This Week in Science

Science  21 Sep 2007:
Vol. 317, Issue 5845, pp. 1645
  1. Completing a Life Cycle of Genomes


    Filarial nematodes, which are important pathogens that infect large numbers of people in developing countries within the tropics, have complex life cycles involving one or more intermediate hosts. Ghedin et al. (p. 1756) now present the genome sequence of Brugia malayi. The free-living Caenorhabditis elegans nematode genome sequences allow comparisons with the sequence for the filarial worm and offer the potential to identify the genes involved in parasitism. The genomes for all of the hosts involved in this parasite's life cycle are now available—human, mosquito, Wolbachia, and the filarial worm itself, which opens the door to system-based studies of these relationships.

  2. Fractional Quantum Hall Effect in 3D

    In a two-dimensional (2D) electron gas, condensation can occur at particular magnetic fields to create a state that hosts remarkable liquidlike properties. Prior work has indicated this fractional quantum Hall effect arises from intrinsically many-body quantum ground states specific to 2D systems. Behnia et al. (p. 1729, published online 16 August; see the Perspective by Huxley and Green) report heat- and charge-transport measurements that suggest that a similar state might also be observed in bismuth crystals at very high magnetic fields, in part because this metal has such a small Fermi surface and electrons travel with very long mean-free paths. The authors found that electron correlations in bismuth are stronger than what has been commonly assumed, and that this elemental metal may host an exotic quantum Hall fluid.

  3. From One Ring, Many

    The Diels-Alder cyclization has proven widely useful in organic synthesis for introducing structural complexity in a single reaction step. Through formation of two carbon-carbon bonds, the reaction presents the opportunity to control the stereochemistry at four different centers simultaneously. Balskus and Jacobsen (p. 1736) show that a chiral oxazaborolidine derivative catalyzes an asymmetric Diels-Alder reaction across preformed organic macrocycles to create ring systems with high stereoselectivity in which three rings with five to eight members are linked by edge-sharing. The broad substrate scope spans a range of macrocycles bearing E,E dienes, and the authors applied this approach specifically to the synthesis of a sesquiterpene natural product.

  4. Optical Nanocircuits


    Increasing the clock speed of electronic circuits will begin to push the operating frequencies toward the optical regime and will require advances in addition to reducing the size of components. Challenges include the unacceptable losses in conventional microelectronics circuitry when operated at optical frequencies as well as a lack of basic circuit elements, such as resistors, capacitors, and inductors, when operating at such high frequencies. Engheta (p. 1698) overviews a proposed route which considers the optical response of subwavelength nanoparticles as the nanocircuit building blocks. By combining the ability to tune the optical response of metamaterials with the ability to join the building blocks together, it may be possible to create “lumped” optical circuitry akin to the circuit diagrams and structures in microelectronics.

  5. Deep Spin-Transition Region

    The structure, composition, and dynamics of the Earth's lower mantle are affected by the quantum spin states of iron within minerals at high pressure and temperature. However, this behavior is not well established experimentally. Lin et al. (p. 1740) have determined the spin state of iron in ferropericlase at lower-mantle pressures and temperatures using an x-ray emission spectrometer with in situ synchrotron x-ray diffraction in the laser-heated diamond cell. A continuous change in spin properties occurs in a region spanning the middle to lower mantle (1300 to 2200 kilometers in depth). In the lowermost mantle, low-spin ferropericlase is stable. The spin transition in the middle to low mantle cannot be described by classical equations of state and may possibly cause steep velocity and density gradients there that may be detected seismically.

  6. All in the Wrist?

    The origins and affinity of the small-bodied hominins, Homo floresiensis, remain widely debated and enigmatic. Are these the fossils of a primitive species that somehow persisted in isolation on Flores until the Holocene, or pathologic modern humans, or something else? Tocheri et al. (p. 1743) show that the wrist bones of the original specimen are markedly primitive and completely unlike those of modern humans or of Neandertals.

  7. Sperm Stem Cells

    Little is known about the mammalian spermatogenic stem cell niche. Yoshida et al. (p. 1722; see the Perspective by DiNardo and Braun) used time-lapse imaging and three-dimensional reconstruction to reveal the localization of the candidate stem cells (undifferentiated spermatogonia) and found that these cells localize to the vasculature surrounding the seminiferous tubules in the mouse testis. Upon differentiation, the germ cells leave these regions to spread all through the seminiferous tubules.

  8. Transporting Zinc

    The divalent zinc cation is found in multiple guises in prokaryotic and eukaryotic cells. Lu and Fu (p. 1746, published online 23 August; see the Perspective by Nies) present the 3.8 angstrom structure of YiiP, a bacterial membrane protein that imports Zn2+ by exchanging it for H+. The structure of YiiP is a Y-shaped homodimer, through which a presumptive pathway for Zn2+/H+ exchange can be discerned. YiiP is a member of the cation diffusion facilitator family of transport proteins. Another family member, ZnT-8, is expressed solely in pancreatic β cells and has recently been associated with risk for type 2 diabetes.

  9. Technicolor Super-Resolution Imaging

    The ability to visualize molecular interactions at 20- to 50-nanometer (nm) resolution requires multicolor super-resolution imaging and has remained challenging. Bates et al. (p. 1749, published online 16 August) report the implementation of multicolor stochastic optical reconstruction microscopy (STORM) using a family of photoswitchable activator-reporter pairs. Combinatorial pairing of three reporters and three activators allowed up to nine distinguishable fluorescent probes. Three-color imaging of a model DNA sample and two-color imaging of fixed cells was obtained at 20- to 30-nm resolution.

  10. Taking a Peek at Platelet Production


    Current models of thrombopoiesis (platelet formation) have largely been derived from in vitro studies and static imaging approaches. Junt et al. (p. 1767; see the Perspective by Geddis and Kaushansky) use dynamic intravital imaging to reveal the behavior of active platelet-producing megakaryocytes within the bone marrow. Megakaryocytes remained in close contact with microvessels of the bone marrow, protruding long extensions into the blood stream. It appears that the shear force exerted by the bloodstream then plays an active role in the release of larger megakaryocyte protrusions that allows platelets and proplatelets to move into the peripheral circulation.

  11. Crossing Kingdoms

    Whole eukaryote genome sequencing projects routinely exclude bacterial sequences on the assumption that they represent contamination. However, Dunning Hotopp et al. (p. 1753, published online 30 August) found examples of genetic insertions from the bacterial endosymbiont Wolbachia into eukaryotic genomes in eleven species across six genera, three insect orders, and two different phyla. These inserts range from almost an entire Wolbachia genome to short 100-base pair inserts and are found as degenerate and transcribed genes. Thus, the movement of DNA between bacteria and eukaryotes may not be as rare as has been assumed.

  12. Reconstituting MicroRNA Gene Regulation

    MicroRNAs are small, ~21-nucleotide noncoding RNAs present in the genomes of almost all eukaryotes. They regulate gene expression in animals by repressing translation of target RNAs and also destabilizing them. The precise mechanism of repression has remained something of a mystery. Mathonnet et al. (p. 1764; published online 26 July) developed an in vitro messenger RNA (mRNA) translation system that recapitulates the ability of miRNAs to down-regulate gene expression. In this cell-free system, Let-7 miRNA acts to block the initial step of translation—recognition of the mRNA 5′ cap. Thus, degradation of mRNA is not critical for gene repression, at least at in the early stages of the process.

  13. Setting Magnetic Walls in Motion

    Magnetic domain walls can be moved either with magnetic fields or by injecting spin-polarized currents. As the magnetization can be used as an information bit and controlled electrically, there is practical interest in understanding the microscopic mechanisms underlying domain wall motion. Yamanouchi et al. (p. 1726) report that domain-wall motion induced by either method is fundamentally different, each belonging to what they describe as separate universality classes. The results should prove useful in developing appropriate logic technologies based on domain-wall motion.

  14. Solution Interactions via Interferometry

    Interferometry is one of several techniques that can be used to probe the concentration and binding interactions of biomolecules, provided that the molecule's binding partner is immobilized on a surface. Bornhop et al. (p. 1732) now show that a fairly simple realization of backscattering interferometry, which makes use of microfluidic channels for sample mixing and as the multipath cell for the laser light, can be used to determine dissociation constants for several pairs, apparently through changes in refractive index upon binding. Dissociation constants were determined for several binding pairs, including protein A with immunoglobulin G and activated calmodulin (CaM) with a small-molecule inhibitor or with calcineurin. Unlike microcalorimetric methods, these assays can be performed with very low amounts of samples; for example, several CaM assays required only 200 picomoles of this protein.

  15. Epigenetic Mechanism Unraveled

    Epigenetic regulation of the genome involves the transmission of information through cell division independently of the underlying DNA sequence. The best understood example is the methylation of cytosine bases in DNA, which acts to silence transposable elements and requires the maintenance DNA methyltransferase DNMT1. Bostick et al. (p. 1760, published online 2 August) show that a member of a recently discovered family of methyl DNA binding proteins, UHRF1 (ubiquitin-like, containing PHD and RING finger domains 1) is also critical for the maintenance of CpG DNA methylation in mammals. UHRF1 binds hemimethylated DNA, interacts and colocalizes with DNMT1, and is required for the stable association of DNMT1 with chromatin. These findings suggest that UHRF1 recruits DNMT1 to facilitate efficient maintenance CpG methylation.

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