This Week in Science

Science  14 Jul 2006:
Vol. 313, Issue 5784, pp. 144
  1. Enough Dust to Go Around


    Supernova explosions are thought to have spread dust (mostly carbon and silicate grains) from the dying embers of stars throughout galaxies and beyond. Observations, however, have failed to find enough dust in supernovae to support this idea. Sugerman et al. (p. 196, published online 8 June; see the Perspective by Dwek) used the Spitzer Space Telescope to map the infrared glow from warm dust around the recent supernova SN 2003gd and found 10 times more dust than has been seen in any such object. The progenitor star that exploded as SN 2003gd was more massive than the Sun and similar to the massive and short-lived stars that would have been the first to explode in the early universe. The quantity of dust found here is sufficient for supernovae to have been the dominant dust factories in the early universe and later spreading heavy elements throughout the first galaxies.

  2. The Ages of the Dinosaurs

    Little has been known about the overall life histories of groups of dinosaurs, especially the fraction that survived into adulthood and old age. Using deposits near Alberta, Canada, that preserve remains of tyrannosaurs that died over a short period of time, and by making comparisons with other tyrannosaurs, Erickson et al. (p. 213; see the news story by Stokstad) constructed a survivorship life table. The results imply that juvenile survivorship was high but that only a small fraction reached extreme size and an old age of between 20 and 30 years.

  3. Routes to Friction Control


    As mechanical systems shrink in size, friction and wear must be treated differently than in macroscopic machines; there is less material to wear away before a device fails, and liquid lubricants tend to become viscous in confined spaces (see the Perspective by Carpick). Socoliuc et al. (p. 207) present a dynamic approach for reducing friction. They slide the sharp silicon tip of a friction force microscope over the surface of NaCl and KBr salt crystals while mechanical exciting the tip in the direction normal to the surface. When the frequency of oscillation matched a mechanical resonance of the tip in the normal direction (or half that value), the friction was sharply reduced; excitation of lateral resonances had no effect. The normal motion likely allows the tip to find regions of interaction where friction is still finite but stick-slip motion disappears. Park et al. (p. 186) found that charge accumulation or depletion modified the friction force between a silicon surface and a metal-coated probe tip of an atomic force microscope. When the sample was positively biased, the friction force increase for positively doped regions of the sample but stayed the same in negatively doped regions

  4. Examining Emulsions

    Emulsions consist of two immiscible liquids that are mixed together, and often the droplets of one component are stabilized by the addition of a surfactant. For small-sized droplets, internal pressure stabilizes the droplets and interactions between droplets are not significant. At large sizes, deformation and hydrodynamic forces dominate, and these forces can be measured by a number of techniques. Dagastine et al. (p. 210) have developed a method to study droplets of intermediate size. Deformation, hydrodynamic drainage, and interaction forces all contribute to the overall behavior of droplet-droplet interactions, and thus current models of emulsion behavior may not be suitable.

  5. One-Dimensional Wigner Crystal

    Wigner crystallization is a natural correlated state for an electronic system whose Coulombic interaction is stronger than the kinetic energy of the electrons and has been seen in two- and three-dimensional systems. Yamamoto et al. (p. 204) performed Coulomb drag experiments in a one-dimensional realization with two closely spaced parallel nanowires, where the injection of current in one wire (the drive wire) drags electrons in the other wire. Usually, the direction of drag is in the direction of current flow, but in this case they observed negative Coulomb drag, where the drag current flows opposite to the driving current. They interpret this negative drag as the Wigner crystallization of the flowing electrons in the drag wire.

  6. Nailing Down the Effects of Arginylation

    Arginylation is a posttranslational modification critical for embryonic development, but the protein targets and molecular effects of arginylation are largely unknown. Karakozova et al. (p. 192, published online 22 June; see the Perspective by Bulinski) show the regulation of a single target protein by arginylation with effects on the molecular and cellular level. β-actin, an abundant, essential intracellular protein, is arginylated in vivo, and this modification regulates actin polymerization, cell motility, and lamella formation in motile cells.

  7. A Study in Character Displacement

    Long-term studies of wild populations of animals are key to the understanding of ecological and evolutionary processes. Previous work has already demonstrated the evolution of beak size in a population of Darwin's finches on a Galápagos island when food supply changes. Continuation of the study by Grant and Grant (p. 224; see the news story by Pennisi) has revealed an evolutionary shift caused by a competitor species that led to character displacement—a divergence in beak size between the two species. It is the strongest evolutionary change recorded in 33 years of study of this system. The demonstration of character displacement in nature strengthens theories of competitive interaction in speciation, adaptive radiation, and the assembly of ecological communities.

  8. Destructive Influence

    Certain bacterial pathogens inject their effector proteins into the target cell to wreak havoc. Nomura et al. (p. 220) now show what a Pseudomonas protein does once it is inside an Arabidopsis plant cell. The virulence protein, HopM1, targets a plant defense protein, AtMIN7, by escorting that protein to its destruction by the plant's own proteasome. AtMIN7 normally functions in the vesicle trafficking that builds up a cell-wall response to pathogen invasion.

  9. This Is How We Catch Scorpions

    Teaching is found in all human societies. Are there unambiguous examples of teaching in other species? Thornton and McAuliffe (p. 227; see the cover) describe observational and experimental field studies on the role of teaching in the development of prey capture in wild meerkats. Teachers modified their behavior in the presence of pups by gradually introducing them to live prey, monitoring their handling behavior, nudging prey, and retrieving and further modifying prey if necessary. Dangerous food items (such as scorpions) were more likely to be killed or disabled than other mobile prey. Helpers gained no direct benefits from their provisioning behavior and incurred costs through giving pups prey that was difficult to handle and that might escape.

  10. A Getting-Inside Story


    Enveloped viruses deliver their genome into the host by fusing with its membrane. Two classes of viral glycoproteins that drive membrane fusion through conformational changes have been identified, but a number of viral fusion proteins do not fall into either of these classes. Roche et al. (p. 187) have determined the crystal structure of the atypical membrane fusion glycoprotein (G) from vesicular stomatitis virus, and Heldwein et al. (p. 217) have determined the structure of glycoprotein B (gB), a conserved component of the complex cell entry machinery of herpes simplex virus (HSV-1). Unexpectedly, G and gB are homologous with both combining features of fusion proteins from classes I and II. This homology identifies gB as the viral fusogen in HSV-1 and has interesting implications in considering the evolution of viral fusion proteins (see the Perspective by Steven and Spear).

  11. New Role for Histone-Like Protein

    Bacteria can incorporate exogenous DNA into their genomes (for example, antibiotic resistance genes and virulence factors), but this process must be control to prevent harmful effects. Navarre et al. (p. 236, published online 8 June) have evidence for a mechanism that regulates the influx of novel genes, but allows the evolution new function. Horizontally acquired DNA can often be recognized in bacteria by its bias in AT-GC content. Interestingly, a histone-like protein from Salmonella, H-NS (histone-like nucleoid structuring protein), has an enigmatic and nonspecific affinity for AT-rich regions, which then inhibits gene expression. It appears that this recognition of AT regions is a form of self-non-self discrimination.

  12. Bacterial Gold Nuggets

    Several studies have shown that microorganisms are involved in the cycling of gold in the environment, and microbial mechanisms for the formation of gold nuggets have been postulated. Reith et al. (p. 233; see the news story by Kerr) now find that active bacterial biofilms are associated with secondary gold grains obtained from Australian mines. They have assessed the community structure of these biofilms and identified key organisms associated with the gold grains as well as potential metabolisms for detoxification and precipitation of the precious metal.

  13. Field Testing of Enzymes

    The summed charges and dipoles surrounding enzymatic active sites often produce sufficiently local electric fields large enough to impact substrate reactivity. The field strengths can be central to understanding structure-function relations and pursuing rational drug design. However, accurate measurement or prediction of these fields is hindered by their extreme sensitivity to subtle conformational variations. Suydam et al. (p. 200) use a nitrile group, appended to an inhibitor, to track local field variations induced by structural mutations in the active site of human aldose reductase. Because the nitrile is not chemically modified by the binding interaction, shifts in its stretching frequency can be attributed directly to changes in the field. The authors achieve good agreement between calculated and observed field strengths by including side-chain conformational equilibration in their theoretical simulations.

  14. Gatekeeping Extracellular Calcium

    The inositol 1,4,5-trisphosphate receptor (IP3R) allows regulated release of calcium from intracellular stores. These receptors have also been reported to exist in other compartments within the cell, but their roles have been uncertain. Dellis et al. (p. 229; see the Perspective by Gill et al.) found that a very small number of IP3Rs—only about two per cell—exist in the plasma membrane, where they appear to account for an appreciable fraction of calcium entry into B cells after stimulation of the B cell receptor. Cells expressing modified IP3Rs had altered properties of the channel currents recorded at the cell surface and responded to externally applied channel regulator. Thus, the authors propose that IP3Rs allow the intracellular second messenger IP3 to cause both release of internally stored calcium from the endoplasmic reticulum, but also from outside the cell, through the plasma membrane. Although store depletion is well known to cause calcium influx across the plasma membrane, the plasma membrane IP3R appears to be insensitive to depletion of internal stores.