Editors' Choice

Science  24 Jan 2003:
Vol. 299, Issue 5606, pp. 475
  1. MOLECULAR BIOLOGY

    Translation by Entrapment

    1. Paula A. Kiberstis

    A group of relatively obscure plant viruses captured the attention of molecular biologists some 30 years ago when it was discovered that their RNA genomes contain a 3'-terminal transfer RNA (tRNA)-like structure (TLS) that can be aminoacylated. Work by Barends et al. now reveals a fascinating role for tRNA mimicry in the viral life cycle.

    Studying turnip yellow mosaic virus, whose proteins are encoded within a single polycistronic messenger RNA (mRNA), they find that the TLS appears to entrap ribosomes in a manner that promotes internal initiation of protein synthesis on the mRNA. This yields a viral RNA replication polyprotein, which incorporates the TLS-bound amino acid (valine) at its N-terminus instead of the customary methionine. From the perspective of the virus, changing the efficiency of this internal initiation mechanism would not only affect the relative levels of the viral proteins, but also alter the balance between viral mRNA translation, RNA replication, and virion assembly. In a broader sense, this finding raises the intriguing possibility that the TLS is a molecular fossil of an ancient RNA world in which chimeric RNAs with properties of both tRNA and mRNA functioned as protoribosomes. — PAK

    Cell112, 123 (2003).

  2. CHEMISTRY

    Hydrogen Bonding to Metal

    1. Phil D. Szuromi

    The adsorption of water molecules on the flat, close-packed surfaces of metals was thought to be simple and well understood. The first layer partially covers the surface and binds through the lone electron pairs of the oxygen atoms, whereas the next layer attaches via hydrogen bonds to these water molecules. However, recent results have suggested that, on the (0001) surface of ruthenium, water can partially dissociate and form a mixed network of water and hydroxyl molecules.

    Ogasawara et al. have combined soft x-ray spectroscopy and density functional theory calculations to show that, on the (111) surface of platinum, half of the water molecules of the first layer bind through their lone pairs, but the other half bind to the metal surface through their hydrogen atoms. The presence of unusual metal-hydrogen bonds to water molecules, in the absence of free hydroxyl, challenges current models of water adsorption on metals and should help further the understanding of processes such as corrosion and the photodissociation of water. — PDS

    Phys. Rev. Lett.89, 276102 (2002).

  3. CELL BIOLOGY

    Little and Large

    1. Stella M. Hurtley

    During neuroblast development in Drosophila, an uneven or asymmetric division occurs, producing daughter cells with different sizes that are destined for different fates. The larger apical cell carries on dividing to produce more of the smaller cells, which differentiate to form ganglion mother cells and, after one further division, two neurons or glial cells. Cai et al. studied the mechanisms involved in asymmetric division and found that mitotic spindle geometry (and thus the unequal daughter cell size) relies on two sets of signaling pathways within an apically localized protein complex. One involves Bazooka and protein kinase C, whereas the other involves Partner of Inscuteable and GMαi. Each pathway can promote asymmetric division, but if both pathways are blocked, symmetric divisions ensue. In the sensory organ precursor cells, both pathways are also active, but they cancel one another because of their localization to opposite sides of the cells. — SMH

    Cell112, 51 (2003).

  4. GEOPHYSICS

    Bohemian Rhapsody in the Crust

    1. Linda Rowan

    Swarms of earthquakes with small to moderate magnitudes are often associated with volcanic activity. In the Vogtland/northwest Bohemia region of the western Bohemian Massif along the Germany-Czech Republic border, major swarms with a periodicity of 74 years have been recorded since the 1550s, yet there has been no volcanic activity in this area for 100,000 years.

    A special issue introduced by Jentzsch et al. synthesizes recent measurements of the 10,000 events during the autumn of 2000 and provides a better understanding of this recurrent crustal rhapsody. Enhanced seismic and geodetic data, as well as stress-field modeling, indicate that the swarms are driven by magmatic fluid migration and degassing along specific areas of fault zones. Perhaps most intriguingly, a hydrologic study by Koch et al. showed changes in water well levels and mineral springs (Bad Brambach) chemistry over a 4- to 5-week period before the seismicity began, due to an unusually prolonged buildup of fluid pressure. During and after the swarm, a large flux of CO2 degassing was detected south of the fault zone. — LR

    J. Geodynamics35, 1; 107 (2003).

  5. IMMUNOLOGY

    Remodeling One's Residence

    1. Stephen J. Simpson

    Tuberculosis (TB) remains an urgent global health problem, rivaling HIV as a cause of human mortality. Resistance to TB depends on cellular immunity to Mycobacterium tuberculosis, and disease readily takes hold in individuals where this has been compromised by other factors, such as coinfection with HIV.

    Geijtenbeek et al. and Tailleux et al. provide evidence that M. tuberculosis binds to the dendritic cell-specific intercellular adhesion molecule-3 grabbing nonintegrin (DC-SIGN), which is a C-type lectin that recognizes adhesion molecules involved in the regulation of dendritic cell migration and T cell activation. Both groups observed that mycobacteria entered dendritic cells through engagement of DC-SIGN by a mycobacterial cell wall component, lipoarabinomannan (LAM). Geijtenbeek et al. also found that the interaction between DC-SIGN and LAM induced a counterinflammatory state, in which dendritic cells did not become activated and were stimulated to produce interleukin-10, which can contribute to the suppression of normal T cell responses and the establishment of persistent infection. — SJS

    J. Exp. Med.197, 7; 121 (2003).

  6. CHEMISTRY

    Made to Order Even More

    1. Marc S. Lavine

    In smectic-phase liquid crystals, rodlike molecules tend to pack parallel to one another in a layered structure. When a semifluorinated chain is attached near the middle of a rodlike aromatic molecule with polar endgroups, a smectic phase can still form, with the molecules lying flat in the layers. Prehm et al. show that if instead a branched chain that bears both fluorinated and hydrocarbon branches is added, additional ordering occurs to create a two-dimensional lattice. Upon cooling from the isotropic melt, indications of a columnar structure were observed in both polarized light microscopy images and x-ray diffraction patterns. Ordering between adjacent layers appears to be caused by the separation of the perfluorinated and linear hydrocarbon units, which prefer to pack with like units only. Analysis of the small-angle x-ray reflections indicated that the secondary packing leads to a noncentered rectangular lattice. — MSL

    J. Am. Chem. Soc. 10.1021/ja0289999 (2002).

  7. CLIMATE SCIENCE

    Warming and Rising

    1. H. Jesse Smith

    During the midst of the last deglaciation, almost 15,000 years ago, sea level rose extremely rapidly during an episode of rapid glacial melting that is called meltwater pulse (MWP) 1a. At approximately the same time another dramatic event, an abrupt increase in Northern Hemisphere air temperature called the Bølling warming, occurred. The relative timing of these two episodes has been a point of active debate and must be resolved in order to develop a mechanistic understanding of the oceanographic, glaciological, and climatic changes that occurred during this period.

    Kienast et al. provide a better picture of the phase relation between MWP 1a and the Bølling warming by examining a marine sediment core from the northern South China Sea. This core reveals that a rapid drop in the supply of terrestrial plant matter during the last deglaciation was paralleled by an equally rapid increase in sea surface temperatures corresponding with the Bølling warming 14,700 years ago. They interpret this sudden drop in terrigenous organic matter delivery as a short-term response of local rivers to rapid sea-level rise, implying that the Bølling warming and the onset of MWP 1a were synchronous. This result indicates that previous studies postulating a weakening of deep-water formation in the North Atlantic due to massive meltwater discharge during MWP 1a need to be reformulated. — HJS

    Geology31, 67 (2003).

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