This Week in Science

Science  04 May 2007:
Vol. 316, Issue 5825, pp. 657
  1. Plastid Distress Signal

    CREDIT: AJIT NOTT

    Plastids, including plant chloroplasts, are built and operated largely under the control of the nuclear genome. Largely, but not exclusively, plastids carry their own residual genome and can talk back when things go awry. Koussevitzky et al. (p. 715, published online 29 March; see the Perspective by Zhang) now show that several signaling pathways that carry news of disaster from the plastid to the nucleus actually converge into one signaling pathway before the news emerges from the chloroplast. Thus, the nucleus receives a coherent report that integrates several aspects of chloroplast function. Gun1 protein is identified as a key integrator within the chloroplast, and ABI4 as a key transcription factor within the nucleus that responds to the news by altering gene transcription.

  2. The Cold Side of Plasmas

    Ionized gases, or plasmas, are present throughout the universe over a wide range of densities and temperatures. Some are extremely hot, such as in the Sun's corona; others are very dense plasmas found within white dwarfs. Killian (p. 705) reviews recent work on an exotic class of plasmas in which the ions move in a neutralizing background of electrons at low temperatures. The ions undergo a rich variety of collective motions that provide challenges to computational efforts, but may answer some of the current questions about planetary interiors and laser-compressed materials.

  3. Mercury's Molten Core

    Mercury is expected to have a metal core and silicate mantle, but thermal models make a wide range of predictions about the physical state of its core. Although the existence of a magnetic field is suggestive of a molten core dynamo, the field could be caused by remnant magnetization. Margot et al. (p. 710; see the cover and the Perspective by Solomon) have used a novel technique of radar speckle interferometry to probe Mercury's rotation dynamics. The planet wobbles in longitude in synchrony with its 88-day orbital period. Couplings between the planet's spin axis, wobble, and orbit suggest that the mantle of Mercury is decoupled from a core that is at least partially molten.

  4. Dynamic Qubit Coupling

    Large-scale quantum information processing will require that the interactions between individual qubits be controlled while retaining quantum coherence. Dynamic coupling of the qubits would simplify the circuitry and allow the system to be broken up into smaller units. Niskanen et al. (p. 723) demonstrate dynamic switching of the coupling of two superconducting flux qubits via a third intermediate qubit, and illustrate the coherence retention by running a quantum algorithm that detects hackers trying to infiltrate the system.

  5. Entangled Quantum Metrology

    The use of entangled quantum-mechanical systems is expected to lead to improvements in precision measurements beyond what can be achieved with classical physics techniques. Nagata et al. (p.726) used sets of four entangled photons to demonstrate how interference measurements can reveal patterns expected of light one-quarter the wavelength of the photon used. Their results open the way for applications to high-precision measurements and the development of quantum metrology with multiple entangled states.

  6. Platinum Nanocrystals with High-Index Facets

    For catalytic applications, metals are often used as nanoparticles to increase their surface area and to create a high density of steps and defects that can act as active sites for reactions. Nonetheless, most of a metal nanocrystal's surface is made up of “low index” surfaces such as {111} facets that are flat and relatively free of defects. Tian et al. (p. 732; see the Perspective by Feldheim) report on an electrochemical route that creates tetrahexahedral (THH) platinum nanoparticles (with a diameter of ~100 nanometers) from larger Pt nanospheres that had been deposited on glass carbon electrodes. A series of square-wave redox pulses create these smaller nanocrystals that exhibit 24 high-index facets such as the highly stepped {730} or {530} facets. Relative to the rounded Pt nanospheres, the THH particles exhibit much higher activity for the electro-oxidation of formic acid and ethanol for the same surface area.

  7. Going Off Road

    CREDIT: USGS

    The density and distribution of roads is at the center of major questions in ecology, use of natural resources, and urban and transportation planning. A useful measure for comparison and analysis of change over time needs to be relatively scale independent and sensitive to both the number of roads within an area and their distribution. Watts et al. (p. 736) developed a metric based on the distance to a road in a defined area, which essentially represents a volume. They have applied this metric, and the per capita equivalent, to all of the counties in the United States.

  8. Extrinsic Developmental Patterning

    Fly, worm, and frog embryos sport maternal factors to specify early asymmetry and axis development, but opinions differ whether asymmetry in the mammalian embryo is prepatterned or if the axis is specified later in development. Kurotaki et al. (p. 719, published online 19 March; see the Perspective by Behringer) used real-time whole-cell tracing and a transgenic mouse line with labeled chromosomes to follow cell lineages in vitro and in vivo. No lineage dependency was observed up to the four-cell stage with regard to the embryo axis; however, cell lineage and the embryonic axis were affected by physical restriction via the zona pellucida. Thus, axis positioning of the early mouse embryo is specified by factors outside the embryo and not by factors within the blastomeres.

  9. Modeling Photosynthetic Energy Conversion

    CREDIT: WANG ET AL.

    The initial energy conversion step in the photosynthetic reaction center transfers an electron from an excited donor to a neighboring acceptor. The rate of this electron transfer has been difficult to model based on a static barrier between initial and charge-separated states. Wang et al. (p. 747; see the Perspective by Skourtis and Beratan) show that protein dynamics dictates the kinetics. They measured protein relaxation dynamics during electron transfer and could quantitatively fit the electron transfer rates of wild-type and several mutant bacterial reaction centers with a reaction diffusion model for electron transfer that incorporates the dynamics.

  10. Tau Reduction and Cognitive Decline

    The brain pathology in Alzheimer's disease involves both neurofibrillary tangles rich in the protein tau and plaques containing amyloid-β peptide (Aβ), but the relative contribution of each to cognitive impairment is unclear. Roberson et al. (p. 750) find that cognitive and neuronal deficits in two transgenic mouse models of Alzheimer's disease are prevented when endogenous tau production is eliminated or reduced by 50%. The effect of tau reduction was robust, despite the absence of tau mutations, hyperphosphorylation, overexpression, or aggregation into neurofibrillary tangles in these models. Tau reduction did not block plaque-associated neuritic dystrophy, indicating that neuritic dystrophy can be dissociated both from tau and from Aβ-induced cognitive impairments.

  11. Adapting the Adapter

    Adapter proteins connect the proteins involved in multiple cell signaling pathways. In T cells, the adapter protein ADAP regulates certain signals from the T cell receptor (TCR) that influence the function of integrin adhesion receptors. Medeiros et al. (p. 754) found that ADAP also associates directly with another adapter, CARMA1, a membrane-associated protein that couples TCR signaling to the activation of the core transcription factor NF-κB. This association involved the assembly of a multi-protein complex that failed to form in the absence of ADAP and corresponded with impaired NF-κB activity. The study introduces a key new step in regulating the pathway that connects TCR activation with the transcriptional response to infection.

  12. Synaptic Communication

    The traditional view of pyramidal neurons, which are excitatory, is that they can only excite their downstream target cells. However, Ren et al. (p. 758) report that cortical pyramidal neurons can elicit an inhibitory synaptic current in another neighboring pyramidal neuron. These unusual responses were caused by axo-axonic disynaptic connections onto γ-aminobutyric acid (GABA)-releasing terminals located on or near the soma of the postsynaptic pyramidal cell. These so-called inter-pyramidal inhibitory postsynaptic currents were remarkably large and frequent, which suggests that they play an important role.

  13. Under Pressure to Grow

    Semiconductor nanowires can often be grown at temperatures below the bulk eutectic temperature, but the mechanisms by which this growth occurs remain controversial. Kodambaka et al. (p. 729; see the Perspective by Schmidt and Gösele) show that the eutectic temperature is not a limitation to vapor-liquid-solid (VLS) growth and that growth pressure, generally not considered a key factor, is actually crucial in determining the growth mode. The state of the catalyst, which is also affected by the thermal history, is key to assessing whether growth will occur.

  14. Fields of Debris

    The Mars Exploration Rover Spirit has in the past year mapped Home Plate, a large light-toned plateau of exposed layered rocks lying within Gusev crater. Using data from all of the Rover's instruments, Squyres et al. (p. 738) conclude that Home Plate originated in a volcanic explosion. Coarse-grained lower strata underlay fine-grained upper strata, exhibiting grain sorting, grain rounding, and cross stratification consistent with eolian deposition. The structure most likely represents layered debris from a volcanic explosion, probably produced when basaltic lava came into contact with subsurface water.

  15. Home and Away

    Because larvae of reef fishes are tiny and spend weeks to months developing in the open ocean, mark-and-recapture studies to assess their dispersal have remained a challenge. Almany et al. (p. 742) address this problem using a method of larval tagging—transmission of a rare, stable isotope from mothers to offspring prior to dispersal—to obtain direct measurements of dispersal in the populations of two species of coral reef fish resident in a small marine reserve. For both species, ~60% of juveniles had returned to the reef of their birth, despite spending weeks to months at sea. The remaining ~40% of juveniles entering the reserve had dispersed from reefs at least 10 kilometers away. Thus, even small marine reserves can ensure the persistence of many marine species and can benefit areas outside their boundaries through the export of juvenile fish born in the reserve.

  16. MILI--The Genome Guardian

    The germline genome, the repository of the DNA that will be passed on to future generations, needs to be protected from the ravages of parasitic DNA sequences, such as transposons. piRNAs are a recently discovered class of small noncoding RNAs that are bound by the Piwi class of Argonaute proteins. piRNAs are expressed exclusively in the germ line, and in Drosophila they play a role in limiting the expression of transposons. Aravin et al. (p. 744, published online 19 April) now show that mammalian piRNAs likely have a similar function in the male germ line. piRNAs expressed early during spermatogenesis and bound by the mouse MILI Piwi protein are, unlike those expressed later, enriched in specific families of repeat sequences. Furthermore, mutants in MILI result in the demethylation and activation of subsets of these same repeat elements. The repeat piRNAs show features consistent with an amplification loop that facilitates destruction of transposon mRNAs and amplifies piRNA expression.

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