This Week in Science

Science  22 Jan 1999:
Vol. 283, Issue 5401, pp. 457
  1. Attractive Calculations

    Many astronomical observations are difficult to model because of the need to compute the gravitational interactions between millions of particles on a wide range of scales and dimensions. Hut and Makino (p. 501) review some key astrophysical results obtained on a series of specialized computers, called Gravity Pipe (GRAPE), that do only the detailed gravitational force interaction calculations between as many as one billion particles, while standard computers handle all the other instructions in the program. The GRAPE family of computers, among the fastest in the world, have refined the physical picture of core collapse in a dense globular cluster, the spiraling of two black holes when galaxies collide, and the process of planet formation from the collision of particles to the collision of planetesimals.

  2. Nanotube Arrays

    Recent advances in nanotube synthesis have allowed the growth of large areas of aligned nanotubes. Fan et al. (p. 512) have moved a step further by showing that through catalyst patterning of the surface prior to nanotube growth, patterns of nanotube blocks can be fabricated with a high degree of control. These blocks are shown to have advantageous electron field emission characteristics, such as low operating voltages and high current stability, that could be used in devices.

  3. El Niño History

    How long has El Niño been operating and affecting climate along the Pacific margin? Rodbell et al. (p. 516; see the news story by Kerr and the Perspective by Sandweiss et al.) present a 15,000-year record from an alpine lake in Ecuador that may provide a clue. The lake sediments contain periodic debris flows that mark times of excessive rainfall into the lake basin. In modern times, the occurrence of these debris flows generally coincides with heavy rains brought about by El Niño. About 5000 years ago, the frequency of debris flows increased to a value near that of the present El Niño frequency.

  4. Upon Reflection

    Antireflective coatings for glass surfaces not only limit unwanted reflections but improve the performance of optical systems by increasing transmission. However, for high performance, materials with extremely low refractive indices are needed. Walheim et al. (p. 520), borrowing from the approach originally taken by Fraunhofer in 1819, create a porous material, thus lowering the dielectric by adding air. Whereas Fraunhofer etched his glass, the authors create nanoporous polymer films. Mixtures of polystyrene and poly(methyl methacrylate) that were spin-coated onto glass were phase-separated, and selective removal of one of the polymers created a low-refractive-index antireflective film. The method was applied to multilayer films and could be extended to mechanically harder materials such as fluoropolymers.

  5. The Soft Side of Dinosaurs

    Most analysis of physiology of extinct animals such as dinosaurs has been based on the fossil record of hard tissues and analysis of presumed extant relatives and trace fossils. Some new rare dinosaur fossils are now providing a direct view of the soft tissues and internal organs and thus of their physiology. Ruben et al. (p. 514; see the news story by Wuethrich) describe some of the remarkable features of the small theropod Scipionyx. Ultraviolet imaging was used to infer the position and extent of the liver and muscles of the visceral cavity. The authors conclude that this dinosaur used a diaphragm-assisted lung ventilation system and had an unusual pattern of exercise physiology.

  6. Stitching Together Polymer Clusters

    Highly anisotropic polymer molecules can now be synthesized by partial cross-linking of smaller oligomers. Zubarev et al. (p. 523) synthesized oligomers in which a coiled styrene block is connected through a short polybutadiene block to a rigid rodlike block. Steric blocking of the polystyrene coils appears to inhibit reactivity of the polybutadiene groups; heating at 250°C of the liquid-crystalline oligomer does not form a fully cross-linked gel but instead produces a polymer with a molecular weight distribution that is highly peaked around 70,000. The polymer objects formed have an anisotropic shape (2 nanometers by 8 nanometers) that leads to even stronger liquid crystalline behavior relative to the precursor oligomer.

  7. Minding the Mix

    Atmospheric and oceanic processes are coupled largely through the upper 100 meters or so of the ocean, where surface heating or cooling, winds, and surface waves contribute to turbulence and water mixing processes. Many of the upper ocean mixing processes are not yet understood in detail. Rudnick and Ferrari (p. 526; see the Perspective by Schmitt) have performed high-resolution measurements in the upper ocean of temperature and salinity simultaneously on length scales from 20 meters to 10 kilometers. Analysis of these data show that horizontal mixing activated by density gradients dominates in the mixed layer, and temperature and salinity gradients compensate on all length scales in their effect on density.

  8. Molecular Redeployment

    Eyespots on butterfly wings are a relatively recent adaptation to avoid predators. Keys et al. (p. 532) show that the developmental organizer, the focus, that controls this pigmentation recruited many of the regulatory molecules already involved in wing development, such as Hedgehog. Cassettes of existing molecular instructions may be co-opted with modification by new structures during evolution, eliminating the need to reinvent entire regulatory pathways each time a new physical feature evolves.

  9. Nerve Begats Blood

    Progenitor cells may face various restrictions on what sorts of cells and tissues they can generate. However, Bjornson et al. (p. 534; see the news story by Muro) show that adult neural stems cells may be considerably less restricted. When injected into irradiated mice, these cells were able to generate hematopoietic cells. Thus, the developmental boundaries between the neuroectoderm and the mesoderm may be more negotiable than once thought.

  10. Tobacco Syntaxin

    With an anchor in the plasma membrane attached to protein-interaction domains, a protein identified in tobacco by Leyman et al. (p. 537) shows similar structure and molecular functions to syntaxins known from yeast and mammalian cells. However, instead of aiding vesicle trafficking as other syntaxins do, this tobacco syntaxin mediates the cellular response to the plant hormone abscisic acid. The roles of vesicle trafficking in abscisic acid signaling, or of syntaxin functions other than vesicle trafficking, remain obscure but intriguing.

  11. Inhibited Rhythms

    Patterns of synchronous neuronal firing can be found in epilepsy and also under normal physiologic conditions like sleep. The reticular nucleus of the thalamus generates a recurrent inhibitory output that may be important in modulating rhythmic and oscillatory firing of neurons. Huntsman et al. (p. 541) analyzed knockout mice that are missing the β3 subunit of the γ-aminobutyric acid type A (GABAA) receptor. This particular subunit is only expressed in specific locations in the brain, such as the thalamic reticular nucleus. In slices from knockout mice, GABAergic inhibition in the reticular nucleus was severely impaired and hypersynchronous neuronal activity could be observed. Thus, lateral inhibition serves as an important mechanism in shaping synchronous neuronal output.

  12. Avoiding Accidental Death

    Normally the tumor necrosis factor receptor type 1 (TNF-R1) requires a trimer of its ligand, TNF, to aggregate the receptor and activate signal transduction through the TNF-R1 death domain. However, overexpression of TNF-R1 induces its activation in the absence of ligand. Jiang et al. (p. 543) detected a protein, called silencer of death domains (SODD), that was constitutively bound to the TNF-R1. After binding of TNF, SODD was released and signaling proteins were then bound. SODD may normally function to prevent the accidental activation of TNF-R1 in the absence of ligand. Overexpression of TNF-R1 may overwhelm limiting quantities of SODD and lead to inappropriate activation.

  13. Predicting Good Performance

    The multiplicity of ion channels in neurons and of neurons in the brain often renders modeling of behavior on the basis of molecules, an open-ended endeavor. Usher et al. (p. 549) have focused on locus coeruleus neurons in monkeys undertaking a visual discrimination task. They observe fluctuations in neuronal activity plausibly associated with behavioral performance. Through modeling, the authors were able to ascribe these fluctuations to changes in gap junction pathways. They then obtained experimental data supporting the proposal that these pathways underlie the neuronal activity that, in turn, influences visual attentiveness.

  14. The Rich Stay Rich

    One explanation for the richness of species in tropical forests is the intermediate disturbance hypothesis: Local disturbances of the forest initiates a sequence of replacement species that ends with canopy trees, and if disturbances occur at an appropriate rate, then the forest will be a patchwork of different succession states. However, an experimental test has found this theory wanting. Hubbell et al.'s (p. 554; see the cover and the Perspective by Tilman), 13-year study of seedling and sapling establishment in natural gaps in a Panamanian tropical forest found that gaps are simply filled by whatever species are locally present. Thus, a species-rich forest is maintained not by dispersal and competitive traits but by the default process of being in the right place at the right time.

  15. Caught Freezing

    Linear alkanes are among the simplest organic molecules, but the melting and freezing behavior of the longer alkanes can be puzzling. An “odd-even” effect seen in their melting temperatures is less pronounced in their freezing temperatures because the alkanes with an even number of carbon atoms supercool and the “odds” do not. Sirota and Herhold (p. 529) used time-resolved x-ray scattering to catch C16 and C18 forming transient rotator phases. Crystallization of the stable triclinic solid is mediated by these transient phases.

  16. Silencing in Plasmid Partitioning

    As with eukaryotic chromosome segregation, plasmid partitioning divides genetic elements among daughter cells. The centromere and transcription silencing around the centromere have been shown to function in chromosome segregation. Transcription silencing is also found in plasmids. Rodionov et al. (p. 546) have now examined the elements and mechanism of centromeric silencing in the P1 plasmid. The protein ParB specifically associates with the silenced region of DNA, and the DNA element ParS acts as a nucleation site for ParB polymerization. Because silencing mutants are also defective in partitioning, silencing may be necessary for plasmid partitioning to occur.

  17. Proofreading by Isoleucyl-Transfer RNA Synthetase: Response

    In their report of 24 April 1998, O. Nureki et al. described the structure of isoleucyl-transfer RNA (tRNA) synthetase. In his technical comment of 11 December 1998, Xiayang Qiu criticized figure 4 and its discussion in the report, which was a” ‘stereoview of the IleRS editing site as a ball-and-stick representation.’ “In their response published today, Nureki et al. provide clarification and further details of their analysis and state that their “most recent experiments have confirmed the importance for editing of this region of [the insertion domain] CP1.” They say that “the outstanding problem in the field is to determine how misactivated substrates move from the active site to the editing site. Most likely, a significant conformational change is involved. “The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/283/5401/459a