This Week in Science

Science  02 Nov 2007:
Vol. 318, Issue 5851, pp. 711
  1. The Root of a Problem


    The Arabidopsis root, which contains 15 cell types, is relatively simple as organ systems go. Brady et al. (p. 801) marked and sorted individual cell types to describe the entirety of gene expression as the root develops, and subsequent bioinformatics analysis revealed surprising complexity of gene expression. Some expression patterns oscillate. Others reveal shared domains that included neighboring cells and shared characteristics of non-neighboring cells.

  2. Quantum Spin Hall Effect Confirmed

    Charged carriers subjected to a magnetic field can be deflected and produce a Hall voltage across a sample. Confining the charged carriers to move in two dimensions can give rise to the quantum Hall effect. However, charge carriers also possess spin, and recent theoretical work indicates that spins can be affected by a voltage bias and give rise to a spin Hall effect for carriers confined to two dimensions. König et al. (p. 766, published online 20 September; see the Perspective by Nagaosa) performed transport measurements on HgTe/(Hg,Cd)Te quantum-well structures and present signatures of the quantum spin Hall effect.

  3. Electrons Tracing Transitions


    Photoinduced transitions in solids such as vanadium dioxide (VO2) that convert from insulators to metals have been studied by a range of spectroscopic methods. However, the underlying rearrangements can involve motion along several different coordinates, and so the deepest understanding is likely to come from techniques that monitor all of these movements simultaneously. Baum et al. (p. 788; see the Perspective by Cavalleri) have made progress toward this goal by applying time-resolved electron diffraction to a VO2 sample in the midst of its transformation. The well-confined probing depth and high signal-to-noise ratio permitted resolution of local atomic motion, as well as the slower and longer range onset of lattice shear over a time scale ranging from ∼1 to 100 picoseconds.

  4. Fast and Low

    Astrophysical explosions can release huge bursts of energy that can be spied at different wavelengths, from the radio band up to gamma rays. Scouring pulsar survey data, Lorimer et al. (p. 777, published online 27 September; see the Perspective by Bower) spotted a new abrupt burst that emitted very powerful radio waves for just 5 milliseconds. The low frequency and lack of repetition of the event suggest that the source lies beyond our own Galaxy, which would make it more powerful and brief than other radio transients known in our Milky Way. Because this flash likely emanated from a single catastrophic event, such as a supernova or the final merging of relativistic objects, similar bright bursts could prove to be valuable probes of the distant universe.

  5. A Change-Up for Sliders

    Friction between sliding surfaces can convert translation energy into vibrational excitations, but to what extent does the nature of the vibrational modes control the ability of a surface layer to absorb energy and control friction? Cannara et al. (p. 780) used an atomic force microscope (AFM) tip to measure the friction force of diamond and silicon surfaces that were terminated with either H or D atoms. This substitution changes the vibrational frequency by a factor between 1.35 and 1.40, and AFM experiments revealed that the shear strengths dropped for both surfaces by factors of 1.26 and 1.30, respectively, upon D substitution. A modeling study suggests that the lower frequency of the deuterium-substituted bond lowers the rate at which kinetic energy can be converted into vibrations.

  6. Piwi and the Argonautes

    At the heart of RNA interference lie the Argonaute (Ago) proteins, which bind small interfering RNA or microRNA and so consign the targeted messenger RNAs to their fate—silenced or destroyed. The Ago superfamily consists of two evolutionarily related groups, the Ago clade and the Piwi clade, and Aravin et al. (p. 761) review the function of this latter group. The evolutionary appearance of Piwi proteins is correlated with the emergence of specialized germ cells in animals, and, indeed, Piwi proteins are required for germ cell development. They are also consorts for a population of recently discovered small RNAs, Piwi interacting (pi)RNAs. The Piwi/piRNA pathway functions as a heritable and adaptive system to protect the germline genome from parasitic nucleic acids.

  7. Of Life and Limb

    The ability of amphibians to regenerate an amputated limb represents a developmental flexibility largely lost for humans. The regeneration process in amphibians depends on interactions with the nerve supply. Kumar et al. (p. 772; see the Perspective by Stocum) have now identified a secreted protein growth factor expressed in the nerves and epidermis of newts during limb regeneration that can serve to support regeneration even when the nerve supply is removed.

  8. Dynamic Developmental Gene Regulation

    Gene regulatory networks control metazoan development and determine which transcription factors will regulate which regulatory genes. However, development is a dynamic process that is driven by continuous change in time and space. Smith et al. (p. 794) now show how a network subcircuit controls a circular pattern of regulatory gene expression that sweeps outward from the vegetal pole of the sea urchin embryo and extinguishes its own activity in the more central regions to establish a dynamic regulatory pattern of gene expression.

  9. Nearest and Dearest


    Both the Scandentia (tree shrews), and Dermoptera (colugos or flying lemurs) have been proposed to represent the closest relatives of primates. A large-scale genomic sequence alignment performed by Janečka et al. (p. 792) now suggests that colugos are the closest living relative of primates. Furthermore, the origin of tree shrews at the Cretaceous-Tertiary boundary indicates that the primates probably evolved at a time when many other mammalian lineages diversified.

  10. An End in Itself

    Telomeres, which cap the ends of eukaryotic chromosomes, solve the “end replication” problem and prevent the cell from mistaking the ends for DNA damage. Telomeres have generally been thought to be transcriptionally inert. Azzalin et al. (p. 798, published online 4 October) now show that, in a range of human and rodent cells, telomeres are in fact transcribed into telomeric repeat containing RNAs, which associate with the telomere heterochromatin. This association is modulated by telomere-associated proteins, which are also linked with the nonsense-mediated decay RNA surveillance pathway.

  11. Molecular Culprit in Gestational Diabetes

    During pregnancy, maternal pancreatic islet β cells expand to accommodate the increasing physiological demands placed on the mother by the growing fetus. The molecular mechanisms controlling this physiological response, which helps prevent the development of “gestational diabetes” in the mother, have been unclear. Studying mouse models, Karnik et al. (p. 806; see the news story by Couzin) now show that menin, a protein previously identified as an endocrine tumor suppressor and transcriptional regulator, inhibits the growth of islet β cells during pregnancy. Transgenic expression of menin in maternal β cells prevented islet expansion and caused the mice to develop several hallmark features of gestational diabetes. Menin appears to be maintained at low levels in islets through the actions of the pregnancy-associated hormone prolactin.

  12. Genetic Variation for Tolerance

    In animals, the distinction between resistance (pathogen limitation) and tolerance (damage limitation) to an infection is rarely made, and there is little understanding of the impact of tolerance on the evolution and ecology of animals and their pathogens. Råberg et al. (p. 812) explored tolerance to a malaria parasite in mice. Taking anemia and weight loss as measures of tolerance in a range of mouse strains that either reduced parasite burden or maximized vigor during infection, a genetic basis for tolerance was revealed. Moreover, resistance and tolerance were traded off against each other when the price of more aggressive immune control of infection was increasing collateral damage. Thus, animals are not always engaged in evolutionary “arms races” with infectious organisms, and it could be just as productive to breed livestock for disease tolerance as for resistance to infection.

  13. Ironing Out C-H Oxidation

    Nature uses iron-based enzymes to catalyze oxidative metabolism of hydrocarbons, but their efficiency and selectivity have stubbornly eluded replication in synthetic molecular catalysts. Chen and White (p. 783; see the Perspective by Crabtree) show that a carefully constructed synthetic ligand, bound through four N atoms to Fe, enables selective hydroxylation of electron-rich tertiary C-H centers in complex organic molecules. Moreover, steric constraints provide a second, complementary mode of selectivity, and carboxylate groups in the substrate can direct a third pathway that leads to cyclized products. Although the catalyst is unstable in the presence of the hydrogen peroxide oxidant, the reaction proceeds rapidly enough that practical yields can be obtained by tailoring the successive additions of catalyst and oxidant.

  14. Dissecting a Core Oscillator

    The core circadian oscillator of the cyanobacterium Synechococcus elongatus can be reconstituted in vitro by mixing the proteins KaiA, KaiB, and KaiC, resulting in oscillations in KaiC phosphorylation. The underlying mechanism of this biochemical oscillator has been elusive. Rust et al. (p. 809, published online 4 October; see the Perspective by Poon and Ferrell) now show that phosphorylation of KaiC at two residues is cyclically ordered and that the abundance of each phosphorylated form determines the phase of the oscillator. This sequential phosphorylation of KaiC, combined with negative feedback caused by one of the phosphorylation states, is sufficient to explain stable oscillation.

  15. Shifting Receptor Sensitivities

    AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazoleproponic acid) receptors mediate excitatory responses to neurotransmitters in the brain and in other neurons of the central nervous system. Menuz et al. (p. 815) report that the presence or absence of auxiliary AMPA receptor subunits has a dramatic effect on the pharmacological properties of these receptors. The compound CNQX (6-cyano-7-nitoquinoxaline-2,3-dione) generally acts as a competitive antagonist of AMPA receptors. However, in receptor complexes in which one of the auxiliary subunits known as TARPS (for transmembrane AMPA receptor regulating proteins) is present, CNQX actually functions as a partial agonist of the receptor. The presence of the extra subunit may help couple conformational changes in the receptor ligand-binding domains to opening of the receptor ion channel.

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