This Week in Science

Science  29 Oct 2004:
Vol. 306, Issue 5697, pp. 775
  1. Getting from the Tip of the Root


    Auxin hormone signaling normally runs from the tip of a plant to its root and directs a wide variety of developmental events along the way. The polarity of auxin transport is established by the asymmetric localization of the auxin efflux carrier PIN. What causes PIN to be localized to one end of the cell and not the other is now elucidated by Friml et al. (p. 862; see the Perspective by Kaplinsky and Barton). The PINOID kinase controls localization of the PIN auxin efflux carrier, placing it at one end of the cell or the other, controlling the direction of auxin flow.

  2. Seeing Waves in the Background

    The cosmic microwave background (CMB), which peaks at a wavelength of about 1 millimeter, is thought to represent the remnants of a hot big bang. The Cosmic Background Imager, a 13-element interferometer perched at an elevation of 5000 meters on the dry, dark Chajnantor plateau in the Chilean Andes, observed the polarization of this radiation from September 2002 to May 2004. Readhead et al. (p. 836, published online 7 October 2004; cover, see the 8 October news story by Seife) found that the polarization data is consistent with a flat universe dominated by dark energy and dark matter (that is, the standard model). Primordial density fluctuations are adiabatic, in accord with inflationary models.

  3. Bendable Hydrogen Bonds in Water

    Molecular dynamics simulations have usually suggested that water molecules in the liquid states form about four hydrogen bonds (two acceptors and two donors), but a recent x-ray Raman and absorption spectroscopy study in conjunction with density function studies argued that the answer is closer to one donor and one acceptor. Smith et al. (p. 851) measured the near-edge x-ray fine structure spectra of normal and supercooled water between −27°and 15°C, and found that the pre-edge feature can form from structures that only distort the hydrogen bond, rather than break them. The authors argue that only about 27% of the hydrogen bond energy is lost in liquid water compared with a fully tetrahedral structure.

  4. Liquid-Liquid Transitions


    Recent evidence has hinted that a given substance may exhibit coexisting liquid phases, and this possibility has now been confirmed by two studies (see the Perspective by Yarger and Wolf). Katayama et al. (p. 848) used x-ray radiography to observe the formation of a second, denser fluid phase of phosphorus at 1 gigapascal of pressure and 1000°C. Kurita and Tanaka (p. 845) tracked the dynamics for the liquid-liquid phase transformation for triphenyl phosphite at low temperatures (212 kelvin) with phase-contrast microscopy and observed the spinodal decomposition of two coexisting liquid phases.

  5. Lower Mantle Modeling

    Seismic velocities must be combined with other data to determine whether wave speeds are caused by thermal or compositional effects. Trampert et al. (p. 853; see the Perspective by van der Hilst) combined seismic models with gravity constraints to create probability densities for temperature, perovskite, and iron variations. Their results indicate that the lowermost 1000 kilometers of the mantle is predominantly buoyed by chemical variations rather than temperature differences. The dominance of chemical buoyancy will affect dynamic models of convection and hot-spot plume development.

  6. Sowing the Seeds of Life on Dry Land

    Producing seeds is a major innovation that allowed plants to colonize dry habitats. Currently, the earliest known seed plants are Late Devonian (365 million years ago). Gerrienne et al. (p. 856) show the existence of an earlier seed in the Middle Devonian (385 million years ago). Their findings are based on the re-examination of material from Runcaria heinzelinii, a small plant structure first described 35 years ago. The morphology of Runcaria also suggests that the earliest ancestral seed plants were wind-pollinated.

  7. Symmetry and Asymmetry in Development

    Left and right sides of an organism may develop as symmetrical, albeit mirror, images of each other, but some organs, notably our own hearts and livers, are not mirror images. Palmer (p. 828) reviews how this asymmetry forms the basis for intriguing insights into evolution. Genetically defined asymmetry, whereby the balance generally falls to one side, has arisen multiple times through evolution. The underlying processes suggest beginnings in genetic assimilation as well as mutations. Analysis of heart looping in vertebrates indicates which portions of the signaling cascade are more ancestral than others.

  8. Blocking Amyloid Formation

    The inhibition of protein-protein interactions remains one of the “Holy Grails” of chemical biology and drug design. Gestwicki et al. (p. 865; see the news story by Wickelgren) describe a strategy to inhibit the protein-protein interactions that lead to amyloid beta (Aβ) oligomerization and Alzheimer's disease. The approach involves recruiting endogenous chaperones to aggregating Aβ using bifunctional small molecules. Several model compounds were produced that acted as potent inhibitors of Aβ aggregation.

  9. New Light on the Vertebrate Eye

    Two types of photoreceptor cell are found in both vertebrates and invertebrates. The photosensitive apparatus in vertebrates has a sensory cilium, whereas the compound eye of invertebrates is of the rhabdomeric type. Arendt et al. (p. 869; see the news story by Pennisi) now examine the evolution of animal eyes by comparing photoreceptor cell types in Bilateria. The vertebrate eye is compared to the primitive polycheate ragworm Platynereis, a modern descendant of the last common ancestor to insects and vertebrates. Platynereis possesses the rhabdomeric photoreceptor cell type, but also has a brain structure with the ciliary photoreceptor. Thus, an ancestral brain photosensitive complex may have displayed both rhabdomeric and ciliary types. Furthermore, rhabdomeric photoreceptors of the invertebrate eyes most closely relate to the vertebrate retinal ganglion cells, whereas the rods and cones of the vertebrate retina relate to a population of ciliary photoreceptors located in the invertebrate brain

  10. BRCA2, a Tale of Long Division


    Women with mutations in the BRCA2 tumor suppressor gene have a greatly elevated risk of developing breast and ovarian cancers. The BRCA2 protein has been implicated in DNA repair and recombination, but the full spectrum of its cellular activities is still unclear. Daniels et al. (p. 876, published online 16 September 2004; see the 17 September news story by Marx) provide tantalizing evidence that BRCA2 may regulate the fidelity of cytokinesis, the process by which two daughter cells separate at the completion of cell division. BRCA2 deficiency causes delays in cytokinesis and an increase in the frequency of binucleate cells, a measure of defective cytokinesis. This potential role of BRCA2 in daughter cell separation may help to explain why BRCA2-deficient tumor cells often contain an abnormal complement of chromosomes.

  11. Depression Meds Affect Emotional Development

    Inhibition of the serotonin transporter is a principal target of many antidepressants. Using genetic and pharmacological strategies, Ansorge et al. (p. 879; see the news story by Holden) show that the serotonin transporter acts during development to establish normal emotional and anxiety-related behavior later in life. Transient exposure to fluoxetine (Prozac)—the most commonly used antidepressant—during early development produces abnormal emotional behavior in adult mice that mimics the behavior of mice lacking the serotonin transporter.

  12. Stress Interferes with Working Memory

    The prefrontal cortex, located just above the eyes, is necessary for the higher order executive functions of the brain, including planning and working memory. Birnbaum et al. (p. 882) studied rats and monkeys performing tasks that require intact working memory. High activity of an intracellular signaling enzyme, protein kinase C (PKC), disrupted the animals' ability to remember. A cellular correlate of working memory, measured by recording neuron's electrical activity, showed this same sensitivity to high PKC. Because stress causes release of norepinephrine, a known activator of PKC, stress may exert its well-established inhibitory effect on prefrontal cognitive function through PKC.

  13. Kinetic Ping Pong

    Thiamine-dependent enzymes use thiamine as a cofactor to catalyze a broad range of important reactions. The enzymes display “ping-pong” kinetics, characterized by two successive half reactions with the first product released before the second substrate is bound. Now, based on crystallographic and mutagenesis data, Frank et al. (p. 872; see the Perspective by Jordan) suggest a mechanism for synchronizing the two active sites in the E1 homodimer of pyruvate dehydrogenase. The two active sites are linked by a conserved acidic tunnel and, by shuttling a proton through the 20 angstrom tunnel, the cofactors act as each other's general acid or base, thus ensuring a coordinated progression of catalysis.

  14. You Scratch My Back...

    The existence of cooperation between unrelated individuals remains a central conundrum in evolutionary biology. Doebeli et al. (p. 859) show that by gradual evolution of the level of cooperative investment, a polymorphism of cooperators and defectors can emerge in the Hawk-Dove (or Snowdrift) game. This result is fundamentally different from those of the classic game theoretical models, which show only that a cooperator and a defector can coexist provided that these alternatives already exist. Classic game theoretical models consider the frequency of cooperators and defectors as the evolutionary variable, whereas in the present model, the level of cooperativeness itself is evolving. Such modeling has implications for the evolutionary dynamics of cooperation and cheating in organisms, and the emergence of cultural diversity in humans.

  15. Cyclin into S Phase

    Cyclin E functions as the activating subunit of the cyclin-dependent kinase2 complex, which regulates the initiation of DNA synthesis (S phase) in proliferating cells. However, the phenotypes of cyclin E knockout mice are not identical to those of Cdk2 knockout mice, and so cyclin E may have other functions independent of its interaction with Cdk2. Matsumoto and Maller (p. 885) report the identification of one such function. Cyclin E was localized to the centrosome through a centrosomal localization signal (CLS), a 20-amino acid portion of the molecule. Expression of a CLS peptide that competes with the endogenous protein for binding to the centrosome, or expression of full-length cyclin E with a mutated CLS inhibited the effect of cyclin E DNA synthesis. Mutations that block interaction of cyclin E with Cdk2, however, did not hinder cyclin E's ability to promote DNA synthesis.