This Week in Science

Science  25 Jul 2008:
Vol. 321, Issue 5888, pp. 461
  1. Coral Reefs Under Threat


    The conservation status of coral reefs can be monitored by assessing the area covered by coral species over time. Carpenter et al. (p. 560, published online 10 July) have estimated that more than a third of the major reef-building coral species are at risk of dying out to the point at which reef viability is lost. The causes of this dismaying decline stem from local insults from physical damage, overfishing, pollution, and sedimentation. These factors, added to the physiological harm done to coral organisms and their symbionts by elevated sea surface temperature rise and water acidification induced by atmospheric greenhouse gas accumulation, can mean that a reef loses viability and quickly turns into a mound of rubble.

  2. Aberration-Corrected Electron Microscopy

    Transmission electron microscopes have been used for a long time to study the structures of materials, but aberrations introduced by the electron optics of the instruments have limited their spatial resolution. Urban (p. 506) reviews recent advances in developing instruments that are largely free of aberrations. These aberration-corrected electron microscopes allow atomic positions to be determined with unprecedented accuracy. Furthermore, they enable determination of the occupancies of atom sites and atomic-scale imaging of chemical composition and bonding. The instruments have been used to study, for example, twin-boundary structures, sublattice structures in multilayered materials, and the atom arrangements in catalyst particles.

  3. Small RNAs and the Argonautes

    Small RNAs play a critical role in regulating a wide range of cellular processes in eukaryotes, the most thoroughly characterized of which occur in the cytoplasm. Small RNAs also function in a number of processes in the cell nucleus, including heterochromatin formation and genome rearrangement. Guang et al. (p. 537; see the Perspective by Meister) describe a genetic screen in the nematode Caenorhabditis elegans for factors required for nuclear RNA interference. The findings suggest that an Argonaute-like protein, NRDE-3, which contains a nuclear localization signal, is involved in many RNA-based nuclear silencing processes. Furthermore, the nuclear localization signal and the binding of small interfering RNAs generated by the action of RNA-dependent RNA polymerases on messenger RNAs in the cytoplasm are required for NRDE-3 to relocate from the cytoplasm to the nucleus.

  4. A Picture Speaks a Thousand Words


    There is a lot of information contained in images, and this complexity can be exploited, for example, in security-key generation and data encryption. Using the quantum properties of light in images, and the capability to entangle those images, would multiply that information capacity many-fold (see the Perspective by Boyd). Boyer et al. (p. 544; published online 12 June) created entangled twin images by passing the light through a cloud of warm rubidium atoms. In the likes of optical tweezers or large-array interferometers, precision measurements are based on monitoring the position of laser beams. Entanglement can also be used to improve measurement. In pursuit of this goal, Wagner et al. (p. 541) were able to entangle two lasers, which could now potentially be applied to enhancing spatial measurements.

  5. Phase Transitions in Bismuth

    Recent work on graphene, a single-layered sheet of carbon atoms in which the electrons move at constant velocity that can be described in terms of relativistic behavior, has triggered interest in other materials that exhibit “Dirac fermion” behavior. Bismuth is of particular interest because its band structure is 3-dimensional and its electrons are also Dirac fermions. As such, exotic electronic phase transition might be expected. By applying high magnetic field at low temperature, Li et al. (p. 547; see the Perspective by Behnia) take bismuth to the quantum limit and observe a novel collective electronic phase in which the pockets of electrons that make up the energy bands align with ferromagnetic ordering.

  6. Birth of the Cool

    Biodiversity during the Ordovician Period (between 490 and 440 million years ago) increased tremendously, in perhaps the greatest evolutionary radiation in Earth's history. Why did this explosion of variety occur? Trotter et al. (p. 550) present a record of sea surface temperatures for the period that shows that the radiation took place only when temperatures had decreased from values as high as 10°C higher to ones similar to modern times. This cooling, which occurred at a fairly uniform pace over the first 25 million years of the Ordovician, was estimated by analyzing the oxygen isotope composition of the calcium phosphate mineral apatite. The data suggest significantly cooler conditions than those derived from older records based on carbonates, bringing estimates more into line with what is understood about the fossil record.

  7. Increasing Thermopower Through Scattering

    Thermoelectric generators and coolers are formed from junctions of semiconductor materials that are good electrical conductors but poor thermal conductors. Efforts to improve these materials have focused mainly on decreasing their thermal conductivity by increasing phonon scattering, but this approach reaches an ultimate limit, that of the amorphous material. However, the figure of merit that describes these materials, ZT, also depends on the thermopower, or Seebeck coefficient S, and theory suggests that S can be increased by an appropriate increase in the density of states of the material. Heremans et al. (p. 554) report that thallium doping of lead telluride, a commonly used thermoelectric, can improve its ZT by about 50% at temperatures of ∼750 kelvin.

  8. Brassinosteroid Signaling

    Brassinosteroids, a type of steroid hormone in plants, regulate a variety of developmental processes. Some genetic targets and signaling components targeted by the brassinosteroids have been identified, but the full chain of command remains unclear. By focusing their attention on proteins in the plasma membrane or proteins modified by phosphorylation, Tang et al. (p. 557) were able to identify two previously unknown kinases (BRK1 and BRK2) that interact with the brassinosteroid receptor at the plasma membrane. BRI1 kinase, the brassinosteroid receptor, phosphorylates these two substrate kinases to activate downstream brassinosteroid signaling.

  9. Crystal Clear Hydrogenation


    The ability to catalyze dissociation of hydrogen efficiently is of great interest in alternative energy technology. One approach is to develop catalysts that mimic the hydrogenase enzymes that catalyze H2-H+ interconversion reactions important in the energy metabolism of many microorganisms. So far, these efforts have been guided by the structures of two of the three classes of hydrogenases: [NiFe]-hydrogenase and [FeFe]-hydrogenase. Now Shima et al. (p. 572; see the Perspective by Armstrong and Fontecilla-Camps) describe the structure of the third class, Fe-hydrogenase, in which a mononuclear iron is part of an iron guanylyl pyridine cofactor without an Fe-S cluster. There is surprising similarity between the active sites of the three enzymes. The convergent features are likely to play an important role in hydrogenation and may provide a clearer framework for the design of new catalysts.

  10. Dissecting a Garbage Disposal Mechanism

    When secretory and membrane proteins misfold in the endoplasmic reticulum (ER), an important mechanism for their disposal, ER-associated degradation (ERAD), is put in train. Misfolded proteins are retrotranslocated into the cytosol, where they are degraded by the ubiquitin-proteasome system. The oxidative environment in the ER generates disulfide bond-mediated high-molecular-weight complexes from misfolded proteins that must be reduced prior to their retrotranslocation during ERAD. Ushioda et al. (p. 569; see the Perspective by Braakman and Otsu) have now identified ERdj5 as a disulfide reductase in the ER. ERdj5 accelerates ERAD by cleaving intermolecular disulfide bonds of misfolded proteins. ERdj5 associates with EDEM, a lectin-like ERAD protein that recognizes misfolded proteins to be degraded. ERdj5 also associates with BiP, an ER molecular chaperone.

  11. Manipulating Mitochondria

    Mitochondria, the metabolic powerhouse of the cell, are relic bacterial symbionts that contain their own vestigial genomic DNA, mitochondrial DNA (mtDNA). Mutations in mtDNA-encoded genes can result in serious cellular dysfunction and disease. MtDNA is present in many copies and inherited in a non-Mendelian manner, making study of defects in mitochondrial metabolism difficult. To address this problem, Xu et al. (p. 575) have developed a system for importing restriction enzymes into the mitochondria of Drosophila germ cells and selecting for embryos containing within their mitochondria mutations in cognate restriction enzyme target sites. Mutations in the cytochrome C oxidase gene, for example, show a range of phenotypes, including male sterility, age-dependent neurodegeneration, and myopathy, similar to those caused by mutations in human mtDNA.

  12. Pairing and Repairing

    Newly replicated eukaryotic chromosomes must be temporarily paired with their homolog so that they can be correctly partitioned during cell division, one to each daughter cell. They may also be paired during repair of DNA double-strand breaks. The sister chromatids are held together by the cohesin complex, which includes the Eco1 acetyltransferase, an essential component. Ben-Shahar et al. (p. 563) and Ünal et al. (p. 566) demonstrate that the target of the Eco1 acetyltransferase is cohesin itself, and specifically the Smc3 subunit. Eco1 acetylation of Smc3 in its adenosine triphosphatase domain is required for the establishment of cohesion between sister chromatids. This acetylation counters the cohesin dissociation promoted by another protein, Wapl, perhaps by preventing Wap1 from destabilizing the cohesin ring.

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