This Week in Science

Science  23 Mar 2007:
Vol. 315, Issue 5819, pp. 1633
  1. Magnifying Superlenses


    Although the spatial resolution of a conventional optical microscope is limited by diffraction to a value of the order of 200 nanometers, a “superlens” based on specially structured metamaterials designed to exhibit a negative refractive index could overcome this limitation. However, the planar superlenses demonstrated to date cannot provide magnification. Smolyaninov et al. (p. 1699) describe a magnifying superlens based on the propagation of surface-plasmon polaritons (SPP). The object to be imaged and magnified is placed inside the center region of their lens, a structure of concentric circles of a polymer deposited on a gold substrate. The light scatters off the object and creates SPPs in the gold film. With the structured lens designed properly, the SPPs propagate radially outward through the “lens” of concentric circles. The magnified image of the object can then be seen at the outermost circle with a conventional microscope. In Brevia, Liu et al. (p. 1686) have used curved nanoscale multilayers of silver and alumina to create a superlens that projects the image of an object onto a far-field surface, where it can be viewed with a conventional microscope.

  2. A Tin Toggle Switch

    Although changes in bonding should affect the conductivity of atomic and molecular scale wires, direct evidence for switching between conducting and insulating configuration of atoms has been rare. Tomatsu et al. (p. 1696), using scanning tunneling microscopy (STM), show that when tin (Sn) atoms are deposited on the (001) of germanium (Ge) surface, they incorporate into the topmost rows of buckled Ge dimers that form one-dimensional conductors. The STM tip can be used to switch the Sn atom from being the “up” or “down” atom of these asymmetric dimers. When the Sn atom is up, the row remains conducting, but when it is switched down, it reflects electrons in the π* state and terminates the wire.

  3. Tropical African Rain Records

    A wealth of marine sedimentary records have been used to document changes in sea surface temperatures (SSTs) between the Last Glacial Maximum 25,000 years ago and the present warm period, the Holocene. The construction of continental records of land surface temperature for the same interval has been more challenging, particularly for tropical Africa. Weijers et al. (p. 1701) have now analyzed terrestrial and marine biomarkers in a marine sedimentary record from near the mouth of the Congo River and developed parallel records of terrestrial and nearby oceanic conditions that facilitated the comparison of conditions in the two regimes. Tropical African land temperatures rose by about 4°C during the last deglaciation, approximately twice the amount that nearby SSTs rose. This changing land-sea temperature difference exerted an important control on precipitation patterns in central Africa.

  4. Damaging Winds and Waves


    In 2005, a pair of strong hurricanes (Katrina and Rita) struck near the mouth of the Mississippi River a few weeks apart. Day et al. (p. 1679) show that their slightly different tracks provide comparative information for assessing the reasons for the damage and flooding of New Orleans and other regions, and also evaluate what needs to be done to rebuild the region for greater resiliency in the future. Hurricanes create damaging storm surges and high waves by transferring wind energy to the sea surface. The magnitude of this transfer normally is estimated from observations of the surface wind fields in a storm, but that approach suffers from artifacts caused by the presence of waves and ocean spray, and is highly uncertain for the high-wind regimes of major tropical cyclones. Jarosz et al. (p. 1707) used full water-column ocean-current velocity data collected during the passage of Hurricane Ivan in 2004 to determine this air-sea momentum transfer directly from the water side. The efficiency of energy transfer has a maximum at a wind speed of around 72 miles per hour (just under hurricane category 1 levels), which decreases to around half of that value once the hurricane reaches the transition from category 2 to category 3 strength at wind speeds of 111 miles per hour. These findings should help improve forecasts of storm track and intensity, as well as those of the associated ocean waves, surges, and tides.

  5. Dating Crust Creation

    The Earth's crust is continually being formed at mid-ocean spreading ridges, where plates roll apart, and above subduction zones, where plate edges can grow by accretion. Was the creation of crustal plates ongoing early in the Earth's history, or was it restricted to the latter half of the Earth's 4.5 billion year existence? Furnes et al. (p. 1704; see the news story by Kerr) show that crustal formation caused by seafloor spreading was under way as long ago as 3.8 billion years. They have identified and dated an ophiolite sequence of rocks in Greenland that is the oldest known example of oceanic crust. The sequence of rocks includes gabbro, pillow lavas, and sheeted dikes, indicating it was formed on the sea floor by processes similar to those seen today.

  6. Division Decisions

    During an immune reaction, T cells divide rapidly and differentiate producing a variety of T cell types that respond appropriately to the particular threat. Memory T cells also emerge from the same population and remain in the body until such time as a new infection calls once again for their attention. Chang et al. (p. 1687, published online 1 March; see the Perspective by Littman and Singh) now show that single T cells undergo an initial asymmetric cell division in response to a pathogen, producing two daughter cells with alternate fates. After forming an immune synapse with an antigen-presenting cell (APC), various proteins, including some responsible for signaling and asymmetric cell division, were reoriented within the T cells. After division, daughters that were proximal to the APC-T cell synapse became effector cells, while their distal sisters became more memory-like and able to confer better protection when transferred to mice.

  7. The Final Crunch

    Regular clusters of repeats separated by spacers of similar length (CRISPR) are widely distributed in the genomes of Bacteria and Archaea, and are distinctively hypervariable. The spacers share sequence homology with bacteriophage and plasmid sequences, and may provide immunity against foreign genetic elements via RNA interference. During the natural generation of phage-resistant Streptococcus thermophilus using lytic phage obtained from yogurt, Barrangou et al. (p. 1709; see the news story by Marx) found that the integration of viral sequences as new spacers into CRISPR loci indeed confers immunity against virulent phages in a specific, acquired, and heritable manner. Addition and deletion of spacers alters sensitivity to viruses, and CRISPR-associated genes may be directly involved in the resistance mechanism.

  8. Staying in Charge


    Under normal conditions, the intracellular concentration of adenosine triphosphate (ATP) is on the order of 1 millimolar. Numerous enzymes and regulatory proteins rely on this universal currency of energy for anabolic, catabolic, and general housekeeping processes. One of the primary enzymes that regulates ATP levels is the AMP-activated protein kinase (AMPK), which senses the relative ratio of ATP to AMP. When this ratio falls, AMPK phosphorylates metabolic enzymes, which then consume less ATP and make more of it. Townley and Shapiro (p. 1726, published online 8 February; see the Perspective by Hardie) have solved the structure of the fission yeast AMPK homolog and demonstrate the competitive binding of ATP and AMP at a single nucleotide site, where the absence of counterions appears to amplify the discrimination between the mono- and triphosphate ligands.

  9. Plant Hormone Signaling Receptor

    The hormone abscisic acid (ABA) regulates a variety of developmental and physiological processes in higher plants. Liu et al. (p. 1712, published online 8 March; see the Perspective by Grill and Christmann) have now identified a membrane-bound protein that functions as an ABA receptor. The protein, GCR2, has features of a G protein-coupled receptor, which have thousands of variants in animal cells, but very few known variants in plant cells.

  10. Diameter Modulators of the Nuclear Pore

    The nuclear pore complex (NPC) controls the exchange of molecules between the cytoplasm and the nucleus. This supramolecular assembly is composed of a set of proteins termed nucleoporins (nups). Melčák et al. (p. 1729) describe the structure of a complex of nup58 and nup45, which are essential components of the central channel of the NPC. The two nucleoporins form stable dimers that further associate into tetramers. Two crystal forms contained four conformers of the tetramer, which differed in the lateral offset between dimers. Thus, these nucleoporins may have dynamic interaction interfaces, and slide relative to each other in order to adjust the diameter of the transport channel to the size of the cargo.

  11. Holding On More Tightly

    Several aspects of the unusual catalytic activity of nanoscale gold clusters on titanium oxide supports are still unclear, including whether oxidized gold atoms play a role in increasing reactivity. Matthey et al. (p. 1692) used high-resolution scanning tunneling microscopy and density functional theory to compare gold nanoclusters supported on reduced, hydroxylated, and oxidized TiO2(110) surfaces. The nanoclusters diffused most readily on the hydroxylated surface, and were bound more strongly to the oxidized surface than the reduced surface. Cationic gold atoms that interact covalently with surface oxygen atoms apparently create more types of clusters that can interact strongly with titania surfaces.

  12. Modeling Competence

    Bacteria undergo a well-studied transient differentiation into a state of competence, during which they can take up DNA from the environment. It is a rare example of a developmental system understood sufficiently well to be analyzed in a rigorous quantitative manner. Süel et al. (p. 1716) use mathematical models and precise experimental measurements to describe key properties of his developmental system. The frequency and duration of episodes of competence could be independently tuned by simple changes in gene expression, and such variations in gene expression could also give rise to distinct dynamic properties of the system.

  13. The Grid in the Brain

    Within the brain, an animal's current location is dynamically represented by a context-independent spatial map encoded by periodic activity in principal cells in layer II of the medial entorhinal cortex. Computational models have predicted that the difference in spatial frequency of grid cells along the dorsal to ventral axis of entorhinal cortex should correspond to a difference in intrinsic temporal frequencies of neurons along this axis. Recordings from stellate cells in layer II of entorhinal cortex by Giocomo et al. (p. 1719) demonstrated a progressive decrease in the frequency of subthreshold membrane oscillations along the dorsal to ventral axis of the entorhinal cortex that is consistent with these models. This gradual decrease was responsible for the corresponding decrease in spatial frequency of grid cells along the same axis.

  14. Generating Color Vision in the Mouse

    For trichromatic color vision, an animal needs three distinct visual pigments expressed separately in individual photoreceptors, and the animal must also be able to process the information in the retina and visual cortex. Mice do not have trichromatic vision, because they only possess two visual pigments. Jacobs et al. (p. 1723) examined the spectral and behavioral properties of mice genetically engineered to express a third visual pigment. The human long-wavelength opsin was knocked-in to form an X-linked polymorphism. When the engineered mice expressed pigments in the correct ratios, they exhibited behavior consistent with functional trichromatic vision.

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