Editors' Choice

Science  19 Mar 2010:
Vol. 327, Issue 5972, pp. 1430
  1. Geology

    Tracking the Deluge

    1. Brooks Hanson

    In the channeled scablands of eastern Washington State, great ripples and scoured rock, visible by satellite, mark periodic massive flood outbursts from a dammed glacial lake. Their recognition early in the 20th century revealed the huge scale of flooding that can occur on Earth and has influenced understanding of even larger flood features on Mars. The floods moved across the rough but low topography in the east and then had to drain through the high Cascade volcanic arc along the Columbia River gorge. Denlinger and O'Connell simulated several of the floods to reveal how rapidly the waters moved and how they scoured the landscape. The model, which starts with the detailed topography of the region and likely positions of the ice sheets, suggests that the floods surged westward, filling the Yakima Basin east of the Cascades to several hundred meters within 38 hours. Drainage through the Columbia Gorge was slower, taking about 3 weeks. Drainage of a single large lake, rather than multiple lakes, would have been sufficient to have formed the scablands.

    Geol. Soc. Am. Bull. 122, 678 (2010).

  2. Chemistry

    Corralling Peptides

    1. Jake Yeston

    There's an inherent entropic challenge in preparing cyclic molecules from linear precursors, because the two ends must be coaxed together before they can form a bond. In zwitterionic peptides, the negative carboxylate and positive ammonium groups at either end attract one another electrostatically, and so, in principle, favor a conformation poised to close the loop. Unfortunately, traditional coupling agents used to facilitate formation of the amide bond eliminate the charged motifs, and with them the convenient conformational biasing. Hili et al. present an alternative coupling scheme that conserves the zwitterionic attraction. Instead of forming amide linkages directly, they interpose an aziridine-bound aldehyde and an isocyanide. In the ensuing reaction, the isocyanide bridges the negative carboxylate at one end of the peptide strand and a positive iminium (formed by amine attack on the aldehyde) at the other end. The reaction tolerates the full range of natural amino acids, with no observed disruption of chirality. Cycles comprising one to five amino acids, in addition to the aziridine coupling unit, were isolated in good yield after simple precipitation, and the aziridine's reactivity allows for further functionalization of the ring perimeter.

    J. Am. Chem. Soc. 132, 2889 (2010).

  3. Virology

    Weathering Influenza

    1. Caroline Ash

    One sneeze and influenza is drifting through the air, plastered across palms of hands and around door handles, poised for its next victim. How long can the virus survive outside a living host? The answer to this question depends on ambient environmental conditions. Shaman and Kohn showed experimentally that low absolute humidity (grams of water per cubic meter of air), which tends to prevail during temperate winters, improves the airborne survival of influenza viruses within aerosolized drops and favors transmission. Shaman et al. modeled how changes in absolute humidity have driven the seasonal peaks and troughs of influenza in the United States during a 30-year period. Epidemics were correlated with the onset of anomalously low absolute humidity, and variations in absolute humidity affected the occurrence of outbreaks during any one season. Thus, it may be just as feasible to forecast short-term influenza risk as it is the weather.

    Proc. Natl. Acad. Sci. U.S.A. 106, 3243 (2009); PLoS Biol. 8, e1000316 (2010).

  4. Evolution

    Polar Bear, Polar Bear

    1. Guy Riddihough

    Polar bears are adapted to living in one of the harshest environments on earth, their range being determined by the extent of Arctic polar sea ice. They arose from the brown bear lineage and are most closely related to a group of genetically distinct brown bears that inhabit the Admiralty, Baranof, and Chichagof Islands of Alaska's Alexander Archipelago (known as the ABC brown bears). Time estimates for the brown bear–polar bear divergence vary considerably. Stratigraphy and dating of a rare polar bear fossil jaw bone, found on the Svalbard Archipelago in Norway, suggest that it is 130,000 to 110,000 years old and reveal that polar bears were a distinct species at this time. Lindqvist et al. extracted DNA from this fossil and sequenced its mitochondrial genome (mtDNA). Comparison of the ancient polar bear mtDNA sequence with a number of mtDNA genomes from modern polar bears and brown bears revealed that the ancient polar bear lies almost directly at the branching point of polar bears and ABC brown bears, indicating that this bear was very close to the common ancestor of both. Time estimates for the divergence suggest a range of 177,000 to 131,000 years ago. Furthermore, stable isotope analysis of carbon and nitrogen in the fossil indicated that the ancient polar bear was a marine predator at the top of the Arctic marine food chain, like its modern counterpart (and unlike brown bears). Thus, polar bears appear to have adapted very rapidly to their Arctic sea ice environment, perhaps within 10,000 to 30,000 years after the split from the brown bear precursor.

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.0914266107 (2010).

  5. Infectious Disease

    Grounding Mosquitoes for Dengue Control

    1. Barbara R. Jasny
    CREDIT: FU ET AL., PROC. NATL. ACAD. SCI. U.S.A. 10.1073/PNAS.1000251107 (2010)

    Dengue fever affects 50 to 100 million people each year and can be accompanied by symptoms so severe that it has been called “breakbone fever.” It is caused by a virus carried by the mosquito Aedes aegypti. There is no vaccine or specific therapeutic drug for dengue fever, and control efforts are complicated by the fact that the female mosquitoes (which bite) are active during the entire day. An active area of investigation in fighting insect vectors has been the possibility of generating sterile or conditional lethal varieties. Now, Fu et al. have created flightless female A. aegypti by linking the promoter of a gene, Actin-4 (which is found in the indirect flight muscles of female A. aegypti), to a tetracycline-repressible transactivator construct. Because of differential alternative splicing, males are unaffected. The flightless females should be more susceptible to predators and unable to attract males with their wing oscillation sounds for mating. It should be possible to distribute engineered eggs, which might then control or eradicate infected mosquito populations by a release of transgenic males—whose female progeny would be flightless.

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.1000251107 (2010).

  6. Cell Biology

    Familial Ties in the Nucleus

    1. Helen Pickersgill

    Inherited mutations in tumor suppressor genes cause an increased risk of developing familial cancer syndromes. Many of these familial tumor suppressor genes are also frequently mutated in somatic cancers. The tumor suppressor gene NF2 is mutated in the familial cancer syndrome neurofibromatosis type 2, which causes multiple brain tumors such as schwannomas and meningiomas. NF2 encodes the protein Merlin, which appears to link cell adhesion receptors at the cell surface to the actin cytoskeleton and is thus poised to inhibit mitogenic signaling downstream of integrins and adhesins. Now, Li et al. have identified a very different function for Merlin, this time in the nucleus. Endogenous Merlin was observed in the nucleus of multiple cell types by virtue of its binding to an E3 ubiquitin ligase, CRL4DCAF1. The binding of CRL4DCAF1 to Merlin inhibited the ubiquitin ligase activity and suppressed cell proliferation. Tumor-derived mutations in NF2 prevented Merlin from inhibiting CRL4DCAF1 activity, and CRL4DCAF1 was required for the malignant properties of primary human tumor cells derived from NF2 patients, thus providing a possible drug target.

    Cell 140, 477 (2010).

  7. Materials Science

    Mass Transit

    1. Marc S. Lavine

    In a crystalline material, plastic (or permanent) deformation involves the movement of dislocations in a series of elementary glide steps. Dislocations are defects in crystalline ordering, and their motion in simple materials is well understood. However, some complex metal alloys can have hundreds of atoms in their unit cell, and it is not at all clear what intricate series of steps guides the plastic deformation that is known to occur in such systems. Heggen et al. used aberration-corrected transmission electron microscopy to track the rearrangement of atoms in the T phase of an Al-Mn-Pd alloy—a lattice with 156 atoms in its unit cell and structural subunits that appear as hexagonal tiling with alternating orientation. In deformed regions, a mix of stacking faults was observed along with neighboring regions of the orthorhombic R phase, which is closely related to the T phase but with parallel hexagonal tiling. Surrounding the dislocation core were defect regions, known as phason defects, which did not possess a strain field. During deformation, the phasons escorted the dislocation core and locally transformed the material, thus allowing the core to move.

    Nat. Mater. 9, 10.1038/nmat2713 (2010).

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