhesions became stationary except at the rear of the cell. The authors propose that a clutch-like mechanism may regulate both the strength of integrin interaction with the substrate and the tension applied by associated actin filaments. A cell could switch from a nonmotile to motile state by stabilizing substrate contacts and increasing its traction. Otherwise, when sitting still, focal adhesions may be in an active “idling” state, ready to engage when a cell wants to move.

  9. Linking Breast Cancer and DNA Repair

    Failure of cell cycle checkpoint mechanisms that prevent cell division in the presence of damaged DNA can lead to an accumulation of genetic errors and can contribute to formation of cancerous cells. Cortez et al. (p. 1162; see the Perspective by Venkitaraman) report that two proteins that are associated with increased risk of breast cancer in humans in their mutated form actually interact with one another in cells and function in the response to DNA damage. They provide evidence that Brca1 (for breast cancer gene 1) is a substrate for ATM (for mutated in ataxia telangiectasia), a protein kinase that is activated in cells with damaged DNA. Phosphorylation of Brca1 by ATM appears to be required for proper function of Brca1 in protecting cells from DNA damage.

  10. Searching for Activity

    Complete genomes for some organisms, such as yeast, are now available, but methods are needed to screen the possible activities of the proteins that are encoded. Martzen et al. (p. 1153) have combined biochemical assays with genomic arrays to create a rapid way to identify the functions of expressed proteins. Open reading frames representing more than 6000 genes were fused to a tag that facilitated affinity purification of gene products, which were then assayed to identify previously unknown proteins involved in the transfer RNA splicing pathway. This technique may shorten the process of functional identification from months or years to days.

  11. Chimp History

    Comparative DNA sequence analysis continues to unravel the story of the evolutionary relationships of humans and the great apes. Kaessmann et al. (p. 1159) have generated a large database of DNA sequences from the X chromosomes of chimpanzees in order to compare the variation found in humans to that in chimps. Chimps have an evolutionary history that is more extensive and diverse than our own, which implies a greater age for the common ancestor of chimpanzee sequences compared to that for humans. Their data also show that chimps do not divide cleanly into several subspecies but that the subspecies are highly intermixed. The bonobo appears to have diverged from other chimp lineages less than 1 million years ago, much more recently than indicated by other measures.

  12. Chunk-Pixelated Vision

    Anyone who has purchased a personal computer will be familiar with the exponential relation between monitor size and cost. The cost arises not only from the actual display size but also from the associated hardware and processing speed needed to support the vastly increased amount of data in a larger number of pixels. Buschbeck et al. (p. 1178) describe a cost-effective solution implemented by a parasitic insect, one whose entire life cycle is spent within the body of the host wasp (in the case of the female) or the wasp nest (male). The eyes of these insects are organized to grab visual data in a small number of chunks, about 50 per eye, with a separate retina subserving each chunk of the visual field. This resolution may be sufficient for the insect's somewhat limited visual requirements, or it may help to concentrate limited light in a fashion reminiscent of trilobite eyes.

  13. Ice Sheets and Climate Change

    The Milankovitch theory states that long-term variations in climate ultimately depend on the seasonal and geographic distribution of solar insolation, which is determined by variations of Earth's orbit around the sun. Although orbital forcing explains many features of terrestrial climate, it fails to account for a number of puzzling aspects, such as the transition from 41,000- to 100,000-year-long glacial cycles that occurred approximately 1.2 million years ago, or why continental ice sheets take so long to grow during glaciations but disappear so quickly during deglaciations. Clark et al. (p. 1104) identify and address some of the ways that ice sheets may have modulated these processes and how the type of terrain beneath ice sheets may have affected their physical and dynamic properties. They conclude that whether a glacier rests on hard beds (crystalline bedrock) or soft beds (unconsolidated sediment) is a critical control on the flow and thus the thickness of the ice sheet. Some key exceptions to the Milankovitch theory can be explained by invoking the effects of ice sheet beds on the dynamics of the Northern Hemispheric continental ice sheets and their subsequent influence on climate.

  14. New Madrid Slip Rates

    Several large-magnitude earthquakes occurred in 1811 to 1812 in the New Madrid seismic zone, a poorly defined strike-slip fault in an otherwise stable mid-continent region located near a bend in the Mississippi River where Missouri, Kentucky, and Tennessee meet. Mueller et al. (p. 1135) have estimated the slip rates for this fault for the last 2300 years by combining structural models with measurements of kinked sedimentary structures in trenches along the Lake County Uplift compressive section of the fault. Their estimated slip rate, although within the error bounds of the recently estimated slip rate from geodetic data, suggests that the slip rate may be higher and that the seismic hazard may be greater than previously thought.

  15. Time to Grow

    As it grows, the worm Caenorhabditis elegans repeatedly molts to accommodate its enlarging body. Jeon et al. (p. 1141) found that the 6-hour period of these molting cycles is disturbed by inactivation of the gene lin-42, which halts development prematurely at the third molt. The RNA for this gene is normally also expressed in 6-hour rhythmic cycles, and its sequence bears an unexpected similarity to the Period family of proteins that are key components of the 24-hour circadian clocks of insects and mammals. Thus, lin-42 may be part of a timing mechanism that is set for 6 rather than 24 hours and that controls periodic events during development.

  16. Wobbly Editing

    Several transfer RNAs contain inosine at the wobble position of their anticodon, which expands the codon recognition capacity during protein synthesis. This inosine is generated by modification, or “editing,” of a genomically encoded adenosine. Gerber and Keller (p. 1146) have isolated a heterodimeric enzyme in yeast that catalyzes this reaction and show that it is essential for cell viability. In terms of evolution, this enzyme appears to be positioned between the cytidine/cytosine deaminases that function in intermediary metabolism and the mammalian RNA-specific adenosine deaminases that edit messenger RNA.

  17. Independent Decisions

    Not all T cells are alike. One of the first developmental commitments a T cell must make is whether it will ultimately express CD4 or CD8, proteins that mark mature T cells primarily as “helpers” or “killers,” respectively. Because this lineage decision is made at about the time when immature cells are selected to either live (positive selection) or die (negative selection), based on the specificity of their antigen receptors, these two decisions could be inextricably linked. Keefe et al. (p. 1149) used a mutant mouse, HD, that does not have CD4 cells to investigate this question. The T cells that normally would be CD4 instead entered the CD8 lineage, yet their positive selection was normal. Thus, these two processes are not linked.

  18. Checkpoint Pathways

    The checkpoint mechanism by which budding yeast Saccharomyces cerevisiae achieve mitotic arrest in response to DNA damage has been studied by Sanchez et al. (p. 1166) In mammalian cells and the fission yeast Saccharomyces pombe, DNA damage causes arrest through inhibitory tyrosine phosphorylation of cyclin-dependent kinase complexes, but such phosphorylation is not required in S. cerevisiae. Nevertheless, the authors show a role for a homolog of the Chk1 protein kinase that functions in checkpoint signaling in S. pombe and mammalian cells. Chk1 acts independently of the Rad53 protein kinase that also participates in the response to DNA damage: Whereas Chk1 apparently phoshorylates the anaphase inhibitor protein Pds1 and thus prevents its degradation, Rad53 appears to prevent exit from mitosis by inhibiting the activity of yet another kinase, Cdc5. The evidence indicates that Chk1 and Rad53 function in branches of the checkpoint pathway that primarily inhibit entry into anaphase and exit from mitosis, respectively, but that they can also support the maintenance of arrest through the alternate branch of the pathway as well.

  19. Examining Extinction

    What factors predispose populations to extinction? Analysis of a series of long-running microcosm experiments with the brine shrimp by Belovsky et al. (p. 1175) have produced some new answers. Although their experiments confirm the more predictable factors that lead to extinction, such as low population size and high environmental variation, they reveal a more complex picture. In particular, the key factor appears to be the variability of the population size over time, which can be driven by environmental variability or nonlinear dynamics. Conservation biology now has a more robust framework within which to plan the protection of threatened species.

  20. Presynaptic Inhibition

    Metabotropic glutamate receptors (mGluRs) constitute an important class of receptors and are widespread in the nervous system. However, their mechanism of action is still poorly understood. O'Connor et al. (p. 1180) describe a series of experiments designed to elucidate the molecular mechanism of how group III mGluRs cause presynaptic inhibition of hippocampal excitatory transmission. They show that the intracellular tail of mGluR7 binds calmodulin in a Ca2+-dependent manner. The intracellular part of the protein can also bind G-protein bγ subunits, but only in the absence of calmodulin, which suggests that binding of calmodulin and bγ subunit is mutually exclusive. The authors present a model in which Ca2+-influx during depolarization of the presynaptic terminal not only triggers neurotransmitter release but also activates calmodulin, which in turn releases bγ subunits from mGluRs, and subsequently induces presynaptic inhibition.

  21. Protonated Methane

    White et al. (Reports, 2 Apr., p. 135) reported the infrared spectrum of CH5+, and Marx and Parrinello (Perspectives, 2 Apr., p. 59) reviewed recent theoretical work which suggests that the bonds in this species are highly fluxional. Kramer comments that previous mass spectrometry studies suggest that, in the absence of intermolecular collisions, CH4D+ and CD4H+ are not rearranging and can be described as a van der Waals complex of a methyl cation and a hydrogen molecule. In response, Oka and White point out some difficulties in the interpretation of the mass spectrometry cited and note that a more static structure would still have a complex infrared spectrum. Marx and Parrinello have performed additional calculations on CH4D+ that show scrambling of the D atom, and note that complexation may “freeze” the scrambling of the H atoms. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/286/5442/1051a

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