Editors' Choice

Science  14 Nov 2003:
Vol. 302, Issue 5648, pp. 1117

    Bacteria into Plowshares

    Crops can be grown successfully in untilled soil by taking advantage of resident microorganisms. Mycorrhizal fungi are important for the healthy growth of most plants, including crop species. Their interaction is not always a simple one, however, and may require the presence of “helper bacteria” for root-fungus recognition, plant colonization, nutrition, and perhaps even to stimulate germination of fungal propagules.

    Artursson and Jansson, recognizing the problems of culturing such specialized soil organisms, made use of an ingenious technique to identify a mycorrhizae-associated bacterium. After the thymidine analog bromodeoxyuridine was added to soil samples (gathered outside Uppsala), newly synthesized DNA containing bromodeoxyuridine was isolated by immunocapture. The DNA was then used as a source of ribosomal RNA genes, which led to the identification of actively growing bacterial species in the soil. Eleven distinct sequences were discerned, but the dominant organism was a Bacillus cereus strain. This was extracted, grown, and labeled with green fluorescent protein, and its behavior in the presence of mycorrhizae was observed. Of several tested, this strain of B. cereus preferentially attached to the surface of mycorrhizae, possibly by polar flagella. — CA

    Appl. Environ. Microbiol. 69, 6208 (2003).


    Metabolism, Cancer, and a Fuel Gauge

    As a key sensor of cellular energy status, the AMP-activated protein kinase (AMPK) has attracted considerable attention among researchers studying metabolic control. In response to cellular stresses that deplete energy stores—such as glucose deprivation, hypoxia, and ischemia—AMPK is activated and switches on biosynthetic pathways that generate ATP while switching off pathways that consume ATP. The AMPK signaling cascade has been implicated in a range of human diseases, including type 2 diabetes, obesity, and cardiac hypertrophy.

    Activation of AMPK requires phosphorylation of Thr172, but the upstream kinase(s) acting on AMPK have eluded detection. In independent studies, Hawley et al. and Woods et al. show that mammalian AMPK is phosphorylated by a protein complex containing LKB1, a serine/threonine kinase previously shown to function as a tumor suppressor protein. Mutations in the LKB1 gene cause Peutz-Jeghers syndrome, a human disorder associated with a 15-fold increased risk of developing cancer. This newly discovered link between two kinases once thought to lie in distinct signaling pathways provides an opportunity for studying the molecular mechanisms that underlie some of the most common human diseases. — PAK

    J. Biol. 2, 28 (2003); Curr. Biol. 10.1016/S0960982203007851 (2003).


    Making Rods out of Spheres

    Semiconductor nanocrystals are of practical importance because their luminescent properties can be finely tuned by changing either the size or the shape of the particle, without needing to alter its composition. For example, rod-shaped particles show very different emissive properties than spherical ones, because the electrons and holes are no longer equally confined in all three directions.

    Talapin et al. show that it is possible to grow rod-shaped shells of CdS on the outside of spherical CdSe cores by using an excess of sulfur and low temperatures during the shell growth process. As the sulfur preferentially interacts with certain facets of the CdSe nanocrystals, the CdS asymmetrically nucleates on the core. The photoluminescence of CdSe nanocrystals is enhanced by having a CdS shell, because electrons delocalize across the entire nanoparticle, whereas holes are more strongly confined to the cores. This results in a much larger Stokes shift (the difference in wavelength between the absorbed and emitted quanta) and increases the quantum yield because the emitted light is shifted to wavelengths where only the CdSe absorbs. — MSL

    NanoLett. 10.1021/nl034815s (2003).


    Segmental History

    Boundaries are established during embryonic development to define cellular compartments that underlie patterning and growth. In the developing Drosophila embryo, the epidermis becomes transiently divided into a series of segments, each with a deep groove that marks an anterior and posterior edge. These boundaries form just posterior to cells that express the transcription factor Engrailed.

    Larsen et al. observed that boundary formation begins when specific Engrailed-expressing cells called the “groove founders” lose apical contact with neighboring cells, constrict, and migrate inward into the embryo, pulling adjacent cells with them to line the sides of the boundary. Once at the bottom of the groove, founder cells stop expressing Engrailed. Boundaries then regress rapidly during dorsal closure, when the epidermis must spread to enclose the entire embryo, suggesting that cells buried in the grooves may provide this extra surface area. Analysis of mutants indicates that groove formation requires Engrailed and the morphogens Hedgehog and Wingless. Hedgehog signaling is required posterior to the boundary but is repressed by Wingless anteriorly in order to prevent a boundary from forming on the wrong side of Engrailed-expressing cells.

    Prud'homme et al. have found that Engrailed and Wingless are also expressed in epidermal cells on opposite sides of boundaries that define morphologically similar segments during the development of a primitive annelid. The molecular and morphological similarities suggest that a segmental unit body plan may have evolved from a common ancestor. — LDC

    Development 130, 5625 (2003); Curr.Biol. 13, 1876 (2003).


    No Fast Route to Greener Cars

    The environmental impact of cars is immense, primarily due to the emissions as they are driven rather than their manufacture, but given our dependence on and preference for light-duty vehicles, can they at least be made “greener?” MacLean and Lave point out several of the challenges to creating a greener automobile fleet. One is the increasing demand, at least in the United States, for larger and less fuel-efficient sport utility vehicles, and a second is the unwillingness to pay a premium for greener cars. A further challenge is conflicting societal goals—for example, using fuel-injected diesel engines would save fuel but clash with strict emissions controls, and making light yet crashworthy cars would be expensive. A final obstacle is that the costs and benefits of materials, processes, and consequences can be difficult to quantify. Nevertheless, their analysis of different fueling and power plant options (diesel, ethanol, battery power, gas-electric hybrids, and hydrogen fuel cells) shows that no one of these options scored highly on all of the potential measures of greenness. — PDS

    Environ. Sci. Technol. 10.1021/es034574q (2003).


    Making the Rounds

    Polyketide synthases are enzymes responsible for the biosynthesis of a variety of macrocyclic compounds, some of which are important pharmaceutical agents. The synthases operate in a modular fashion: Each enzyme (or module) is believed to catalyze extension of the chain by one unit and subsequent modifications such as dehydration. Manipulation of the assembly process may lead to more potent pharmaceutical agents.

    Olano et al. show that the simple “one extension, one module” rule needs to be revised. In the biosynthesis of the angiogenesis inhibitor and possible antitumor agent borrelidin, one module catalyzes not one but three consecutive rounds of chain extension before passing the intermediate structure to the next module. The authors constructed several mutant enzymes to show that this is not due to insertion of several copies of the module; rather, the same module is used repeatedly. Genetic evidence suggests that such behavior may also occur in other polyketide synthases. — JFU

    Chem. Comm. 10.1039/b310648a (2003).


    See oskar Move

    The regional specification of intracellular proteins within the cytoplasm is a well-established organizational principle, and there are an increasing number of cases of localized messenger RNAs (mRNAs), for instance, in developing Drosophila eggs. Bratu et al. describe the direct imaging in living eggs of the distribution of oskar mRNA, which helps to define the posterior end of the oocyte. They developed a fluorescent tag called a molecular beacon, which is an oligonucleotide probe that lights up only when hybridized to its complementary RNA target. oskar mRNA was observed in real time to move from its site of synthesis in the surrounding nurse cells into the oocyte and thence to the posterior pole. The transport process appears to require microtubules and an intact 3′ untranslated region within the oskar mRNA. Such molecular beacons will be useful for real-time imaging of other spatially regulated mRNAs. — SMH

    Proc. Natl. Acad. Sci. U.S.A. 100, 13308 (2003).