This Week in Science

Science  30 May 2003:
Vol. 300, Issue 5624, pp. 1337
  1. Seeing More with Scattering

    Two studies focus on the use of scattering techniques to reveal the structures of nanoscale systems. The analysis of the controlled-growth nanoparticles and nanowires has been limited mainly to “off-line” techniques, which require a halt in the growth and a detailed analysis with methods such as electron microscopy. Renaud et al. (p. 1416; see p. the Perspective by Thornton) used grazing-incidence x-ray scattering to study the growth of palladium on the (001) surface of magnesium oxide and of cobalt on the (111) surface of gold. They observed the growth dynamics in “real time” and were able to probe buried interfaces. A problem with standard diffraction methods is the loss of phase information. Zuo et al. (p. 1419) have developed a technique in which they use coherent electron diffraction from very small areas and oversampling in an iterative process to retrieve the phase. They successfully determine the structure of a single double-walled carbon nanotube.

  2. Revealing High-Energy Quasi-Particles

    In superconductors, quasi-particles (QPs) are excitations involving Cooper pairs that have been broken. In the high-transition temperature cuprates, the excitation energy needed to form QPs depends on the direction of their momentum in the crystal. Previous studies have examined low-energy QPs, but details of the higher energy QPs may be more relevant to the pairing mechanism. Gedik et al. (p. 1410) have developed a new pump-probe technique that allows the diffusion and lifetime properties of these higher energy QPs to be determined. Initial results reveal a significant difference in the scattering behavior of the low- and high-energy QPs.

  3. Fickle Contacts

    Scanning tunneling microscopy studies of the conductivity of aromatic thiols adsorbed on gold substrates have revealed intermittent changes in conductivity. Explanations for these changes include tilting of the molecules with respect to the substrate and electron-localization effects. Ramachandran et al. (p. 1413) found that similar stochastic switching can occur in both aromatic and alkane thiols terminated either with organic groups or with gold nanoparticles. These measurements were performed with an atomic-force microscope tip at constant force and at different temperatures. The authors show that some of these events are caused by fluctuations of the top contact but that most are caused by intermittent breaking of the bottom contact, the Au-S bond. Switching increases by 50% at 60°C, the temperature at which these films are commonly annealed.

  4. Going Down in Tibet

    The collision of India with Asia produced the high Tibetan plateau, which has been a natural laboratory for studying crust and mantle deformation. Tilmann et al. (p. 1424) provide a high-resolution image of the crumple zone in the upper mantle that lies below the thick crust. The images show a high-velocity, vertical lens-shaped region from about 100 to almost 400 kilometers depth. This region may represent Indian lithosphere that was dragged down into the mantle at the start of the collision about 50 million years ago. This downwelling can account for all of the convergence and shortening between India and Asia.

  5. Satellites Hear Earth's Grumblings

    When an earthquake occurs, seismometers all around the world record the various arrivals of the body waves and surface waves. Unfortunately, large earthquakes or earthquakes near a seismometer ruin the waveform records because the instruments go off-scale or contain too much additional surface-related noise. Larson et al. (p. 1421) have discovered another method to measure the amplitude and velocity of seismic waves using the global positioning system (GPS) network of satellites and receivers. A set of eight receivers in North America, set to receive data in the 1- to 0.1-second range (geodetic-based receivers are normally set to collect data every 30 seconds), recorded the surface waves produced by the 2002 Denali Fault earthquake as far away as 3800 kilometers.

  6. The Wild Side of Adaptive Mutation

    The evidence for adaptive mutation in response to environmental stress has been obtained from laboratory strains of organisms that tend to be more homologous than the wild type. Bjedov et al. (p. 1404; see p. the Perspective by Rosenberg and Hastings) collected about 800 natural isolates of the bacterium Escherichia coli and found that the majority display stress-inducible (or stationary-phase) elevation of mutation in response to growth-limiting conditions. Their analysis shows that carbon starvation is the physiologically relevant trigger and that removing oxygen largely eliminated enhanced mutation in aging colonies. Genetic analysis of a single natural isolate revealed the roles of the carbon-sensing genetic regulators, the stationary-phase and stress-response regulon, the RecA protein, high-fidelity DNA polymerase II, and compromised mismatch repair. Computer simulations showed that the remarkably high frequency of aging colony mutators in natural isolates could be accounted for by the indirect selective advantage of increased genetic variability.

  7. Genome Sequences of the SARS Virus

    International groups have been conducting massive efforts to gather information about the virus associated with SARS (severe acute respiratory syndrome). Two groups, Marra et al. (p. 1399) and Rota et al. (p. 1394; see p. the Perspective by Holmes and Enjuanes), present genome sequences representing two isolates of this coronavirus. Although the genome organization is consistent with other coronaviruses, the sequences are significantly different from the other known coronaviruses. The information gleaned from the sequence will be important for development of diagnostic tests and identification of targets for development of antiviral drugs and vaccines.

  8. Phospholipid Signaling in Plant Development

    Cellular morphogenesis is particularly important in plant development because these cells do not migrate but remain where they are formed. The highly patterned formation of root hairs from some (but not all) epidermal cells of the growing root in Arabidopsis offers an opportunity to study tissue patterning as well as cellular morphogenesis. Ohashi et al. (p. 1427; see the cover) searched for the genes downstream of the group of transcription factors that regulate whether a root hair develops. Beginning with GLABRA2, the furthest downstream of the transcription factors, the authors identified a phospholipase D gene as its direct target. Experimental alterations of the phospholipase D gene expression indicate that phospholipid signaling contributes to cellular morphogenic choices.

  9. Attacking the Stomach Lining

    The majority of peptic ulcers are the result of infection by Helicobacter pylori, a bacterium that can adhere to and weaken epithelial cells that line the stomach. Amieva et al. (p. 1430) have determined that the corrupt behavior of H. pylori is unlike that of other bacteria, which attack epithelial cells from the outside by secreting proteases. The attack begins at adherens junctions (AJs), points of adherent cell-cell contact. Upon adhesion, the bacterium injects the protein CagA, which relocates a key intercellular component of AJs called ZO-1 to the sites of attachment. The subsequent breakdown of AJs causes changes in cell shape and leakiness of the epithelial cell layer.

  10. A View from Within

    One of the challenges in biological imaging has been to develop innocuous probes that emit intense signals from within living tissue or, better yet, a living animal. Larson et al. (p. 1434) image highly fluorescent quantum dots via two-photon absorption (which allowed the use of penetrating near-infrared excitation) within the capillaries of mice. The optical fluorescence signal could be seen clearly through intact skin and also through adipose tissue.

  11. Movement, Timing, and the Cerebellum

    The cerebellum plays an important role in numerous motor tasks, but its precise function in the timing of movements is still not fully understood. Spencer et al. (p. 1437) examined the performance of patients with cerebellar lesions on discrete versus continuous movements. The cycle duration in discrete movements was more variable in the impaired limb than in the unimpaired limb. In continuous movements, however, the performance of the impaired limb was as good as the unimpaired limb. The authors conclude that the cerebellum is essential in providing an explicit temporal goal for discrete movements but is not essential for continuous movements. The timing of continuous movements arises instead as an emergent property from other brain areas.

  12. Deciphering DNA's Double Entendres

    Annotation of DNA sequences containing the sequence UGA is problematic because it serves as a signal for both termination of protein synthesis and as the selenocysteine codon. Kryukov et al. (p. 1439) developed a computational approach to this problem based on structural and thermodynamic information as well as information about evolutionary conservation from sequenced genomes. New mammalian selenoproteins were identified and validated by metabolic labeling; two are localized on the plasma membrane.

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