Editors' Choice

Science  16 Apr 2004:
Vol. 304, Issue 5669, pp. 361

    Return of the Ring

    1. Andrew M. Sugden

    A classic example of an evolutionary pattern may need revision after a new analysis based on mitochondrial DNA. In the ring species model, an organism expands its geographical range around an uninhabitable area, so that when the extremities of the population finally meet, they have diverged sufficiently to become reproductively isolated from each other (“isolation by distance”). An oft-cited example is the herring gull complex, which contains more than 20 species and subspecies of large gull distributed around the Northern Hemisphere. According to the original ring species model, the modern complex originated in the Caspian/Aral Sea region and spread northwest to Scandinavia, Britain, and France, giving rise to lesser black-backed gulls, and eastward to Siberia, Alaska, and North America, giving rise to herring gulls. Finally, the North American herring gulls were thought to have spread eastward across the Atlantic, where they coexist with but are reproductively isolated from the European black-backed gulls.

    A phylogeographic analysis by Liebers et al. suggests that the picture is more complex. Rather than evolving via a series of interconnected populations, the gull taxa differentiated through events involving long-distance dispersal, isolation, and fragmentation of populations. The DNA evidence indicates that the ring was never closed by colonization of Europe from North America; in fact, lesser black-backed gulls are currently expanding westward and may close the ring in the opposite direction. — AMS

    Proc. R. Soc. London Ser. B 10.1098/rspb.2004.2679 (2004).


    Saving Nickels and Coppers

    1. Marc S. Lavine

    The expensive metals palladium and platinum are used to catalyze many chemical reactions. Because reactions occur only at the surfaces of catalysts, nanoparticles are of interest because they offer a high surface-to-volume ratio and because it may be possible to use a cheaper metal for the core. For bimetallic particles including copper or nickel, palladium often ends up in the nanoparticle core, probably because Pd ions are more readily reduced.

    Son et al. have developed a metal-surfactant decomposition reaction that leads to Pd-coated Ni nanoparticles. When they heated a mixture of metal-trioctylphosphine complexes (Pd-TOP and Ni-TOP), they observed that Ni-TOP decomposed at a lower temperature, resulting in Ni nanoparticles onto which Pd deposited as the temperature was raised. Catalytic activity was assessed for the Sonogashira coupling reaction, which involves the coupling of terminal alkynes with aryl or vinyl halides. At equal amounts of Pd, the Ni/Pd core/shell particles showed greater reactivity than those of pure Pd and could be recycled many times. — MSL

    J. Am. Chem. Soc. 10.1021/ja039757r (2004).


    Snapping Up Sulfate

    1. Phil D. Szuromi

    Sulfate-binding proteins hold their ligands by means of a large number of neutral hydrogen-bonding interactions, but designers of small synthetic receptors generally have had to resort to electrostatic interactions to capture the sulfate ion. Bondy et al. have modified a system for binding sulfate that relies on Pt2+ centers (which provide an electrostatic interaction) that sport four nicotinamide arms (which form hydrogen bonds). Now, by using iso-quinoline ligands derivatized with urea groups, they can stabilize sulfate binding in deuterated dimethyl sulfoxide (DMSO-d6) enough so that they can follow anion binding on the nuclear magnetic resonance time scale. Halides formed 2:1 complexes with these receptors, but the four urea moieties converged on a single sulfate ion in a 1:1 complex featuring eight hydrogen bonds, as revealed by x-ray crystallography. The association constant for sulfate in DMSO exceeded 105. — PDS

    J. Am. Chem. Soc. 10.1021/ja039712q (2004).


    Recombination Combine

    1. Stephen J. Simpson

    The resolution of DNA double-strand breaks (DSBs) is regulated in a context-dependent fashion that guards against aberrant genetic recombination. For example, although homologous recombination of DSBs is permitted during meiosis, it would be inappropriate in the somatic V(D)J recombination events that occur at the immunoglobulin loci. Instead, nonhomologous end joining (NHEJ) of DSBs takes place between V(D)J segments. This process involves formation of a postcleavage complex containing the DNA ends and the recombination activating gene (RAG) proteins.

    Using an experimental system in which in vivo NHEJ and homologous recombination could be measured, Lee et al. found that expressing mutant RAG proteins (impaired in joining activity) produced a significant number of homologous recombination and alternative NHEJ events. These mutants formed unstable postcleavage complexes, suggesting that instability of the complex might render V(D)J intermediates available for inappropriate end-joining events; under normal circumstances, RAG proteins would guide DSB ends toward the NHEJ pathway. These results raise the possibility that RAG proteins could stimulate homologous recombination via single-strand nicking of DNA, perhaps leading to chromosomal translocations. — SJS

    Cell 117, in press (2004).


    Putting Sugar on Chips

    1. Gilbert J. Chin

    Microarrays have transformed experimentation by allowing molecular catalogs to be searched in parallel, instead of serially. Constructing these catalogs is routine for nucleic acids and becoming easier for proteins, yet carbohydrates are a rather different kettle of fish. Effort thus far has focused on chemical synthesis of oligosaccharides and their attachment (via thiols or amides) to suitably modified surfaces.

    Park et al. describe a combined chemical and enzymatic approach to more lifelike sugar chains, using three successive glycosylations to make sialyl Lewis × (Lex). At this rate, it might not be far-fetched to imagine a chip displaying the complement of cell surface carbohydrates. Why this might be useful is aptly illustrated by the demonstration by Matrosovich et al. that avian influenza targets ciliated cells in human airway epithelium, whereas human influenza infects the nonciliated cells; the former express 2–3 linked sialic acids, and the latter, 2–6 linked sialic acids. — GJC

    J. Am. Chem. Soc. 10.1021/ja0391661 (2004); Proc. Natl. Acad. Sci. U.S.A. 101, 4620 (2004).


    Emollient Eicosonoids

    1. Lisa D. Chong

    Cystic fibrosis (CF) is an inherited condition that causes progressive lung disease. Although functionally deleterious mutations in the chloride ion transporter CFTR have been identified in CF patients, the sodium and chloride concentrations in their airway surface fluid can be normal, indicating that faulty ion transport is not the whole story.

    Karp et al. suggest that anti-inflammatory lipids called lipoxins may link CFTR mutations to CF pathogenesis in the lung. Lipoxins are synthesized by airway epithelial cells and neutrophils in response to infection and are crucial mediators of inflammatory response maturation. Many CF patients suffer from chronic bacterial infections and inflammation. Lipoxins inhibit the acute neutrophil response and also activate macrophages during the second phase of bacterial clearance. In CF patients, however, lipoxin levels in airway fluid are reduced, which may allow excessive neutrophil-mediated damage of the epithelium and subsequent bacterial proliferation. In a mouse model of chronic airway infection, treatment with a lipoxin analog inhibited neutrophil accumulation and decreased bacterial burden in the lungs. As for the link between lipoxin and CFTR, eicosonoid metabolism in epithelial cells may be affected by electrolyte or pH, as decreased expression of lipoxygenases was observed in mice expressing a mutated form of CFTR. — LDC

    Nature Immunol. 5, 388 (2004).


    Blue Blob in the Red Rectangle

    1. Linda Rowan

    The Red Rectangle nebula is an unusual protoplanetary nebula. Its central star, HD44179, is dying, and substantial mass is being blown off the star in a bipolar outflow, creating the rectangular shape and a large amount of dust. It is the brightest source of unidentified infrared band emission (UIR), and much of this comes from carbon compounds. Given that 20 to 30% of the Milky Way's infrared radiation is in the UIR and that carbon is an important component of stars, planets, and terrestrial life, it is desirable to identify the carbon species that account for the emissions.

    At shorter wavelengths, in the visible to ultraviolet, the Red Rectangle looks like a blue blob, produced by dust-scattered light from the central star. Vijh et al. obtained spectra at these wavelengths, and the peaks correspond to those in fluorescence spectra of electronic transitions in neutral polycyclic aromatic hydrocarbons (PAHs) with three to four rings, such as anthracene and pyrene. Thus, not only is the Red Rectangle producing a lot of PAH dust, but blue luminescence may be used to identify many carbon and silicon species that fluoresce at specific wavelengths. — LR

    Astrophys. J., in press (astro-ph/0403522).

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