This Week in Science

Science  14 Mar 2008:
Vol. 319, Issue 5869, pp. 1453
  1. Maintaining Duplicate Nuclei


    The intestinal parasite Giardia has two nuclei in its vegetatively growing form (the trophozoite), and both nuclei seem to be fully functional and essentially indistinguishable. Remarkably, despite no previous evidence for genetic exchange between the two nuclei, they somehow appear to contain essentially identical copies of the genome. Poxleitner et al. (p. 1530) show that efficient DNA transfer occurs during encystations and provides a mechanism for maintaining homozygosity between the two nuclei.

  2. Migration Dates to the New World

    The Americas were the last major habitable continents to be settled by humans. Goebel et al. (p. 1497) review recent archaeological finds, more accurate dates, and genetic evidence that have changed some ideas about how this migration occurred. The data favor an initial migration along a coastal ice-free corridor about 15,000 years ago from populations that were in Beringia, including Alaska, and perhaps a second migration around 13,000 years ago that led to the Clovis culture in North America.

  3. Protoplanetary Parts Lists

    Our understanding of the initial conditions of our early solar system is based mainly on samples of material that has condensed and accumulated into grains, comets, asteroids, and planets. Carr and Najita (p. 1504; see the Perspective by Ciesla) present spectral observations obtained with Spitzer Space Telescope of the gas in the protoplanetary disk around the star AA Tauri, which is thought to be similar to our early Sun. Even the inner portion of this disk of contains high concentrations of HCN, CO2, C2H2, OH, and H2O, which suggests that organic chemical reactions are ongoing. These results raise the possibility that even more complex molecules are synthesized in these regions, and may also help in understanding the origin of Earth's water budget.

  4. Hydrogen Surrogates Under Pressure

    The issue of hydrogen metallization at extreme pressures is not only of fundamental interest but also bears on our understanding of the deep interior of gas giant planets. This issue is difficult to address experimentally in that pressure cells, which allow for detailed observations, cannot yet induce elemental hydrogen into its metallic phase. Eremets et al. (p. 1506) turned to compounds containing hydrogenin this case, silaneswith the motivation that because the hydrogen is chemically precompressed, it should be easier to induce phase changes at lower pressure. Silane can indeed be pressured into a metallic phase and is also superconducting at lower temperature. The results suggest that hydrogen-rich compounds can be a good surrogate for studying elemental hydrogen.

  5. Look Beyond BCS Theory?


    The Bardeen-Cooper-Schrieffer (BCS) theory of low-temperature superconductivity in metals stands as a monument in solid-state physics, and its cornerstone is a pairing interaction between electrons mediated by lattice vibrations, or phonons. The availability of high-resolution neutron spectroscopy now allows measurement of phonon lifetimes from micro-electron volt resolution studies throughout the Brillouin zone. Aynajian et al. (p. 1509, published online 21 February; see the Perspective by Scalapino) studied the elemental superconductors lead and niobium with such methods and found results which are suggestive of many-body correlated effects that require explanations beyond the standard BCS framework.

  6. Blocking Out Cyclic Polymers

    Synthesis of cyclic polymers requires some way of bringing the ends of a linear polymer together before competing reactions occur. For this reason, such reactions are usually performed under high dilution, and for long polymers, this requirement often leads to low yields. Schappacher and Deffieux (p. 1512) overcome a number of these limitations by synthesizing a triblock copolymer with a long middle block and two smaller end blocks with reactive groups needed for the cyclization reaction. The central block could also be decorated with side chains, which aided in visualization with atomic force microscopy. By using two monomers to decorate the cyclic polymers and by choosing a selective solvent for one of them, it was possible to form supramolecular tubular assemblies in solution.

  7. Constraining Earth's Chlorine Budget

    Most of Earth's chlorine is now in the crust (in evaporiates and brines) and the oceans. Determining how and when it became concentrated there requires some assessment of the mantle chlorine concentration and isotopic value. Such measurements are often difficultfor example, volcanic rocks from the mantle, such as mid-ocean ridge basalts (MORBs), are rapidly contaminated by seawater. Bonifacie et al. (p. 1518) examined Cl stable isotopes in 22 MORB samples and found, in contrast to previous studies, that the mantle chlorine has relatively less 37Cl than 36Cl. This difference implies either that fractionation accompanied separation from the mantle, and later mixing in of altered ocean crust, or that the crust of the Earth contains isotopically different Cl from a late accretion event.

  8. Yeast Prion Protein Structure Revealed

    Prion proteins are linked to several diseases, including bovine spongiform encephalopathy in cattle, scrapie in sheep, and Creutzfeldt-Jakob disease in humans. Infectious prion-like proteins that also form amyloid fibrils are found in yeast and other fungi. Based on solid-state nuclear magnetic resonance data, Wasmer et al. (p. 1523) provide a structural model of amyloid fibrils from the prion-forming domain of the fungal HET-s protein. An earlier preliminary model gave no information on intermolecular -sheet propagation. The current model shows that the amyloid fibrils form a left-handed -solenoid with two windings per molecule that is stabilized by hydrophobic and polar interactions and salt bridges.

  9. Bacterial Conjugation in Living Color


    Although conjugation in bacteria has been studied for decades, there are many outstanding issues about how it works. By using a direct visualization technique, Babi et al. (p. 1553) show that DNA is transferred through the pilus and efficiently incorporated (>95) into the recipient's bacterial chromosome; unrecombined DNA is degraded. Tracking of the acquired DNA showed that the resulting chromosomes appeared to recombine frequently, about once per cell generation, and was able to generate a diverse set of progeny from a single ancestor.

  10. Stress Relief in Alcoholism

    Despite the success of psychosocial interventions such as Alcoholics Anonymous, chronic alcohol abuse remains a significant public health problem. Noting that stress is a relapse trigger in alcoholism, George et al. (p. 1536, published online 14 February) explored whether pharmacological inhibition of the neurokinin 1 receptor (NK1R), a mediator of the brain's stress response, would alleviate symptoms associated with alcohol dependence. Mice genetically deficient in NK1R consumed less alcohol than controls. A drug that antagonizes NK1R was used in a small, controlled study of recently detoxified alcoholic inpatients and showed promising activity, such as reducing alcohol cravings. However, these latter results need to be confirmed in much larger, longer-term studies.

  11. Synthetic Biology in Action

    One goal of synthetic biology is to remodel intracellular processes at will. Bashor et al. (p. 1539; see the Perspective by Pryciak) wanted to engineer new regulatory properties into the well-studied mitogen-activated protein kinase pathway in yeast that mediates response to mating pheromones. By creating proteins with customized protein interaction domains, which altered the association of signaling proteins with a key scaffold protein that assembles proteins that participate in the signaling pathway, the authors changed complex regulatory properties of the signaling pathway. A gradual dose response could be modified into a switchlike all-or-none response, the time course of system response could be varied, or adaptation (a process in which output diminishes even in the presence of continued stimulus) could be added.

  12. A Working Model of Working Memory

    Working memory, the system that enables us to hold information temporarily for processing purposes, is thought to be stored primarily in the form of elevated, persistent neuronal firing rates. Mongillo et al. (p. 1543; see the Perspective by Fusi) now suggest that working memory may instead be stored in the short-term facilitation caused by a temporary calcium increase in synapses from neurons that code for a particular memory. Short-term facilitation on the necessary time scale has recently been observed in prefrontal cortex, lending itself to memory maintenance on the scale of seconds. During the memory span, and also during recall, neuronal activity then shows population spikes. If memory is indeed stored in the form of synaptic facilitation, few spikes need to be expended in maintaining it, which would reduce the metabolic cost of working memory.

  13. Calcium Mantle Maps

    Seismic discontinuities in Earth's mantle mark changes in density typically produced by reactions among major minerals induced by the increasing pressure with depth. One peculiar discontinuity lies at about a depth of 520 kilometers, but the depth varies geographically, and in some regions, two discontinuities are seen. Saikia et al. (p. 1515) show through high-pressure, high-temperature experiments that this discontinuity is reflecting the exsolution of calcium from garnet with increasing pressure to form a new phase, Ca perovskite. This reaction proceeds only in calcium-rich mantle, so the geographic and depth variations primarily reflect the average calcium content of the mantle. Combining the mineral physics and seismic data provides a means to map the composition of the mantle in the area of this transition, a depth at which many subducting slabs may be ponding.

  14. The Rise and Fall of Coral Reefs

    Despite wide variation in the species diversity of tropical coral reefs in different parts of the world, rates of coral growth and development are broadly similar. To test whether this disconnection has a long-term basis, Johnson et al. (p. 1521) present an analysis of coral diversity and reef building during the past 30 million years in the Caribbean. The chief finding, made possible by more than two decades of fossil data collection, is that reef-coral diversity and the extent of reef development have independent responses to long-term environmental change. The data include two episodes of accelerated extinction and reorganization of reef ecosystems, which may help to explain the relation between current anthropogenic environmental change and the observed worldwide demise of coral reef ecosystems.

  15. Shaping Tomatoes

    Domestication has generated a huge variation in the size and shape of fruit compared to wild progenitors. In the tomato, several varieties have large, elongated fruits relative to its small round ancestors. Xiao et al. (p. 1527; see the cover) now identify the molecular basis of fruit elongationan increased expression of a transposed copy of the SUN gene. This ability of a transposable element to move a host gene to another location, which leads to differential gene regulation and generates a novel phenotype, may represent a more general driving force in phenotypic diversity.