This Week in Science

Science  29 Feb 2008:
Vol. 319, Issue 5867, pp. 1157
  1. A Blue-Emission Special


    The fluorescence of trans-stilbene can be used as a probe of its surrounding environment. In solution, it fluoresces only weakly because the excitation energy drives a rapid isomerization to the cis form; in confined environments that inhibit isomerization, fluorescence can be seen more readily. Debler et al. (p. 1232; see the Perspective by Armitage and Berget) have reexamined the intense blue luminescence from a stilbene-antibody complex that was initially described as fluorescence from a complex that formed between the excited state of stilbene and a tryptophan (Trp) residue. The excited-state complex forms via charge transfer to form an anionic stilbene and a cationic Trp. Charge recombination is the source of the intense blue emission.

  2. Asymmetric Explanations

    The intense explosions known as “gamma-ray bursts” (GRBs) may be associated with supernova explosions after the death and collapse of a star. Some GRB events could be explained if these explosions are asymmetric, with strong jets emerging from the fireball, and if the jets are in our line of sight. Maeda et al. (p. 1220, published online 31 January) have looked at the spectra of several such events at times late in the evolution of the emission, when expansion lowers the density of the ejected matter and allows optical photons to escape. This approach permits a glimpse of the far side of the explosion. Analysis of these results indicates that explosions are aspherical for many GRBs.

  3. Incurring Carbon Debts in Biofuel Production

    Although biofuels have the potential to reduce CO2 emissions, secondary effects of biofuel production must also be considered, such as how much CO2 is released by the conversion of land to the production of biofuel stock. Fargione et al. (p. 1235, published online 7 February) analyze the carbon balance of the conversion of a variety of carbon-rich land types to food-based biofuel croplands and find that the carbon debt incurred by the conversion process can be as much as 420 times that of the annual greenhouse gas emission reductions that result from the displacement of fossil fuels from the energy generation process. Biofuels made from waste biomass, or grown on abandoned agricultural lands, can avoid most, or even all, of that carbon debt, however. Searchinger et al. (p. 1238; published online 7 February) have modeled greenhouse gas emissions in the production of corn-based ethanol. Instead of generating a roughly 20% reduction in greenhouse gases, as typically is claimed, emissions would approximately double during the first 30 years of implementation and create an emission increase that would take more than 160 years to recoup.

  4. An Atomic View of Current Flow


    The magnetic properties of ultracold atom clouds can be used as minute compass needles for the detection of small changes in magnetic field. As current flows through a wire, scattering of the electrons is usually confined to short length scales, and long-range ordering would not necessarily be expected. However, Aigner et al. (p. 1226) report a surprising finding using cold atom magnetometry to study the flow of current in polycrystalline gold wires. Ordered current fluctuations occur along the length of the wire angled at 45° to the current flow. They interpret and model the observed patterns as arising from scattering of the electrons around defects.

  5. Imaging a Tight Squeeze

    One form of controlled nuclear fusion uses high-energy lasers to compress small capsules of hydrogen to densities and temperatures where fusion reactions can occur. Achieving optimum compression will require measurements of the shape and distribution of matter during the implosion, as well as an understanding of how local electromagnetic fields might affect the fusion plasma. Rygg et al. (p. 1223; see the Perspective by Norreys) used proton beams to create images of imploding fusion targets that map out the density and field distribution as a function of time during compression. The maps show unusual structures in the plasma that consist of filaments and strong radial electric fields that have a clear influence on the implosion dynamics.

  6. Exosome Assembly Pathway

    Exosomes are vesicles of endocytic origin that are released into the extracellular environment after fusion of multivesicular endosomes with the plasma membrane. Trajkovic et al. (p. 1244; see the Perspective by Marsh and van Meer) found that the exosomal cargo segregates together with lipid-raft components into distinct microdomains on the endosomal membrane. The transfer of these microdomains into the lumen of the endosome did not depend on the function of the known intravesicular budding ESCRT (endosomal sorting complex required for transport) involved in the degradative pathway, but required ceramide. Exosomes were enriched in ceramide, and exosome formation was sensitive to the inhibition of neutral sphingomyelinases. In giant unilamellar liposomes, addition of sphingomyelinase was sufficient to induce the inward budding of lipid rafts into a liposome. Thus, lipid rafts may act as collecting devices for the lateral segregation of cargo in the limiting membrane of endosomes, and the formation of ceramide from sphingomyelin within these microdomains could trigger membrane budding into the multivesicular endosome.

  7. Synthetic Bacterial Chromosomes

    The synthesis of genomes de novo will provide a powerful tool for understanding the basic biology of living organisms and designing genomes for medical and environmental applications. Gibson et al. (p. 1215, published online 24 January; see the Perspective by Endy) have assembled the complete 580,076-base pair genome of Mycoplasma genitalium, starting from cassettes of 5 to 7 kilobases in size. The synthetic genome contained short “watermark” sequences at intergenic locations. Although it was possible to do some of the assembly in vitro, the larger fragments were assembled (that is, quarter genomes assembled into halves and wholes) by recombination in yeast.

  8. Explaining Polarized Growth Patterns

    The ability of certain cells to grow in a polarized fashion has been studied for many years, but the mechanisms involved in the process, particularly in plant cell systems, remain unclear. Takeda et al. (p. 1241) report the discovery of a positive feedback mechanism that regulates the development of a polarized cell shape in the commonly studied model plant, Arabidopsis thaliana. The positive feedback system is generated by the local interaction of Ca2+ and reactive oxygen species in root hair cells and is central to the maintenance of active growth at spatially restricted sites during polarized cell elongation.

  9. Bat Flight Plan


    Recent work has found that bat wings generate very high lift coefficients at low flight velocities. However, the aerodynamic mechanism responsible for this excess lift has remained obscure. Muijres et al. (p. 1250) have visualized and measured the air flow above the wing surface of actively flying bats. The main lift-enhancing mechanism observed is a leading-edge vortex, which stays attached to the wing throughout the downstroke. The same unsteady mechanism is also responsible for high lift generation in insects.

  10. Memory Breakdown

    The phenomenon of memory reconsolidation has made people question the traditional view that long-term memories become more stable and resistant to perturbation with time. Reconsolidation indicates that memory change is a continuous process and that change is initiated by retrieval experiences themselves. However, the cellular events and mechanisms underlying this phenomenon have not been clear. Lee et al. (p. 1253, published online 7 February) provide evidence for degradation of postsynaptic proteins in hippocampal synapses thought to participate in the formation of contextual fear memories. Blockade of this degradation is accompanied by blockade of the retrieval-induced reorganization of the original memories. Thus, reconsolidation is like a breakdown of original memories while new elements are incorporated by new protein synthesis.

  11. Dissecting Function in the Living Brain

    Because the brain is composed of many interconnected cell types in close proximity, it is not easy to determine the precise function of any one class. Existing methods such as lesions or pharmacological inhibition are relatively crude, and their effects cannot reliably be used to eliminate the contribution of one cell type. Even genetic approaches, which can be targeted to certain cells, have only been used to inhibit one receptor subtype, and the deficits are often present throughout development, confounding interpretation. Nakashiba et al. (p. 1260, published online 24 January) constructed a tetanus-toxin-based, triple-transgenic mouse that allows reversible inhibition of all the synaptic activity of one type of cell in the hippocampus, the CA3 pyramidal cell. When the CA3 hippocampal cells were silenced, the mice could still learn a spatial task but could not recall certain memories or perform rapid learning tasks.

  12. Cutting Graphene into Nanoribbons

    In graphene (individual layers of graphite), electrons are already confined to two dimensions, and narrow ribbons of this material should exhibit additional quantum-confinement properties. Li et al. (p. 1229; published online 24 January) have developed a chemical method to produce graphene nanoribbons with widths below 10 nanometers. Exfoliated graphite was heated for 1 minute to 1000°C in an argon atmosphere containing 3% hydrogen, and dispersed in a polymer-containing solvent and sonicated. Structural characterization by atomic force microscopy revealed that the nanoribbons had smooth edges. Electrical transport measurements showed that all of the nanoribbons tested were semiconductors and exhibited high on-off ratios in field-effect transistors.

  13. Shaping Up

    How is the characteristic shape of an organelle generated and maintained? Integral membrane proteins of the reticulon and DP1-Yop1 families have been implicated in shaping the tubular structure of the endoplasmic reticulum. Hu et al. (p. 1247) now show that these proteins are sufficient to generate tubules in vitro. These “morphogenic” proteins appear to form oligomers in the plane of the membrane that result in the creation of narrow tubules.

  14. Fruit Flies, Mosquitoes, and CO2 Sensations

    Many insects have neurons dedicated to CO2 sensation. The fruit fly Drosophila is repelled by CO2 and has its sensing neurons on its antennae. Mosquitoes, however, are attracted to CO2 and have their sensing neurons on their maxillary palps. Cayirlioglu et al. (p. 1256) found that mutating a microRNA in the Drosophila genome produces a new class of CO2-sensing neurons on the maxillary palps. These olfactory receptor neurons also express two odorant receptors that are normally expressed in the fruit fly palps, and they project both to normal palp target glomeruli and to the glomerulus, which receives projections from the CO2 neurons in the antennae. Thus, the anatomy of the fruit fly CO2-sensing system is switched to an arrangement that resembles a hybrid between the fruit fly and the mosquito.

  15. Imaging Small Brainstem Nuclei

    The reward prediction error theory of dopamine function is well established in nonhuman primates. Using newly developed sophisticated brain-imaging techniques to examine blood oxygen level dependent (BOLD) responses in a very small brain area, the ventral tegmental area, D'Ardenne et al. (p. 1264) successfully imaged midbrain dopaminergic nuclei in humans. Although consistent with the theory of reward prediction error, the BOLD response in midbrain dopaminergic nuclei differed from that observed in the ventral striatum. The BOLD response in the ventral tegmental area reflected a positive reward prediction error but not a negative reward prediction error, both of which were observed in ventral striatum.