This Week in Science

Science  09 Dec 2005:
Vol. 310, Issue 5754, pp. 1581
  1. DNA Twisted into Tetrahedra


    One strategy for building molecular nanostructures in three dimensions is to exploit the connectivity afforded by nucleic acid structures. In many cases, the steps needed to select particular base pairing to create structures such as cubes lead to long, multistep syntheses. Goodman et al. (p. 1661) have developed a rapid self-assembly process that creates DNA tetrahedra that have 10 to 30 base pairs on each edge. Four single strands that contain the complementary sequences for six edges anneal in seconds in 95% yield, and single diastereomeric products are formed. The authors also present atomic force microscopy studies of the compression of a single DNA tetrahedron.

  2. Restoring the Forests

    Deforestation in the tropics has had seriously adverse consequences for biodiversity, ecosystem services, and the human inhabitants of the tropical forest. In recent years, projects have been set in motion to restore degraded forest lands in some countries. Lamb et al. (p. 1628) review the range of approaches to restoration and assess the extent to which these approaches might be successful in achieving their aims, particularly with respect to human well-being.

  3. Superconducting Qubit Interferometry

    Mach-Zehnder interferometry is a powerful technique to probe quantum optical effects. Such interferometers contain two beam splitters. The first sends two beams of photons along separate paths. The acquired path or phase difference the two beams may acquire creates interference fringes after the second beam splitter recombines the two beams. Oliver et al. (p. 1653, published online 10 November) show that a two-level superconducting qubit can also be made to exhibit similar interference fringes. In this case, the anti-crossing between the ground and excited states acts as the beam splitter, and the energy level splitting between them corresponds to the optical path difference. Multiple photon transitions (up to 20) can be induced, thus illustrating a potentially useful route for the manipulation of superconducting qubits in quantum computing schemes.

  4. Going Softer


    Whether added deliberately or by accident, impurities or solutes have long been used to strengthen metals. A more recent discovery was that impurities can soften some metals, but the underlying reasons have not been fully understood. Using simulations, Trinkle and Woodward (p. 1665; see the Perspective by Chrzan) show that for molybdenum, certain transition metal solutes can influence the energy barriers for dislocation motion, and in some cases, these changes lead to a softening of the metal. By reducing the strength, and thus the tendency to fracture abruptly, these modified metals may find expanded use in structural components.

  5. Rapid Glacial Erosion

    Determining the relative importance of incision by rivers and glaciers in the creation of alpine valleys is often hampered by difficulties in quantifying rates of glacial erosion. Shuster et al. (p. 1668; see the cover) assessed the timing and rate of glacial erosion by 4He/3He thermochronometry. Using an example from the Coast Mountains of British Columbia, Canada, they determined erosion rates both before and during alpine glaciation. The Klinaklini Valley deepened rapidly by 2 kilometers or more around 1.8 million years ago when it became glaciated, at least six times as fast as during its preglacial state.

  6. Moon Magma

    A giant impact into the early Earth is thought to have ejected a huge amount of debris into orbit that coalesced to form the Moon. Heat from the impact also apparently melted much of the Moon and created a huge ocean of magma. One means of dating these processes is by detecting 182W, the daughter product of a short-lived isotope, 182Hf. Differences in the abundances of 182W are produced when magma, rocks, and crystals separate while 182Hf is still present. Kliene et al. (p. 1671; published online 24 November) report accurate measurements of tungsten isotopes by analyzing metals returned in Apollo samples (metals provide the most accurate measure). The data imply that the giant impact occurred about 30 million years after the formation of the solar system and that the magma ocean had solidified by about 50 million years.

  7. Protein Interaction in the Gaseous Phase

    The identification of transient or readily reversible interactions between proteins is a difficult problem that has been addressed with a variety of methods. Ruotolo et al. (p. 1658; published online 17 November) have now applied mass spectrometry to the problem in order to exploit its advantages of sensitivity and speed. They show that the trp RNA-binding attenuator protein (TRAP) maintains its 11-membered ringlike structure in the gas phase and that binding of RNA and tryptophan influences the shape and stability of the ring in a fashion consistent with its known behavior in aqueous solution.

  8. The Liver and the Control of Glucose Metabolism

    The protein kinase and tumor suppressor LKB1 is a potential activator of the adenosine monophosphate. activated protein kinase (AMPK), a kinase that senses cellular energy levels by binding the metabolite AMP. Shaw et al. (p. 1642; published online 24 November) engineered mice so that LKB1 expression could be acutely blocked only in the liver; they found that its expression plays a critical role in the control of metabolism in the liver and in glucose homeostasis. In the absence of LKB1, AMPK was almost completely inactive. Animals lacking LKB1 in the liver showed hyperglycemia and increased expression of genes encoding enzymes of gluconeogenesis and lipogenesis.

  9. Predicting Responses on the Death Pathway


    Multiple signaling pathways can influence whether a cell commits to the cell death program known as apoptosis. For many years, it has been possible to categorize signals as contributing to the “gas” or to the “brakes.” However, predicting the biological outcome of multiple signals that apply some gas here, and a stomp on the brakes there, has remained a challenge. Janes et al. (p. 1646) applied a systems-level approach to this problem and created a model to analyze coupling between almost 8000 measurements of signaling parameters in cultured cells with about 1500 measures of the various stages of apoptosis in cells treated with various combinations of cytokines. The model allows the cellular apoptotic response to be correctly predicted under a variety of conditions.

  10. Land-Use Effects on Climate

    Climate models are still only rather crude representations of real climate systems, and one class of important feedbacks not adequately realized in them is that of land processes. Fedemma et al. (p. 1674; see the Perspective by Pielke) investigate the role of biogeophysical land processes, which directly affect the absorption and distribution of energy at the Earth's surface, by integrating them into a global climate model. Increases in atmospheric CO2 concentrations during the next century and associated greenhouse gas-induced warming led to significant regional impacts directly associated with land cover, mostly in mid-latitude and tropical areas. However, global average temperature was not affected much by land cover change because regional variations that led to more or less warming tended to cancel out.

  11. Lipids and Neurotoxins

    The venom of certain snakes includes neurotoxins capable of paralyzing their victims. Upon intoxication, snake presynaptic phospholipase A2 neurotoxins (SPANs) cause motor nerve terminals in the neuromuscular junction to enlarge and induce exocytosis of neurotransmitters from synaptic vesicles. Rigoni et al. (p. 1678; see the Perspective by Zimmerberg and Chernomordik) now find that a mixture of lysophospholipids and fatty acids, which are released by SPANs acting on phospholipids, closely mimics all of the biological effects of SPANs. Thus, at the presynaptic membrane, lysophospholipids and fatty acids help to generate a membrane conformation that promotes vesicle exocytosis and also inhibits synaptic vesicle retrieval.

  12. Ambiguity Averse

    In a 2002 news briefing, U.S. Secretary of Defense Donald Rumsfeld famously distinguished between known knowns, known unknowns, and unknown unknowns. The last group remains difficult to discuss, but neuroscientists and economists have joined forces to examine the distinctions between the first two. Hsu et al. (p. 1680; see the Perspective by Rustichini) challenged subjects to choose between risky and ambiguous payoffs, where the former type of choice contains outcomes with known probabilities and the latter type features the same outcomes but with unknown probabilities. Even under conditions where the expected payoffs are equal, normal humans prefer risk over ambiguity, and brain-imaging results suggest that the amygdala and orbitofrontal cortex (OFC), which both become more active with ambiguity, modulate a third area of the brain, the striatum. Notably, patients bearing lesions in the OFC did not exhibit an aversion to ambiguity.

  13. Checkpoint for Synapsis

    The complex mechanics of the eukaryotic cell cycle is monitored at a number of points to ensure that everything is going according to plan, before the next step in the process is executed. Known checkpoints include DNA replication, DNA damage, and spindle function. Bhalla and Dernburg (p. 1683) identify a checkpoint that monitors synapsis, or pairing between homologous chromosomes during the meiotic (haploid gamete-producing) cell cycle in Caenorhabditis elegans, which ensures the accurate chromosome segregation during division. The checkpoint requires chromosomal sites known as pairing centers, where synapsis initiates, and is distinct from the DNA damage checkpoint monitoring meiotic recombination. The checkpoint involves the PCH2 gene, which is also involved in the pachytene checkpoint in budding yeast.

  14. Getting to Grips with G Protein Structure

    Heterotrimeric G protein signaling is important in a wide range of physiological processes; however, little is known about how activated heterotrimer subunits (Gαβγ) are oriented at the membrane during signal transduction. Tesmer et al. (p. 1686) provide a snapshot of activated G proteins in a 3.1 angstrom crystal structure of G protein-coupled receptor kinase 2 (GRK2) bound simultaneously to activated Gαq and Gβγ. GRK2 is critical to the phosphorylation-dependent desensitization of many G protein-coupled receptors. In the complex, Gαq is fully disassociated from Gβγ, oriented away from its position in the heterotrimer, and forms an effector-like interaction with GRK2.