This Week in Science

Science  02 Jan 2009:
Vol. 323, Issue 5910, pp. 12
  1. Gold Nanoparticles on a Roll


    The optical or electronic properties of nanoparticles can differ from those of the bulk material through confinement effects and through their interactions with one another through space. The possibilities for tuning and exploring such interactions would be enhanced if nanoparticles could be attached to frameworks so that they are arranged in three dimensions. Sharma et al. (p. 112) attached gold nanoparticles to single DNA strands and incorporated them into DNA networks that could curl up to form tubules. Electron tomography revealed that the nanoparticles were arranged on the surface of the tubules in a variety of spiral morphologies. In addition, through their own steric repulsions, the nanoparticles actively influenced the assembly process.

  2. Keeping Fungal Invaders at Bay

    Plant cells mobilize a cell-autonomous response to the sites of attempted fungal penetration, resulting in the polarization of the peroxisome. The mechanism driving this polarization has remained unclear. Now Clay et al. (p. 95, published online 18 December) and Bednarek et al. (p. 101, published online 18 December) show that the pre-invasion resistance mechanism is due to the coordinated and infection-induced biosynthesis of a specific glucosinolate molecule. This molecule is made and delivered to the outside of the cell through the action of the peroxisomal proteins (PEN2 and PEN3) and is produced via a biochemical reaction that uses myrosinase products as a substrate. Bednarek et al. show that derived compounds of PEN2 play a key role as antifungal compounds and that this route of synthesis occurs in living cells and only in response to infection. Clay et al. have discovered how these myrosinases and their related breakdown products act as signaling molecules in the Arabidopsis defense response. Thus, a family of metabolites controls this nonhost resistance pathway.

  3. Ultra Clean, Ultra Small

    Carbon nanotubes have generally been found to fall into two categories—metallic or insulating. Deshpande et al. (p. 106) now report transport measurements on ultraclean individual single-walled carbon nanotubes. Even though the synthesized nanotubes are a mixture of nominally metallic and insulating types, all of the nanotubes were actually insulating, exhibiting an energy gap dependent on the radius of the tubes. This insulating behavior may be due to a Mott-insulating state resulting from electronic correlations in the tubes.

  4. Adding Injury to Insult


    Coral reefs worldwide are under terrible pressure from rising sea-surface temperatures, pollution, and overexploitation. Another emerging threat to their health is ocean acidification—the declining pH of surface seawater layers caused by the absorption of increasing amounts of atmospheric CO2. Many studies have demonstrated recent declines in the coverage and numbers of live coral reefs, as well as reduced coral diversity, but few have examined how rates of coral calcification have been affected. De'ath et al. (p. 116; see the news story by Pennisi) examined growth patterns of 328 massive Porites corals from the Great Barrier Reef of Australia and found that their rates of calcification have declined by nearly 15% since 1990, to values lower than any seen for the past 400 years. The main causes of this continuing decline appear to be increasing water temperatures and ocean acidification.

  5. A Warped Route to Cloaking

    Transformation optics and metamaterials allow the manipulation of light with unprecedented control, giving rise to possibilities like invisibility, hyperlensing, and cloaking. However, most approaches so far have been based on mappings of Maxwell's equations in a Euclidean, flat space geometry, which has generally limited the extent to which these properties can be realized to monochromatic light. Leonhardt and Tyc (p. 110; see the Perspective by Nicolet and Zolla, published online 20 November) describe a theoretical approach based on mappings in a curved, or non-Euclidean, space that may open up applications to a broad range of wavelengths. Furthermore, the physical properties required of materials to achieve broadband invisibility need not be as extreme as in previous approaches.

  6. Right on Target

    Cotranslational protein targeting to membranes involves signal recognition particle (SRP) and its receptor. SRP contains a noncoding RNA, which catalytically accelerates the interaction of SRP with its receptor. Bradshaw et al. (p. 127) now show that SRP RNA is a molecular switch that is triggered by signal sequences on secretory and membrane proteins. When activated in response to signal sequence binding, SRP RNA profoundly accelerates the forward rate of SRP binding to the SRP receptor. Previously, this effect was masked because detergent mimics the signal peptide, activating SRP RNA constitutively. Thus, cargo (the signal sequence) controls the SRP protein targeting machine.

  7. Bright Shiny Flowers

    The study of flower color has primarily focused on chemical pigmentation. Whitney et al. (p. 130) explore how the structural features on the petal surface of a tulip flower generate color independently of chemical pigmentation via iridescence. Floral iridescence may result in an ultraviolet signal that is visible to insects. Bumblebees can learn to use information from iridescence to select among potential food sources. Thus, iridescence may contribute to plant-pollinator interactions.

  8. Life's Too Short

    Our ability to control scourges like dengue virus and the malaria parasite is undermined by a lack of vaccines and the evolution of drug and insecticide resistance. Control measures, which eliminate the older insects that have accumulated more parasites, not only reduce the transmission rate of the parasite but also remove the selection pressure for the evolution of resistance, because young adult insects are still allowed to reproduce. McMeniman et al. (p. 141; see the Perspective by Read and Thomas) studied the potential value of a life-shortening strain of Wolbachia, as a biological control agent. This bacterium was established in laboratory populations of the dengue-transmitting mosquito Aedes aegypti after a 3-year adaptation program. The Wolbachia did not eliminate egg production by the mosquito, but it did halve the adult's life span, which is probably enough to slow the transmission of a pathogen that takes several days to mature within its insect host.

  9. Chaperone to Neurodegeneration


    Although autophagy in general is known to play a role in the process of neurodegeneration, it is not clear what role, if any, chaperone-mediated autophagy, which selectively regulates the levels of specific cytoplasmic proteins, plays in cellular survival and death. Members of the MEF2 (myocyte enhancer factor 2) family are nuclear transcription factors involved in neuronal survival, differentiation, and synaptic function. Now Yang et al. (p. 124) provide in vitro and in vivo data from knockout mice, transgenic mice, and human brains to show that chaperone mediated autophagy directly targets MEF2 for lysosomal degradation. During the process, MEF2 is translocated from the nucleus to the cytoplasm. The chaperone-mediated autophagy of MEF2 is sensitive to the level of α-synuclein, increased levels of which can cause Parkinson's disease. Thus, chaperone-mediated autophagy can play a direct role in modulating the neuronal survival machinery, and may be involved in the process by which mutant and/or overexpressed α-synuclein can undermine neuronal viability.

  10. Trading Toxins

    Some bacterial viruses (phage) scoop up host DNA as they are packaged into particles. Chen and Novick (p. 139) show that this capacity can involve bacterial genes encoding virulence determinants, leading to their transfer to another species of pathogenic bacteria. Several temperate phage, able to integrate their DNA into bacterial chromosomes and reproduce without lysing their hosts, mediated the transfer of toxin genes from Staphylococcus aureus to Listeria monocytogenes. Both pathogens can occur simultaneously in bovine mastitis, and phage transduction of the pathogenicity determinants was seen to happen in raw milk. This veterinary condition is succumbing to antibiotic resistance, and phage therapy has offered an alternative bactericidal treatment; however, it may have the unwanted side effect of promoting the transfer of bacterial virulence factors among pathogens and thereby pose a risk to humans.

  11. Rare Air

    The oxygen-isotopic composition of sulfate minerals is affected by a variety of important physical, chemical, and biological processes that occur on Earth. As such, it is a valuable tracer of key environmental conditions throughout geological time. Bao et al. (p. 119) report measurements of extreme isotopic fractionation during the Neoproterozoic era (circa 1000 to 542 million years ago), at the time of a “snowball Earth” episode. The unique set of oxygen isotopic values observed in sulfates and their accompanying carbonate host rocks represents evidence of either an atmosphere with exceptionally high CO2 content or a completely unique pattern of O2 flux between that atmosphere and the biosphere.

  12. Peer Support

    In a large classroom, the use of individual response systems can enhance the interaction between one instructor and many students. Smith et al. (p. 122) study students using “clickers” to respond to various questions through the class time. When the students are allowed to discuss among themselves, their answers improve. Analysis of the basis for this improved response suggests that students achieve better understanding and are not simply following the cues of other students who seem to know the answer.

  13. Viewing the Action of DNA Polymerase

    Direct DNA sequencing methods are based on the observation the stepwise action of a DNA polymerase along a primer template. Direct methods can provide information about the kinetics of nucleotide incorporation, but are often limited by the ability to obtain sufficient fluorescent signal in single-molecule realizations and changes in kinetics caused by steric hindrance of the labels. Eid et al. (p. 133, published online 20 November) have developed a method that allows the observation in real time of the incorporation of nucleotides, tagged at the terminal phosphates, as a single surface-tethered polymerase acts on a primer template. The use of this method for genomics applications will require tuning of the polymerase reaction chemistry and further improvement in the optical system, but currently offers the ability to sequence small viral and bacterial genomes.

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