This Week in Science

Science  15 Dec 2000:
Vol. 290, Issue 5499, pp. 2029
  1. Parity Violation and Strange Magnetism

    The magnetic properties of protons find widespread use in techniques such as magnetic resonance imaging, but there remains a lack of understanding of the fundamental physics underlying the generation of the proton's magnetic moment based on its internal structure. Hasty et al. (p. 2117; see the Perspective by Rosner) used the preferential scattering by protons of electrons with particular polarizations to probe strange quark interactions within the nucleus. Their results place constraints on the contribution of the strange quark-antiquark interactions to the proton magnetization to be −0.1 ± 5.1% of its total. The authors also present evidence for the existence of the anapole moment of the proton.

  2. Densely Packed Magnetic Nanowires

    One route for making metallic nanowires is to grow them electrochemically in a nanoporous template. Although several types of templates can be made, it is often difficult to create templates that have narrow pores that are also closely and regularly spaced. Thurn-Albrecht et al. (p. 2126; see the news story by Service) field-aligned micrometer-thick block copolymer films of poly(methyl methacrylate) (PMMA) nanocylinder arrays within a polystyrene (PS) matrix; ultraviolet exposure removed the PMMA and cross-linked the PS. They could then grow densely packed, high-aspect ratio nanowires of copper or cobalt (∼2 × 1011 wires per square centimeter) and observed enhanced coercivities for the ferromagnetic cobalt wires compared with a continuous cobalt film of the same thickness.

  3. Indy-Dependent Life-Style

    Aging and life-span are still poorly understood aspects of basic biology, although it is widely accepted that genetic factors play a role. To identify specific genes that influence life-span, many researchers have turned to model organisms such as yeast, worms, and flies. Rogina et al. (p. 2137; see the news story by Pennisi) have found that altered expression of a single gene in the fruit fly Drosophila nearly doubles the life-span of the flies without adverse effects on fertility or physical activity. This gene, called Indy (for I'm not dead yet), encodes a protein with sequence homology to mammalian sodium dicarboxylate cotransporters, transmembrane proteins that transport Krebs cycle intermediates across the plasma membrane. Based on this sequence homology and the gene's expression pattern in the flies, the authors postulate that Indy affects life-span by altering absorption and utilization of metabolites, perhaps creating a metabolic state similar to caloric restriction.

  4. Nanowires Ready for Lift-Off

    Flexible wire interconnects for nanodevices appear to be in reach. Zach et al. (p. 2120; see the cover) have made molybdenum wires on graphite that range from 15 nanometers to 1 micrometer in diameter and are half a millimeter in length. They could control the wire's diameter during its electrodeposition as an oxide, which was later reduced to the metal with hydrogen. The wires can be lifted off in polystyrene films and exhibit metallic conductivity—and they can be bent like the bulk metal. The method is applicable to other non-noble metals such as nickel.

  5. Better After a Stretch

    One potential use for self-assembled monolayers is as coatings for the tailoring of wetting behavior. However, for some materials, such as elastomeric silicones, it has proven difficult to generate sufficient attachment points for molecules to achieve packing densities high enough to change their surface properties. Genzer and Efimenko (p. 2130) show that by first stretching a silicone and then generating additional surface hydroxyl groups with ultraviolet light exposure and ozone, they could create high surface densities of semifluorinated hydrocarbon chains. The treated silicones retained high hydrophobicity and avoided surface reconstruction even after being stored under water for a week.

  6. Twisting and Pushing

    The bacterial flagellum is composed of subunits that are inserted at its tip. The basic mechanism of growth involves the transfer of subunits through a hollow channel along the flagellum, but how assembly at the tip is controlled has not been obvious. Yonekura et al. (p. 2148; see the Perspective by Macnab) evaluated the process using image reconstruction of electron micrographs. Their results yield startling information on how the cap structure at the tip of the flagellum appears to rotate to allow the insertion of new subunits while always maintaining at least one point of contact with the flagellum.

  7. Lessons from Arabidopsis Genomics

    Analysis of the genome of the tiny Arabidopsis plant is revealing new insights not only into plants and their evolution, but also into relations across kingdoms of organisms (see the Policy Forum by Somerville and the news stories by Pennisi and Mlot). The transcription of certain genes changes in a regular pattern throughout the circadian cycle. Harmer et al. (p. 2110) analyzed 8000 Arabidopsis genes with oligonucleotide microarrays and found that about 6% showed daily cycles of expression. Entire metabolic pathways exhibit coordinated circadian rhythms of their components. In this comparative survey of genomes, Riechmann et al. (p. 2105) analyzed major families of transcription factors. Comparisons of whole-genome sequences from Arabidopsis, Caenorhabditis elegans, Drosophila, and yeast show that some families of transcription factors are held in common across kingdoms, whereas other families appear to belong to only one kingdom. For those factors that are represented in all of the kingdoms, the DNA binding domain shows the most similarities—but these factors can show divergent functions. A comprehensive analysis of the Arabidopsis genome by Vision et al. (p. 2114) shows that this tiny plant went through an age of multiple, large-scale genome duplications before settling down into its current phase of relative stability. Chromosome fusions, inversions, and translocations have also helped shape the current Arabidopsis genome.

  8. Bacterial Cell Cycles

    The array of genes that are expressed during the life cycle of a cell can be pictured as a complex genetic circuit. Laub et al. (p. 2144) provide a global survey of the part of that circuit that is expressed during the cell cycle of Caulobacter crescentus, a bacterium that differentiates during the course of its life. Out of nearly 3000 genes, 553 show cell cycle regulation. Temporal controls over genes involved in specific cell functions and coordinate regulation of proteins that are part of complexes were also observed. CtRA was shown to be a central regulator of the cell cycle in experiments with overexpressing or loss-of-function mutants. A histidine kinase and two RNA polymerase sigma factors were identified as possible regulators of early S phase.

  9. Brain-Building MHCs

    In some ways, the nervous system and the immune system solve similar problems: They both have to distinguish and respond to an extremely large array of input from the external world, and both are exceedingly complex. Huh et al. (p. 2155; see the news story by Helmuth) show that class I major histocompatibility complex (MHC) molecules, used by the immune system to respond to antigens, are also necessary for accurate assembly of the brain. In mice genetically deficient for class I MHC molecules, the neural connections between the retina and their targets in the central nervous system are abnormal. Long-term potentiation, a form of cellular learning, is enhanced, and another form, long-term depression, is eliminated. The diversity and specificity of class I MHC molecules makes them attractive candidates for a role in establishing neural connections.

  10. Getting Warmer

    Global annual mean near-surface air temperature increased during the 20th century in two major steps, the first between roughly 1910 and 1940 and the second (which is still continuing) after about 1975. It has been difficult to understand the causes of this overall rise, partly because anthropogenic forcing by fossil fuel combustion has grown steadily during that interval and partly because it was not as important a forcing factor in the first half of the century as in the second. Stott et al. (p. 2133; see the Perspective by Zwiers and Weaver) have used a state-of-the-art climate model, HadCM3, to examine the reasons for this increase. An ensemble of four simulations of the last 140 years indicates that a combination of natural climate variations and human-induced variability can explain the observed temperature rise, and that most of the multidecadal-scale global variations are not due to internal variability of Earth's climate system, but are externally forced.

  11. Dendritic Cell Origins

    The expression of CD8α—a cell surface molecule ordinarily found on cytotoxic T cells—has long been used to discriminate between two anatomically and functionally distinct subsets of dendritic cells (DCs). Much debate has revolved around whether each class of DC is derived from a discrete lineage of progenitors. It has been proposed that CD8α DCs are derived from a myeloid progenitor and that the less abundant CD8α+ DCs are lymphoid in origin. Thus, some of the differences in physiology of these cells have been attributed to these alternative pathways of development. Using transfer of clonogenic common myeloid progenitor cells, Traver et al. (p. 2152) demonstrate that both CD8α+ and CD8α can develop from a common myeloid ancestor. The distinct biology of each type of DC must depend on factors other than early differences in lineage commitment.

  12. The Pros and Cons of Transgenic Plants

    The use of transgenic plants in agriculture and food crops is a matter of current and energetic public debate. In a review article, Wolfenbarger and Phifer (p. 2088) work through the complex suite of arguments for and against and provide a thoughtful synthesis of what is, and is not, known about the risks attendant to use of genetically modified plants.

  13. Inkjet Printing of Organic Electronics

    Although inkjet printing has been used to fabricate low-cost organic light emitting diodes and displays, its application to more complex thin film transistors (TFTs) has been hampered by its resolution limit (∼50 micrometers). In order to maintain sufficient drive-current and switching times and reasonable turn-on voltages in polymer TFTs, channel lengths less than 10 micrometers are needed. Sirringhaus et al. (p. 2123) introduce a method using a prepatterned substrate in which the hydrophobic properties of polyimide are used to define the critical features. They demonstrate the ability to fabricate TFTs with 5-micrometer channel lengths and on-off ratios in excess of 105 at operating voltages of 10 volts.

  14. An Orphan No More?

    Nuclear receptors are a superfamily of proteins that function as ligand-activated transcription factors. Among these proteins are the so-called “orphan receptors,” such as the retinoid X receptor (RXR), for which the endogenous activating ligands have not been identified. Using a cell-based screening assay, Mata de Urquiza et al. (p. 2140) show that docosahexaenoic acid (DHA), an abundant polyunsaturated fatty acid in mammalian brain, is a potent activator of RXR. DHA deficiency has previously been linked to defects in spatial learning capacity in rodents. The new data raise the possibility that DHA's effects are mediated through an RXR signaling pathway.

  15. Genetic Polymorphism in CX3CR1 and Risk of HIV Disease

    Faure et al. (Reports, 24 March, p. 2274), in a study of French Caucasian patients from three HIV-infected cohorts, found that patients homozygous for a specific single nucleotide polymorphism (SNP) in the chemokine receptor CX3CR1—which is also a coreceptor for human immunodeficiency virus (HIV)—progressed to AIDS more rapidly than did those with other haplotypes. In a comment, a group including the senior author of the Faure et al. study report that they were unable to confirm these associations in a subsequent study of three North American cohorts. They suggest a number of possible explanations for the discrepant results, including the comparatively small number of patients homozygous for the suspect allele in both studies and known compositional differences (in characteristics such as gender, HIV risk category, and median length of patient follow-up) between the French and North American cohorts. “Nonetheless,” the comment authors conclude, the results of the two studies, “taken together, do not support a clear and consistent role for CX3CR1 in HIV pathogenesis.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/290/5499/2031a

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