Editors' Choice

Science  22 Sep 2006:
Vol. 313, Issue 5794, pp. 1704

    A Plastic Genome

    1. Gilbert J. Chin

    Ralstonia eutropha H16 is a bacterium that can adjust to life on a variety of nutrients (as carbon and energy sources) and can survive periods of anoxia. Two skills are of particular interest: the ability to perform the Knallgas reaction and the storage of carbon in polyhydroxyalkanoate (PHA) granules. The former refers to the explosive combination of H2 and O2 (in a 2:1 ratio), which Ralstonia carries out in a traditional respiratory fashion, passing protons and electrons separately through membrane-bound carriers until they are added to O2 in a terminal oxidase complex to produce water. The latter was first detailed almost half a century ago and has led to the biodegradable thermoplastic Biopol and to polythioesters.

    Pohlmann et al. report the sequence of the two Ralstonia chromosomes (∼7 Mb), providing an inventory of the many candidate enzymes involved in the synthesis, polymerization, depolymerization, and catabolism of PHAs. The large number of genes encoding β-ketothiolases and acetoacetyl-CoA reductases offers the potential for tinkering with substrate specificity to create an intracellular library of three- to five-carbon hydroxyacid monomers. — GJC

    Nat. Biotechnol. 24, 10.1038/nbt1244 (2006).


    A Pick-Me-Up for Cancer

    1. Stephen J. Simpson

    A major avenue that is being explored in the treatment of cancer is the possibility of mobilizing the immune system to attack tumor cells. However, for reasons that are only slowly becoming clear, encouraging immune cells to destroy tumors remains relatively inefficient.

    Ohta et al. provide evidence that tumors protect themselves from immune attack via extracellular adenosine generated within the hypoxic environment of the tumor mass itself. Previous studies have suggested that during inflammation, the activation of the adenosine receptor (A2AR) on T cells leads to levels of intracellular cyclic AMP that inhibit cell function. In the current experiments, 60% of mice lacking A2AR rejected their tumors, as compared to unimpaired tumor growth seen in mice with immune cells able to signal through the receptor. A2AR antagonists—including caffeine—had similar, albeit less robust, tumor-inhibiting effects that depended on interferon-γ-producing CD8+ T cells. These results lend support to the contested notion that the immune system continuously monitors for malignancy and raises the question of an A2AR-mediated contribution to early spontaneous tumor growth. If this is the case, then inhibition of this pathway might be helpful as an adjunct to immune-based therapies for some cancers. — SJS

    Proc. Natl. Acad. Sci. U.S.A. 103, 13132 (2006).


    Stand Up, Line Up, Charge!

    1. Phil D. Szuromi

    The miniaturization of electronic devices to the nanometer scale requires the fabrication of extremely narrow wires. One approach has focused on the synthesis of conducting metal or carbon nanotubes. A second approach is the self-assembly of small molecular components into conduits held together by noncovalent interactions. Stacked aromatic molecules such as tetrathiafulvalene (TTF) could potentially achieve this function. However, when adsorbed on graphite, the highly conjugated TTF molecule interacts strongly with the substrate and lies flat, which minimizes interactions between molecules; hence, the stack motif is unstable. Puigmartí-Luis et al. have derivatized TTF by capping two of the terminal sulfur atoms with amide groups, which are in turn bonded to long alkyl chains. The intermolecular hydrogen-bonding interactions between the amides allow the TTF moieties to form long one-dimensional chains in which the π electron-rich cores are tilted at a high angle to the surface. Scanning tunneling microscopy revealed that parallel wires are formed, spaced ∼5 nm apart, which is consistent with distances predicted by molecular modeling. Both quantum mechanical calculations and scanning tunneling spectroscopy suggest that the nanowires should be highly conducting. Furthermore, rectifying behavior was observed in the -1- to 1-V range, with a 10-fold increase in current at negative versus positive substrate bias. — PDS

    J. Am. Chem. Soc. 128, 10.1021/ja0640288 (2006).


    Negative Index Made Easy

    1. Ian S. Osborne

    The realization of designer materials, or metamaterials, in which the electrical permittivity and magnetic permeability can be made negative simultaneously has generated much interest, primarily due to theoretical proposals for remarkable applications such as perfect lenses and, most recently, the ability to hide, or cloak, objects from electromagnetic radiation. After the initial demonstration in the microwave regime, much of the experimental effort in metamaterial design has focused on pushing the response of these materials toward shorter wavelengths. However, the design of choice, a split ring resonator coupled to a metal wire, is somewhat limited when it comes to size reduction, and other approaches are being pursued. Chettiar et al. propose the use of a simpler structure to achieve negative refraction: a pair of metallic nanostrips separated by a thin dielectric layer, easily fabricated with existing deposition and lithographic technology down to feature sizes in the 10- to 100-nm range. Their simulations show that coupling such pairs of nanostrips to continuous metal films should provide a negative refractive index in the optical and infrared regimes. — ISO

    Opt. Express 14, 7872 (2006).


    Stripped and Eliminated

    1. Stella M. Hurtley

    Protozoan parasites such as Plasmodium and Toxoplasma invade host cells and divide within a parasitophorous vacuole. The vacuolar membrane is modified by the invading parasite in order to forestall its fusion with host endocytic and degradative organelles (lysosomes). Ling et al. have examined how mouse macrophages, after being infected by T. gondii, can break through this parasite-constructed defensive wall. Cells from mice lacking an interferon-γ-inducible p47 GTPase (IGTP) failed to eliminate the pathogen. In contrast, in wild-type cells the parasitophorous vacuole membrane was disrupted during the degradation process, and the parasite plasma membrane was stripped away. The parasite was then engulfed by a double-membrane autophagosome, which fused with lysosomes, leading to destruction of the parasite. Recently, IGTPs have been shown to play a similar role in the elimination of intracellular Mycobacterium in mice and in humans (Singh et al., Reports, 8 September 2006, p. 1438). — SMH

    J. Exp Med. 203, 2063 (2006).


    Signs of Collapse

    1. David Voss

    When their nuclear fuel is exhausted, stars die, and the residual iron core collapses on itself. The outcome of a star's death throes depends on mass, however. Stars with between 10 and 20 times the mass of the Sun collapse in a spectacular explosion known as a supernova, leaving behind a neutron star, whereas those larger than 20 solar masses implode to form black holes in a “hypernova.” In both cases, copious bursts of neutrinos are released along with optical, x-ray, and gamma radiation. Most scenarios for hypernova collapse involve rapidly rotating stars, but recent studies indicate that some massive stars may be rotating only slowly or not at all.

    Sumiyoshi et al. have carried out simulations showing that such stars may lead to explosions that are very dim in the electromagnetic spectrum, but that still lead to black hole formation and powerful neutrino bursts. These neutrino signals are sensitive indicators of the equation of state of matter in the collapsed star. (The equation of state relates basic quantities such as pressure, density, and temperature.) As a result, these neutrino bursts could offer a valuable diagnostic tool for studying the properties of stellar matter. — DV

    Phys. Rev. Lett. 97, 091101 (2006).

  7. STKE

    An Open and Shut Case

    1. L. Bryan Ray

    Stomata are openings on the surfaces of leaves that mediate the exchange of gases, which is essential for respiration and osmotic balance. However, these doorways also provide a route by which infectious bacteria can gain access to plant internal tissues. Stomata open and close in response to changes in exposure to light, humidity, and other stimuli, but Melotto et al. show that they can also be shut as a defense against bacterial invasion. Arabidopsis closed their stomata within 2 hours of exposure to the pathogenic bacterium Pseudomonas syringae, but reopened them a few hours later. Microscopic observation showed that the bacteria were able to detect and migrate toward open stomata, perhaps sensing nutrients or other molecules released from the plant interior. Flg22, a peptide derived from the bacterial flagellin protein, or lipopolysaccharide, a component of the bacterial outer cell wall, could trigger stomatal closure, and plants are known to have immune receptors that recognize these molecules. The subsequent reopening of the stomata led the authors to test whether P. syringae produced a virulence factor that could override the host plant's protective mechanism. Indeed, they found that the bacterially produced polyketide toxin coronatine was required for reopening of the stomata. These results reveal that plants have developed an innate immune mechanism to protect themselves against bacterial invasion and that in response some bacteria have developed a virulence factor that reopens doors. — LBR

    Cell 126, 969 (2006).

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