This Week in Science

Science  23 Jun 2000:
Vol. 288, Issue 5474, pp. 2093
  1. Bent-Core Liquid Crystals

    In smectic liquid crystals, the molecules (or “mesogens”) organize into layers, and in some cases the molecules adopt a particular orientation or polarization within the layers. Polarization is more likely if the molecules have a net chirality, and the design of ferroelectric liquid crystals (in which layer polarizations are all aligned) usually has relied on chiral molecules. Recently, it was shown that even achiral molecules, if they adopted a “bow” shape, could form antiferroelectric alignments (see the Perspective by Lubensky). Walba et al. (p. 2181) have now designed bow-shaped achiral mesogens that can adopt a ferroelectric alignment by paying careful attention to how the molecules interact between layers. Pratibha et al. (p. 2184) show that achiral bent-core mesogens can be added to an anisotropic smectic liquid crystal to convert the uniaxial material into an orthogonal biaxial smectic liquid crystal, which has been seen previously only in polymeric systems. These results suggest that dissolving bent-core molecules into lipid bilayers may produce unusual membrane order structures.

  2. Making Stronger Polymers

    Polypropylene and polyethylene are among the most widely used plastics in the world, with applications ranging from food packaging to artificial knees. Recent advances in organometallic catalyst development have allowed a high level of control over the properties of the polymers. Chaffin et al. (p. 2187) report that synthesis using such metallocene catalysts leads to a much higher interfacial strength in layered materials. As a result, the layers are more difficult to separate by mechanical means than those synthesized by means of conventional catalysts, an advantageous property for multicomponent composites, which are important in many advanced applications of the polymers.

  3. Gut Reaction

    The large intestine is home to a vast and complex microbial community that is present throughout the life of the human host, and that is important to normal bowel function. Mammals produce large quantities of immunoglobulin A (IgA) from the gut wall to prevent the flora from becoming invasive and to maintain the symbiosis. Despite the volume of IgA produced, little is known about it. Macpherson et al. (p. 2222) have discovered that these nonpathogenic organisms stimulate specific anti-commensal IgA from large numbers of B1 cell-derived plasma cells in the lamina propria of intact, healthy guts, even in the absence of T cell help or of any particular tissue organization. This capacity appears to be an evolutionarily primitive form of specific immune response.

  4. Nearby Asteroids

    Near-Earth asteroids (NEAs) are fragments that have departed from the main asteroid belt, and now cross Earth's orbit, and thus have the potential to collide with us. Bottke et al. (p. 2190) combined numerical simulations of NEA orbits with observations of NEAs from the Spacewatch telescope to estimate the number and distribution of NEAs. They conclude that there are about 750 NEAs with diameters of about 1 kilometer and that the observational surveys have not yet detected many of the NEAs that are in highly eccentric and highly inclined orbits.

  5. The Strain in Grains

    The rate of mountain building is usually described by macroscopic structures such as folds and faults, but with improvements in microanalysis techniques, it is possible to use microscopic structures within the rock fabric to define more accurately the rate of deformation (strain rate). Müller et al. (p. 2195; see the Perspective by Ramsay) used the orientation of millimeter-long curved fibers of calcite, chlorite, and quartz around pyrite grains (called strain fringes) from a thrust fault in the Northern Pyrenees along with Rb-Sr dating of the fibers to determine the strain history of the orogeny. In particular, they noted a sudden increase in the strain rate over a short interval of 2 million years in which the deformation changed from gravitational collapse to horizontal crustal shortening.

  6. Low Latitude Ups and Downs

    The warm climate of the last 12,000 years (the Holocene epoch) has been relatively stable compared to that of the last glacial period, but it is becoming clear that the Holocene has not been without its own thermal ups and downs. The magnitude, timing, and nature of low latitude climate variability is the subject of work by deMenocal et al. (p. 2198). Using a marine sediment core drilled off the coast of West Africa, they show that numerous episodes of abrupt cooling occurred in surface waters, synchronously with millennial-scale variations in the North Atlantic.

  7. About the Origin of Feathers

    The earliest well-developed feathers have been described in Archaeopteryx, a Late Jurassic bird (circa 145 Ma). On the basis of younger fossils, there are suggestions that earlier forms of feathers may have developed in theropod dinosaurs. Jones et al. (p. 2202; see the news story by Stokstad) now suggest that a Late Triassic (220 Ma) archosaur (a primitive reptile) had several long feathers extending from its body. The feather-like appendages seem to preserve the calamus (the hollow base of the feather), rachis (shaft), and barbs. The purpose of these feather-like features is unclear as is their relation to the later feathers seen in birds.

  8. Remodeling on the Run

    Choreographed cell migration forms the basis for normal development of a variety of structures. In the tiny worm Caenorhabditis elegans, the hermaphroditic gonad forms in the path laid down by its lead cell, which migrates back and forth along the body. Nishiwaki et al. (p. 2205; see the Perspective by Hardin) have now identified a protein, MIG-17, that is a component of the surrounding basement membranes and is critical for correct pathfinding by this leading cell. Because activity is dependent on an intact metalloprotease domain, the results suggest that MIG-17 may remodel the basement membrane in the course of pathway guidance.

  9. Seeing the Light

    Rhodopsin is a G protein-coupled receptor that is activated by light to induce the enzymatic cascade leading to vision. Isomerization of the 11-cis chromophore to all-trans triggers the visual transduction process. Borhan et al. (p. 2209) have trapped the photoisomerization intermediates and have traced changes in the interaction between the chromophore and the protein by photoaffinity labeling. They show that the isomerization occurs with minimal movement of the chromophore ring, giving a highly strained intermediate. This energy is used to flip the ring, which is accompanied by conformational changes in the protein.

  10. Evolution of the Circle Game

    Archaebacteria represent a lineage distinct from prokaryotes and eukaryotes and can provide insights into the evolution of fundamental processes. Myllykallio et al. (p. 2212) have found evidence for a single origin of bidirectional replication in the circular genome of several Archaea and a region of termination of replication opposite it. The region surrounding the replication origin did not encode homologs of bacterial replication machinery, but had a cluster of eukaryotic-like enzymes that may function in initiation of replication. There was a lot of genetic variability in the region around the terminus, which is reminiscent of the genome shuffling seen in bacteria.

  11. Events at the Centromere

    At cellular mitosis, duplicated genetic material segregates by attaching to microtubules of the mitotic spindle. This association is facilitated by kinetochore proteins that bridge microtubules to regions in the DNA called centromeres. Takahashi et al. (p. 2215) show that in fission yeast, a histone-like protein called SpCENP-A is important for proper separation of duplicated chromosomes to occur. Its specific localization to centromeres depends on Mis6, another centromere-specific protein.

  12. Choosing a Helper

    Immune responses can be mediated by cells (cell-mediated immunity) or through circulating factors like antibodies (humoral immunity), depending on whether naïve T cells differentiate into T helper 1 (TH1) or T helper 2 (TH2) cells. Li et al. (p. 2219) report that the small guanosine triphosphatase Rac2 increased the expression of interferon-γ (IFN-γ), a cytokine that promotes T cell differentiation, and that it appeared to signal through the mitogen-activated protein kinase p38 and the transcription factor NF-κB.

  13. Selective Memory Loss

    Learning and memory are accompanied by changes in gene expression in the brain. One of the greatest challenges in neuroscience is the elucidation of the signaling pathways that underlie these changes. A clue comes from the work of Garcia et al. (p. 2226), who show that mice deficient in a single transcription factor, NPAS2, are normal in a wide variety of behavioral tests but are impaired in a specific type of long-term memory, as revealed in a cued and contextual fear assay. Thus, NPAS2 is likely to regulate genes required for consolidation of long-term memory.

  14. Time-Variable Cratering Rates?

    Studying the age distribution of spherules from Apollo 14 lunar soil samples, Culler et al. (Reports, 10 March, p. 1785) concluded that “over the last ∼3.5 billion years [Gy], the cratering rate decreased by a factor of 2 to 3 to a low about 500 to 600 million years ago, then increased by a factor of 3.7 ± 1.2 in the last 400 million years.” Hörz points out that the distribution data “might be consistent with a constant impactor flux” over the past 3 Gy if the time evolution of lunar soils is considered. He also notes that, because “lunar soils are the products of stochastic processes,” a local sample “might not faithfully represent the average cratering history through geologic time.” Muller et al. respond that “local effects can indeed distort the local record” and thus that measurements of material from other sites are currently under way. Meanwhile, they argue that, although Hörz's model is “an alternative interpretation of our data that deserves testing,” analysis of the existing data at the improved precision allowed by the Culler et al. study strongly suggests variation in the impact rate over time.The full text of these comments can be seen at http://www.sciencemag.org/cgi/content/full/288/5474/2095a/

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