This Week in Science

Science  10 Aug 2007:
Vol. 317, Issue 5839, pp. 717
  1. High-Salt Survival Tactics


    The Na+/H+antiporter NhaA is an inner-membrane protein in Esherichia coli that is required for survival in high salt or under alkaline stress. NhaA uses energy from proton transport down an electrochemical gradient into the cell to excrete Na+, but not K+, from the cytoplasm. Starting from a recently determined structure of NhaA, Arkin et al. (p. 799) performed molecular dynamics simulations to examine the mechanism of ion transport, pH regulation, and cation selectivity. The simulations, together with existing experimental data and mutagenesis experiments, indicate that three aspartates are essential to NhaA function: Asp164 is the Na+ binding site, Asp163 controls alternating cytoplasmic or periplasmic access to the binding site, and Asp133 is involved in pH regulation.

  2. A Layered Response

    Actuators convert energy into mechanical force, and Masmadinis et al. (p. 780; see the Perspective by Blencowe) present a method for the actuation of nanomechanical resonators based on piezoelectric semiconductors, in this case, cantilevers of epitaxially grown GaAs that form a pin diode (an intrinsic layer, which is charge-depleted and an active piezoelectric material, sandwiched between p-type and n-type layers). The strength of the mechanical resonance induced by an applied ac voltage can be controlled by a dc bias, which alters the charge-depleted layer. The actuation can be tailored by altering the device's band structure, geometry, and crystallographic direction.

  3. Heating a Monolayer


    Heat is carried through crystalline solids via very low frequency acoustic vibrations. In contrast, the rates and mechanisms whereby isolated molecules transport heat are more obscure, but are potentially important for applications such as molecular electronics. Wang et al. (p. 787; see the Perspective by Nitzan) have used coherent vibrational spectroscopy to quantify the rate of heat conduction from a flash-heated gold substrate to the top of an alkanethiol monolayer assembled on its surface. The technique is sensitive to the disordering of terminal methyl groups that spreads across the ensemble in picoseconds as heat flows through the hydrocarbon chains. A theoretical analysis is consistent with ballistic heat transport at an effective rate of ~1 kilometer per second.

  4. Straight to Amides

    The prevalence of amides in biomolecules and commercial polymers places high value on a flexible and efficient synthetic route to molecules bearing this C(=O)N motif. In general, oxidative coupling of available alcohols and amines to amides requires the use of wasteful quantities of stoichiometric activators or corrosively strong acids and bases. Gunanathan et al. (p. 790) show that a ruthenium complex catalyzes the direct coupling of a wide range of primary alcohols and amines to the corresponding amides, with the endothermic reaction driven by liberation of H2 as the sole by-product. High yields were obtained from boiling toluene solutions in less than 10 hours.

  5. Ice Through 11 Glacial Cycles

    The ice core drilled at Dome C, Antarctica, provides the longest continuous record of climate and atmospheric composition of any polar ice site, encompassing 11 glacial cycles. Jouzel et al. (p. 793, published online 5 July) present the deuterium isotopic profile for the entire 3260-meter length of the core, which allows the construction of a climate record that extends back to 800,000 years before the present. The high resolution of this record allows the relation between shorter, millennial-scale climate events in the northern and southern high latitudes to be examined. The authors also used an atmospheric General Circulation Model to calculate an improved temperature record for the entire interval, finding temperatures during warm intervals as much as 4.5°C warmer, and during cold intervals as much as 10°C lower, than preanthropogenic Holocene values.

  6. Looking Within

    Global climate models have been criticized as being overly simple representations of an extremely complex system; for example, they include only natural and anthropogenic external forcing terms (like those for solar radiation, atmospheric aerosols, and the concentrations of greenhouse gases) and neglect to incorporate the effects of nonanthropogenic internal climate variability that could have a significant effect on predictions. Smith et al. (p. 796; see the news story by Kerr) report model hindcast results that include the effects of that internal variability. They found that surface temperature can be predicted with substantially more skill over a decade, both globally and regionally. Their forecast of global annual mean surface temperature for the decade beginning in 2005 indicates a reduced rate of warming for the next few years, followed by continued and more rapid warming, with at least half of the years after 2009 predicted to be warmer than the warmest year currently on record.

  7. Activity of the Depressed Brain

    To try to understand how brain circuits malfunction during depression, Airan et al. (p. 819, published online 5 July; see the Perspective by Insel) tracked neural activity within the hippocampus with voltage-sensitive dyes in brain slices from rats experiencing depression-like states. Certain aspects of neural activity predicted the degree of “depression” exhibited by the animal, including behavioral improvements after administration of antidepressant drugs. This indication that hippocampal neural activity reflects the behavioral state of these animals provides a starting point for further understanding of the malfunctioning circuits in depression and suggests an approach for the study of other mental illnesses.

  8. Wnt, the Fountain of Youth?


    Wnt proteins are secreted ligands that bind to cell surface receptors and have important effects during development, which it now seems may also contribute to phenotypes of stem and progenitor cells associated with aging. Liu et al. (p. 803) examined stem cell properties in mice carrying a mutation in the Klotho protein, which show characteristics of accelerated aging. They observed increased senescence of stem cells in the mutant mice and found that the Klotho protein physically interacted with and inhibited Wnt proteins in transfected cells. Exposure of mouse embryo fibroblasts in culture to excessive Wnt signaling enhanced senescence, and in transgenic mice also promoted senescence of skin cells. Brack et al. (p. 807) found that Wnt signaling appeared to be more active in aging animals. Injection of Wnt3A into young regenerating muscle reduced proliferation and increased deposition of connective tissue. Thus, antagonizing Wnt signals could provide a strategy to ease the effects of aging and age-related diseases.

  9. Birds of a Feather

    The European Union's Birds Directive was a pioneering international policy instrument that was enacted in 1979 with the aim of providing a framework for the conservation of bird species considered to be rare or vulnerable. Donald et al. (p. 810) evaluate the effectiveness of the directive in terms of the population response of all the species at which it is targeted. As was hoped, positive population changes were observed between 1990 and 2000 for listed species compared to nonlisted species. Such international initiatives are indeed capable of delivering measurable conservation benefits.

  10. Regulatory Change and Phenotypes

    Phenotypic changes between related species are thought to arise from changes in gene composition and alterations in their mode of regulation. Borneman et al. (p. 815; see the Perspective by Kruglyak and Stern) have studied the divergence in transcription networks across four yeast species by directly determining the binding site distribution of orthologs of the transcription factors Ste12 and Tec1. Gene regulatory binding sites were considerably more variable than the genes themselves, suggesting that such changes may drive phenotypic variation.

  11. Flu Variations

    A major question in the transmission of the H5N1 strain of influenza between humans is how mutations might influence the ability of the virus to enter human cells and how this might affect detection by our immune systems. Yang et al. (p. 825) reveal that specific mutations in the influenza hemag-glutinin gene can alter host receptor binding and the recognition of the virus by neutralizing antibodies. However, new neutralizing antibodies specific to the mutants could be elicited by immunization, which will be important in the design of vaccines to combat the virus.

  12. What You See and What You Don't

    What are the limits to the visual information that can be consciously accessed by a human being at any one time? Huang et al. (p. 823) analyzed single momentary acts of conscious perception specifically with respect to location and color of briefly presented visual stimuli. They found that we can be aware of more than one location, but not more than one color, per visual scene. The results are interpreted within the framework of “labeled Boolean maps,” which postulates that people can attend to multiple locations simultaneously, but only a single feature.

  13. Singling Out Molecular Binding

    Detection of very low concentrations of molecules usually requires molecules to be naturally fluorescent or to be labeled with a fluorescent marker. Armani et al. (p. 783, published online 5 July) now show that the resonance shifts of a type of optical sensor, a whisper gallery microcavity sensor, can detect the binding of a single molecule when its silica surface has been functionalized with binding partners such as antibodies. The resonators have very high quality factors Q (> 108), and molecular binding shifts the resonance frequency quadratically in Q because the high circulating intensity of light leads to local heating and favorable thermo-optical effects. Single-molecule detection was verified by detecting dye molecules that could also be quantified by fluorescence. Interleukin-2 could be detected in the hundreds of attomolar range in calf serum.

  14. Positively Clonal Competitors

    Evolution by natural selection is driven by the continuous generation of adaptive mutations. There are currently several direct and indirect estimates of the deleterious mutation rate in different organisms, but data are lacking for beneficial mutations. Perfeito et al. (p. 813) show that the adaptive mutation rate for the bacterium Escherichia coli is a thousand times higher than previously thought and that competition between clones carrying different adaptive mutations is extremely intense. Such competition is a major phenomenon constraining evolution and helps to explain why mechanisms that allow genetic exchange are maintained among bacteria.