This Week in Science

Science  20 Apr 2007:
Vol. 316, Issue 5823, pp. 335
  1. That's Far Enough


    In the developing root tip of Arabidopsis, the transcription factor SHORTROOT (SHR) moves from cell to cell. However, the cells that are the target of this mobile transcription signal form a layer that is only one cell thick. Cui et al. (p. 421; see the Perspective by Dolan) now elucidate the interactions that send the mobile signal just so far, and no further. When SHR moves to the adjacent layer of cells, it meets its mate, the SCARECROW (SCR) protein, to which it binds. Together, SHR and SCR move to the nucleus and regulate transcription of the SCR gene. An excess of SCR protein scavenges the peripatetic SHR, and with more SCR available on demand, SHR and its developmental signal cannot move out any further than that first cell layer.

  2. Mantle Melt Mixing

    The heterogeneity of the convecting mantle is a result of recycling of oceanic and some continental crust within it. Sobolev et al. (p. 412, published online 29 March; see the Perspective by Herzberg) have developed a method for separating out the recycled crust components in basalts by combining measurements of a variety of elements, including Ni, Ca, Co, Mn, Cr, and Al. They amassed a large sample of olivine phenocrysts collected from basalts from mid-ocean ridges, ocean islands, and large igneous provinces. Ion microprobe analyses were carried out for 17,000 grains from 232 samples. By analyzing their compositions jointly, variations were seen for the different basalt types that can be ascribed to crustal mixing. Recycled material is detected in almost all melting environments, and its contribution to the melt can be related to potential temperature and the thickness of the lithosphere.

  3. Measuring More Than Saturn's Rotation

    Saturn's true inner rotation rate is hard to determine because the gas giant planet is swathed in thick layers of clouds that rotate more slowly. One measure that has been used is the modulation of intense radio emission (Saturn kilometric radiation) caused by the rotation of the planet's magnetic dipole. However, some changes in timing are seen that suggest there are slippages in the rotation of the plasma surrounding Saturn that may also modulate the radio signals. Gurnett et al. (p. 442, published online 22 March; see the Perspective by Bagenal) show that most of the variation occurs at particular longitudes and near the orbit of the moon Enceladus, where vapor is also being injected. The authors propose that twin convection cells are set up on either side of the plasma disk that funnel ions preferentially in one direction, which increases the electron density and radio emission at one longitude.

  4. Cuprate Contortion


    Advances in both x-ray and electron-diffraction technology have offered an increasingly detailed view of the sudden structural changes that solids can undergo upon optical excitation. Using electron diffraction with picosecond time resolution, Gedik et al. (p. 425) uncover a previously unappreciated light-induced expansion in a cuprate ceramic (a superconductor below 32 kelvin). Specifically, a pulse of 800-nanometer wavelength photons above a certain threshold intensity induces a discrete lattice expansion along the c-axis within tens of picoseconds; the solid then recontracts on a 10-fold slower time scale. The expansion scales linearly with photon fluence, which is consistent with a preliminary model of the transient geometry that attributes repulsive interactions to photoinduced oxide-to-copper charge transfer.

  5. Bending Visible Light Backward

    Artificial materials, or metamaterials, with tunable electric and magnetic responses can give rise to a negative index of refraction, in which electromagnetic radiation is bent in the opposite direction from that expected for natural materials. Other effects such as perfect lensing and cloaking have also been demonstrated at longer wavelengths. Lezec et al. (p. 430, published online 22 March) now demonstrate negative refractive index in the visible regime, providing the potential of practical devices, such as super-lenses, for this important wavelength regime.

  6. Beating Creep in the Heat

    Heat engines are usually more efficient when they operate at higher temperatures, but materials properties, particularly creep strength, tend to degrade at extended elevated temperatures. Nickel-based superalloys can overcome this problem but are too expensive for widespread use. Some steels have sufficient creep strength, but their protective chromium oxide layers do not have sufficient oxidative resistance at high temperatures. Aluminum oxide provides better oxidative resistance, but at high Al content, steel forms a weaker ferrite phase. Yamamoto et al. (p. 433) show that by eliminating titanium and vanadium from the steel alloy, and using only niobium to form the carbide precipitates, a lower fraction of Al can be used and will form a protective scale. Thus, a high-temperature, lower-cost, creep-resistant steel alloy can be formed.

  7. Lightweight Cleaver

    The facility with which most transition metals can cleave H2 has been attributed to a primary donation of electrons from hydrogen to metal, coupled with a secondary donation in the opposite direction. Frey et al. (p. 439) show that a similar process promotes H2 addition to an electron-rich alkyl(amino)carbene. Like a transition metal, the stable singlet carbene has both an electron pair and vacant orbital, but calculations suggest that the primary process is nucleophilic electron donation from carbene to hydrogen. This nucleophilic mechanism also allows the carbene to cleave the N-H bond of ammonia, a much rarer reaction at electrophilic metal centers.

  8. Color-Coded Mitotic Spindle Assembly


    The ability to screen the whole genome within Drosophila cells with RNA interference technology provides the opportunity to examine aspects of intracellular processes in unprecedented detail. Goshima et al. (p. 417, published online 5 April) present a whole-genome screen that used computational analysis to aid the phenotypic screening of high-throughput microscopy images to look for proteins involved in mitotic spindle function. The mitotic spindle represents the most complex intracellular structure to be screened to date, and the automated screening technology and detailed phenotypic analyses provide insight into how spindle microtubules are generated and how centrosomes are positioned.

  9. Insight into Autism's Heritability

    Although twin studies have shown that autism is “the most highly heritable” of all neuropsychiatric disorders, the majority of cases have no family history of autism, for reasons that are not understood. Linkage and association studies have not unambiguously identified strong gene candidates, which suggests that heritable risk factors for autism are complex. Rather than looking at patterns of inheritance (the focus of traditional genetic approaches), Sebatet al. (p. 445, published online 15 March) have looked for the occurrence of spontaneous variation in copy number by microarray comparative genomic hybridization. They found a 10-fold increase in the frequency of copy number variants in individuals with autism relative to controls. The chromosomal changes were in small regions and included changes in single genes.

  10. Just Enough Oxygen Activation

    Many iron-containing enzymes activate O2 and harness the oxidizing power of the reactive oxoferryl species, whereas others protect the cell from activated oxygen. Two crystallography studies provide insight into the mechanisms of both O2 activation and deactivation (see the Perspective by Wilmot). Kovaleva and Lipscomb (p. 453) report the structures of superoxo, alkylperoxo, and bound-product intermediates in the O2 activation and insertion reaction of an extradiol ring-cleaving dioxygenase. Together, these structures define the major chemical steps of the dioxygenase mechanism. Katona et al. (p. 449) trapped iron peroxide intermediates in superoxide reductase and suggest how this enzyme forms the reaction product, H2O2, rather than producing reactive oxo-ferryl intermediates.

  11. The Winners Recall Memories

    Electrophysiological and cellular imaging studies show that only a portion of neurons are involved in a given memory. Why is one neuron, rather than its neighbor, included in a particular memory? Han et al. (p. 457) found that neurons in the lateral amygdala that contain the highest levels of function of the transcription factor CREB at the time of the encoding of an auditory fear memory are those that preferentially express the activity-regulated gene Arc after the recall of the memory. Thus, neurons compete during memory formation, and CREB helps to determine the winners.

  12. Hardness Without High Pressures

    Superhard materials find use as abrasives and cutting materials. The hardest material, diamond, cannot be used to cut iron alloys because of the formation of iron carbide at elevated temperatures, and this deficiency spurred a search for other superhard materials with greater operational stability. However, these materials generally require synthesis under very high pressures. Chung et al. (p. 436) report the arc-melting synthesis of rhenium diboride at ambient pressures. This material shows very high hardness and incompressibility along its c-axis that rivals that of diamond.

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