This Week in Science

Science  27 Jun 2008:
Vol. 320, Issue 5884, pp. 1691
  1. Mesoporous Metal Composites from Nanoparticles

    CREDIT: COURTESY OF SCOTT WARREN AND ULI WIESNER, CORNELL UNIVERSITY

    Metallic materials with internal pores on the scale of 10 nanometers could be useful as high-surface-area electrodes or catalysts, but many metals have high surface energies that tend to encourage the closure of such pores and decrease surface area. In principle, it should be possible to assemble metal nanoparticles into composites, but the surface coatings on nanoparticles can lead to low overall metal content in the final composite. Warren et al. (p. 1748) chose a block polymer that could bind ligand-stabilized platinum nanoparticles and form lamellar or inverse hexagonal phases with high nanoparticle loadings. Pyrolysis created an ordered, highly conductive platinum-carbon composite with open pores ≥10 nanometers in size; almost all of the carbon could be removed from thin section samples with a plasma treatment to yield porous platinum mesostructures.

  2. After the Asteroid

    About 35 million years ago, a large asteroid hit the continental shelf of North America in what is now the southeastern tip of the Delmarva Peninsula, just east of the Chesapeake Bay, forming a crater nearly 90 kilometers across. Gohn et al. (p. 1740) describe results from two cores drilled from the crater. Within a few minutes of the impact, huge granite blocks were rapidly transported several kilometers inward. The top of the crater was filled with a large debris flow and seawater from the time of the impact remains trapped in the pores of the crater. Although the high heat of impact probably sterilized the local rocks and sediments, the seawater has never been flushed since soon after the impact, and in the deepest part of the crater the water contains abundant microbes.

  3. Let There Be Light

    Very high energy gamma rays are thought to be produced as matter falls into supermassive black holes in distant galaxies. Using a large gamma-ray telescope, the MAGIC Collaboration (p. 1752) detected an extremely energetic source of gamma rays more than 5 billion light-years from Earth. En route, the gamma rays will have interacted with relic light from older stars and galaxies that formed during the evolution and expansion of the universe, which allows an independent measurement of this extragalactic background light. The results are consistent the amount of light expected from galaxy counts.

  4. Birds, Birds, Birds

    The relationships among bird families and orders have been difficult to establish due to the existence of conflicting phylogenies from morphology and to the availability of a relatively limited number of samples of avian DNA sequences. Hackett et al. (p. 1763; see the news story by Pennisi) have performed a phylogenetic analysis of 25 kilobases of nuclear DNA sequence per species representing multiple chromosomes, genes, and noncoding regions of genomes across the Aves. While many established relationships were upheld in this phylogeny, several surprising results were noted, which suggests that bird evolutionary relationships are complex and that previously grouped species may not be closely related but instead show convergence of particular life history traits.

  5. Electron Relay Station

    CREDIT: SHIH ET AL.

    Many proteins promote rapid electron transfer over surprisingly long distances. One proposed mechanism to account for such efficient rates of transfer has been the transient oxidation and/or reduction of intermediate residues along the path from donor to acceptor. Shih et al. (p. 1760; see the Perspective by Bollinger) present quantitative evidence for this type of relay mechanism in a series of engineered azurin mutants bearing a tryptophan, tyrosine, or phenylalanine residue between coordinated copper donor and rhenium acceptor complexes. Transient absorption spectroscopy in the visible and infrared revealed that the intervening tryptophan accelerated the electron transfer more than a hundredfold. In comparison, neither tyrosine nor phenylalanine had the same effect, highlighting the narrow range of intermediary redox potentials necessary for the acceleration.

  6. Linking Cell Contact and Polarity

    Polarization of the early embryo enables essential patterning events such as gastrulation and cell specification. In certain animals, including worms and mammals, cell-cell contact polarizes early embryos by inducing asymmetries in cortical PAR proteins. Anderson et al. (p. 1771) identify a pathway linking cell-cell contact to the PAR asymmetries that polarize early embryos of the nematode, Caenorhabditis elegans. A key member in this pathway is PAC-1, a Rho guanosine triphosphatase-activating protein. PAC-1 is recruited to cell contacts and locally excludes PAR proteins, thereby restricting the PAR proteins to contact-free surfaces. In turn, PAR proteins induce radial polarity that asymmetrically positions cytoskeletal proteins needed for gastrulation.

  7. Developing the Lateral Line

    Fish use the lateral line to sense changes in pressure and water movements that may signal the arrival of food or predators. Nechiporuk and Raible (p. 1774) now use genetics, cell transplantation, and live imaging to study lateral line development in zebrafish. In zebrafish, the lateral line develops from front to back, with a migrating zone of progenitor cells laying down repeated clusters of hair cells in its wake. Fibroblast growth factor signaling is involved in several aspects of the process: migration, deposition of a primordium, and differentiation.

  8. Expanding β-Arrestin's Remit

    CREDIT: JEFFREY J. KOVACS

    The β-arrestin proteins help restore G protein-coupled receptors to an inactive state after stimulation. Recently, positive signaling roles for the arrestins have been revealed, as well as roles in signaling by other types of receptors, including the receptor Smoothened (Smo), which has structural similarity to G protein-coupled receptors, but signals in a distinct manner. Kovacs et al. (p. 1777, published online 22 May; see the Perspective by Rohatgi and Scott) report that β-arrestins 1 and 2 in cultured cells were detected in a complex with a tagged version of the Smo protein and with Kif3A, a kinesin motor protein. Depletion of β-arrestin 1 or β-arrestin 2 inhibited localization of Smo to primary cilia, which function as “antenna”-like structures rich in receptor proteins. Smo-dependent transcriptional activation was also inhibited if the arrestins were depleted. This Smo-dependent interaction of β-arrestins with Kif3A may link Smo to the transport apparatus that brings it to the primary cilium.

  9. Dissecting Spliceosome Functions

    Many eukaryotic genes are split into several pieces by the presence of noncoding introns. Introns are spliced out of transcribed RNA by a large RNA protein complex, the spliceosome. Very similar transesterification reactions are carried out by self-splicing Group II intron RNAs. Self-splicing reactions are fully reversible. The potentially much more complex spliceosome reactions should, in principle, also be reversible, but this has not been demonstrated. Now Tseng and Cheng (p. 1782) use a spliceosome variant unable to release the spliced RNA products of the splicing reaction to show that in the absence of monovalent ions, the second transesterification step of splicing can run backwards at high efficiency. Spliceosomes arrested after the first transesterification reaction can also carry out the reverse reaction, this time in the presence of KCl, raising questions about the function of various components of the spliceosome.

  10. Same But Different

    The genetic code is inherently redundant, with synonymous codons encoding the same amino acid. A level of bias has evolved in terms of how often particular codons are actually used to encode a given amino acid because, in the context of the whole sequence, particular codon usage affects secondary structure and/or translation of the transcribed RNA. Similarly, the many ways in which synonymous codons are paired with one another can show particular leanings (probably for similar reasons), which is known as codon-pair bias. Coleman et al. (p. 1784; see the news story by Enserink) explore how bias of synonymous codon pairs at a genome level might influence the biology of a system. An algorithm was designed to recode the DNA sequence for the poliovirus capsid protein, while maintaining the original amino acid sequence, to generate two recoded chimeric strains of the virus carrying either under- or overrepresented codon pairs. Although marginal effects were seen with the latter, chimeric viruses carrying underrepresented codon pairs had reduced translation, were less able to infect cells, and showed reduced pathogenicity in mice.

  11. Human Migration Unraveled

    The ability to follow the migration and demography of the earliest human settlers helps us understand the significance of cultural remains. Gilbert et al. (p. 1787, published online 29 May) have sequenced a human hair sample from approximately 3000 years ago and assembled a complete mitochondrial genome from one of the oldest human inhabitants of Greenland. Comparisons with existing human mitochondrial genomes suggest that this individual, unlike the current population, was related to eastern Asian populations. Thus the first human occupation of the far north may have been by migrants from East Asia, and these early inhabitants were subsequently replaced by a later migration within America giving rise to the existing Eskimo populations.

  12. Getting the Measure of Trapped Atoms

    The ability to stop and store single atoms or ions in optical traps has made contributions to a variety of applications ranging from precision measurements to quantum-state engineering. Ye et al. (p. 1734) review recent experimental work demonstrating how much control can be exerted over the position and energy levels of the trapped particles and how that control leads to better precision.

  13. From Microscopic to Macroscopic Properties

    A longstanding goal of materials modeling and simulation is to be able to connect studies and data from the atomistic scale with large-scale mechanical properties. For metals, this would connect the local behavior of dislocations with the strength, toughness, or ductility of the metal. Devincre et al. (p. 1745; see the Perspective by El-Azab) examine the three-dimensional nature of dislocation glide within a dislocation forest in metallic crystals. They devise a quantitative definition of the mean-free-path for dislocation motion in terms of three dimensionless parameters that are directly determined from dislocation simulations. Using these parameters, they show that by accounting for dislocation orientations and motion they can represent the behavior of face-centered cubic metal crystals during strain-induced hardening at the continuum level.

  14. Reassessing Defects on Titania Surfaces

    Although titanium dioxide might at first appear to be a poor candidate as a catalyst for chemical reactions, given that it is a wide-band gap insulator, defect states at its surface provide sites of greater reactivity. The Ti3d defect of rutile TiO2 (110) has long been assumed to arise solely from bridging oxygen vacancies. Wendt et al. (p. 1755, published online 5 June), using high-resolution scanning tunneling microscopy and photoelectron spectroscopy, argue that the Ti3d defect state in the band gap does not originate mainly from bridging oxygen vacancies but from Ti atoms trapped in interstitial sites in the near-surface region. These donor sites provide the electronic charge needed for molecular oxygen to adsorb and dissociate. These experimental observations, which are supported by density functional theory calculations, suggest the need for a reexamination of defect states on other transition metal oxides.

  15. Mountain Forest Ecosystem Change

    Current changes in the elevational distribution of plants with changing climate are usually measured at the upper and lower distributional boundaries. Lenoir et al. (p. 1768) document changes of plant distributional range within the whole altitudinal gradient covered by European temperate forests, working beyond the traditional focus on altitudinal range boundaries. Climate-driven changes are important in affecting changes within the altitudinal range of species, not only on their distributional limits. Although species' responses to climate change are widely variable in terms of magnitude, species that share the same ecological (area of occupancy) and biological properties (life-history traits) do show similar patterns of change. Over the last century climate warming has resulted in an average upslope movement of plant species' optimum elevation of 29 meters per decade.

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