This Week in Science

Science  23 Nov 2007:
Vol. 318, Issue 5854, pp. 1213
  1. Short DNAs Stack and Order


    For a material to form a liquid crystalline phase, it is generally believed that its constituent molecules need to have strong shape anisotropy (morphologies of rods or discs) and need to be fairly rigid. Long DNA fragments have been shown to form mesophases, but it is unclear that shorter fragments would do so. Nakata et al. (p. 1276) show that short DNA segments (6 to 20 base pairs) that lack shape anisotropy can form a wide range of ordered mesophases during the cooling of a solution of random fragments. Hydrophobic forces allow the base pairs to associate end-to-end to create long rod-shaped aggregates. By this route, it is possible that mixtures of fragments can undergo a separation as complementary fragments associate and locally concentrate into ordered droplets.

  2. Insulating Cooper Pairs

    The standard model of superconducting metals (the Bardeen-Cooper-Schreiffer, or BCS theory) describes how electrons pair up into a phase-coherent state and lose all of their electrical resistance. In the context of studying how quantum- driven phase transitions differ from thermally driven ones, a question now being addressed is what happens to the pairing gap and the long-range coherence when the superconductor is driven out of the superconducting regime by disorder or magnetic fields. Stewart et al. (p. 1273) present magnetoresistance measurements on amorphous ultrathin films of bismuth perforated with an array of nanometer-sized holes. Cooper pairs that give rise to zero resistance superconductors also give rise to high-resistance electrical insulators.

  3. Reconsidering Supercontinent Movement

    Paleomagnetic reconstructions of the latitudinal position of Pangea, the supercontinent that contained most of Earth's landmass for 100 million years from the Permian into the Jurassic, have been interpreted as showing that Pangea was originally centered roughly on the equator and moved northward by about 20° by the Early Jurassic. Rowe et al. (p. 1284) reconstructed apparent wind patterns derived from aeolian deposits in the southwestern United States and found that wind direction over the Colorado Plateau remained constant throughout the entire period, which indicates that Pangea remained at or near the equator. Among the several resolutions to this disagreement are that the latitudinal movement of Pangea based on paleomagnetic data is misinterpreted, our understanding of how winds shape sand dunes is incorrect, existing paleogeographic reconstructions cannot reproduce adequately the wind fields responsible for dune formation, or the climate controls on winds in the Jurassic were different than they are today.

  4. Tracking Electrons on the Go

    The detailed dynamics of current flow in a metal are challenging to study because the onset time of a potential gradient to drive the electrons forward tends to be much longer than the duration of the collisions and scattering events of interest. Güdde et al. (p. 1287) overcame this problem by inducing a transient current at a copper surface using the coherent interference effect of one-and two-photon laser excitation pathways, a phenomenon previously demonstrated in semiconductors. The femtosecond dynamics of transient current generated on an ultrashort time scale was tracked by imaging the momenta of photoelectrons ejected by a third laser pulse. The technique should generalize to a range of conducting and semiconducting substrates.

  5. Superconducting Terahertz Generators


    Practical sources of electromagnetic radiation between 0.5 to 2 terahertz have been lacking, but this frequency range has potential uses for molecular recognition, spectroscopy, and imaging applications. Josephson junctions, in which two superconducting layers sandwich an insulating layer, naturally oscillate with a frequency that depends on the superconducting gap and the applied bias, but this potential terahertz source cannot produce sufficient power for applications. With the clever design of the emitter geometry analogous to that of a laser cavity, Ozyuzer et al. (p. 1291; see the Perspective by Kleiner) report on the generation of intense terahertz radiation from a stack of Josephson junctions formed by the weakly coupled CuO2 layers in the Bi2212 high-temperature superconductor.

  6. Energizing Polymer Shape-Memory Materials

    In a shape-memory material, internal stresses that are captured during deformation processes are used to recover the shape of the material as it is heated above a transition temperature. For polymer-shape memory materials, large strain changes are possible, but these changes are accompanied by only small recovery stresses. This combination leads to a low energy density and has made the polymer-based materials poor candidates for actuators. Miaudet et al. (p. 1294) formed a composite from poly(vinyl alcohol) and 20 weight percent carbon nanotubes whose stress recovery approaches that of metal-alloy shape-memory materials but still exhibit large strain capabilities.

  7. β2-Adrenergic Receptor Structures

    G protein-coupled receptors (GPCRs) are transmembrane proteins that respond to diverse extracellular stimuli to trigger many different cellular responses. They are important pharmacological targets, but drug design has been hampered because structural information is only available for the rather distinctive member of this family, rhodopsin (see the Perspective by Ranganathan). Two studies now provide structural insights into another GPCR, the human β2-adrenergic receptor. Rosenbaum et al. (p. 1266, published online 25 October) stabilized the protein for crystallization by protein engineering and analyzed mutagenesis data in the context of the structure to provide insight into how ligand binding is coupled. Cherezov et al. (p. 1258, published online 25 October; see the cover) describe in detail the 2.4 angstrom structure of the fusion protein and how it compares to the rhodopsin structure. It appears that conserved helices provide a common core in class A GPCRs while variable helices provide binding-site plasticity.

  8. Nudging-In on Stem Cell Niches

    The ability to transplant hematopoietic stem cells (HSCs) in bone marrow is of huge importance in a wide number of clinical settings. However, incoming HSCs do not easily conquer niches already occupied by host HSCs. Czechowicz et al. (p. 1296) use a mouse transplantation model to demonstrate that endogenous HSCs do indeed prevent the engraftment of donor HSCs through the occupancy of appropriate niches. However, by using an antibody that can transiently deplete endogenous HSCs, transplanted animals could be preconditioned to allow chimerism levels to rise to as much as 90%. Further work will be needed to determine if similar conditioning can improve HSC transplantation in humans.

  9. Fair's Fair


    It is not uncommon for an employee to reflect both upon how much he or she has been paid, and on how much colleagues have received. A purely rational being would calculate the balance between income and expenses, in order to decide upon happiness or misery, yet ample experience suggests that we do care about our relative remuneration. Fliessbach et al. (p. 1305) used side-by-side brain imaging scanners and a behavioral task in which equal performance was rewarded inequitably. Neural activity in the ventral striatum, a brain area notable for its central role in responding to and predicting rewards, was indeed sensitive to the relative amount of money paid. Furthermore, this response occurred even when no decisions were made, which suggests that the calculation of social standing—as indexed by payment—may be automatic.

  10. Decisions and Impulsivity

    Deep brain stimulation of the subthalamic nucleus and dopaminergic medication both represent successful therapies for Parkinson's disease. However, both interfere with normal cognitive strategies, such as decision-making in conflict situations. Frank et al. (p. 1309, published online 25 October; see the 26 October news story by Miller) provide evidence for two distinct computational roles of these treatments in decision-making. Dopaminergic medication altered patients' relative tendency to learn from positive versus negative outcomes without impacting conflict-induced slowing. In contrast, deep-brain stimulation sped up high-conflict choices, without affecting learning biases. Both of these findings were predicted in a computational model of the basal ganglia in learning and decision-making.

  11. Rupture in Action

    Parkfield, California, sits on the San Andreas fault and has experienced a number of magnitude 6.0 earthquakes, the most recent in 2004. The seismic and geodetic sensor network in this region allows detailed views of the earthquake rupture process to be created. Applying a full-waveform back-projection method to the Parkfield data, Allmann and Shearer (p. 1279) mapped the 2004 event in both space and time. As well as the main rupture, they detected a secondary burst of high-frequency energy about 5 seconds into the quake and lying 12.5 kilometers north of the rupture initiation away. Thus, the Parkfield event consists of two distinct subevents. The high-frequency event occurred at the southern edge of a patch of high slip, which suggests that slip grew rapidly at this point, perhaps through the rupture of an asperity.

  12. Domesticated Mobile Elements Modulate Light Responses

    Plants constantly gauge their environments and adjust their growth and development in order to survive. Plants can “see” their surroundings using a battery of photoreceptors to determine the color, intensity, direction, and duration of light around them. Phytochrome A (phyA) mediates various far-red light-induced responses through a cascade of signaling events. Lin et al. (p. 1302) report that two proteins co-opted from ancestral Mutator-like transposable elements act as transcription factors essential for phyA-controlled gene expression. These results provide insight into the mechanisms and evolutionary history of phytochrome signaling in higher plants and lend support for a role of mobile elements as driving forces for host evolution.

  13. InsP7 and Insulin Release

    Inositol 1,4,5-trisphosphate is a well-known second messenger in cellular signaling. More highly phosphorylated forms of this molecule, inositol pyrophosphates, may also function in cellular regulation. Illies et al. (p. 1299; see the Perspective by Nagamatsu and Ohara-Imaizumi) now show that InsP7 (diphosphoinositol pentakisphosphate) is required for full exocytotic release of insulin from mouse pancreatic β cells. Depletion of the kinase that generates InsP7 inhibited exocytosis, whereas overexpression of the enzyme stimulated exocytotic release of insulin. Pancreatic β cells may maintain high amounts of InsP7 to help assure ready release of insulin in response to metabolic demands.

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