This Week in Science

Science  29 Aug 2008:
Vol. 321, Issue 5893, pp. 1129
  1. Rainbow Signals


    Many microorganisms produce antibiotics that kill other microbes. Indeed, human beings and other organisms have become adept at exploiting these natural products to guard themselves from infection. But it seems these substances are not just disinfectant waste products; there are hints that some have quite specific functions, for example, in facilitating the uptake of metals. Dietrich et al. (p. 1203) analyzed the production of antibiotic pigments called phenazines from two classes of bacteria, pseudomonads and actinomycetes, and found that the pigments play important roles as signaling molecules in regulating the structure of the microbial community. This activity seems to be mediated via the SoxR regulon, classically thought to be a mediator of oxidative stress responses.

  2. Polarized Crab Nebula

    Pulsar systems containing neutron stars accelerate particles to immense energies, typically one hundred times more than the most powerful accelerators on Earth. It is uncertain exactly how these systems work and where the particles are accelerated. Dean et al. (p. 1183; see the Perspective by Celotti) detected polarized gamma-ray emission from the vicinity of the Crab Nebula—one of the most dramatic sights in deep space. The results show polarization with an electric vector aligned with the spin axis of the neutron star, demonstrating that a significant fraction of the high-energy electrons responsible for the polarized photons are produced in a highly ordered jet structure close to the pulsar. These findings provide a powerful diagnostic tool to expose the inner structure of this class of powerful cosmic machines.

  3. Wildfire Past and Present

    How much oxygen must there be in the air for wildfires to burn? Charcoal, the most ubiquitous proxy for wildfires, is found throughout the geological record, and other direct evidence of the abundance of O2 is rare. Most of what is understood about the evolution of atmospheric O2 comes from models, some of which have concluded that the concentration of atmospheric O2 was as low as 10% during some intervals of the Mesozoic (251 to 65 million years ago). In comparison, the present-day atmosphere contains about 21% O2. Belcher et al. (p. 1197) present results from laboratory combustion experiments over a range of O2 concentrations to show that fire is not sustainable in an atmosphere with an O2 content below 15%. By combining their findings with a record of paleowildfires in the Mesozoic, they conclude that prolonged intervals of atmospheric oxygen levels between 10% and 12% could not have occurred in the Mesozoic.

  4. A Clean H/F Swap


    Many applications of synthetic fluorocarbons rely on their inertness, which stems partly from the high strength of C-F bonds. The flip side, however, is a tenacious resistance to environmental degradation once the compounds join the waste stream—a problem of growing concern in light of their strong atmospheric greenhouse absorptions. Douvris and Ozerov (p. 1188; see the Perspective by Perutz) present an efficient catalytic scheme for hydrodefluorination (C-F to C-H bond conversion) based on chlorinated or brominated carborane anions that can robustly stabilize Si- and C-centered cations in solution. By mixing trialkylsilanes with fluorocarbons in the presence of carborane, they exchange F substituents on C with H substituents on Si, presumably via successive abstractions by silyl and carbocations. Unlike metal-catalyzed defluorinations, this process selectively converts aliphatic sites over aromatic ones.

  5. Electrons Sprung Slowly

    In the same way bullets do more damage than arrows, surface chemistry induced by incoming molecules has tended to scale in efficiency with those molecules' incident velocity. Nahler et al. (p. 1191) document a surprising exception to this dictum, in which highly vibrationally excited NO molecules liberate electrons from a sparsely cesium-coated gold surface more effectively when they approach more slowly. A model in which the electron affinity of the NO varies depending on the N-O bond length, with electrons attracted at the outer limit of the stretching cycle and then expelled upon compression can account for the observations. A low translational velocity affords more time for the charge transfer to occur at the requisite separation distance from the surface.

  6. Seamounts and Earthquakes

    To what extent does topography across subducting plates control the extent and distribution of great subduction zone earthquakes, for example, where the earthquakes start and stop, and whether uneven topography increases earthquake size? The largest features likely to influence such earthquakes are typically underwater seamounts, but whether these enhance or suppress large quakes has been uncertain. Mochizuki et al. (p. 1194; see the Perspective by von Huene) examine the Japan Trench, where a series of seamounts have been subducted and there is a long earthquake record. Seismicity is enhanced in front of the seamount but reduced in its wake, implying only weak coupling between the two plates.

  7. Sweet Scent of Outcrossing

    Floral compounds that affect scent and nectar have long been assumed to increase a plant's outcrossing rates by attracting insect pollinators, but experimental evidence for this idea is scarce, and the function of repellents is unknown. Kessler et al. (p. 1200, cover; see the Perspective by Raguso) studied the genetic basis and evolutionary consequences of pollinator choice in wild tobacco, using a combination of manipulations of secondary products from chemistry, genetics, transgenesis, natural history, and field experiments affecting attractants and repellents. The attractive scent was found to be necessary to bring pollinators to the flower. The plants also used toxins (nicotine) in their nectar in order to enforce modest drinking behavior in their pollinators, allowing for more pollinator visits. Thus, floral compounds play a dual role both to attract and to repel pollinators, and both roles are necessary to optimize a plant's reproductive output.

  8. Ancient Urban Wisdom

    Amazonia was more densely populated with indigenous peoples before the arrival of European colonists. Using a combination of remote sensing and archaeological techniques, Heckenberger et al. (p. 1214) now show that the pre-Columbian societies of the Upper Xingu river basin in Brazilian Amazonia lived in an urban landscape of distributed towns, villages, and hamlets organized by a road network that connected settlements arranged in a gridlike pattern. The inhabitants created a highly productive and heterogeneous cultural landscape through careful, multigenerational resource creation and management. These arrangements may provide lessons for current attempts at sustainable development in the region.

  9. Channel Opening


    The Escherichia coli mechanosensitive channel, MscS, opens in response to membrane tension to allow ion efflux, so that bacteria can survive hypo-osmotic shock. Now two papers provide insight into the molecular basis of channel gating (see the Perspective by Gandhi and Rees). Wang et al. (p. 1179) determined the crystal structure of the MscS channel in an open conformation and Vásquez et al. (p. 1210) obtained electron paramagnetic resonance measurements on the open conformation in a lipid bilayer. Comparison with a previously determined closed-state structure, combined with functional data or computational analysis, allowed modeling of the movements of the transmembrane helices that cause channel opening.

  10. Stem Cells from ALS Patients

    Stem cells hold promise both for replacing damaged cells and for opening up avenues for research into disease processes. These avenues would benefit from cells derived to match a specific patient or to reflect a specific disease. Dimos et al. (p. 1218, published online 31 July; see the Perspective by Brown) have now derived induced pluripotent cells from skin samples taken from two elderly patients who carry a genetic mutation associated with a familial form of ALS, amyotrophic lateral sclerosis. The induced pluripotent cells, which resemble stem cells in their flexibility, were then enticed in culture to develop into differentiated cells. Of particular interest was derivation of cells that resemble motor neurons, the cell type that is much afflicted as the disease ALS takes its course.

  11. Amyloid-β in Living Human Brain

    A great deal of interest has been directed at Alzheimer's disease, and the amyloid-β peptide (Aβ) has been at the center of much of this attention. Yet, despite over 20 years of study since the discovery that Aβ is the principal constituent of the hallmark senile plaques, virtually nothing is known about the concentration or regulation of Aβ in the extracellular space of the human brain, where these plaques form and neurotoxic effects are likely to occur. Now Brody et al. (p. 1221) present measurements of the concentrations of Aβ in the living human brain, and show that Aβ is dynamically regulated in concert with neurological status. The findings were obtained using intracerebral microdialysis in brain-injured patients and will contribute to future pathophysiological and pharmacodynamic studies of brain injury and Alzheimer's disease.

  12. Light and Mirrors

    Optical interferometers have been the workhorse of precision measurements, capable of logging displacements shorter than the wavelength of light used in the setup. The light in the interferometer, however, produces a “pressure” on the mirrors, causing a displacement in the system itself. Kippenberg and Vahala (p. 1172) review recent work in which this “back-action” coupling between the light and mirror can be probed, and discuss possibilities of the effect for applications and metrology.

  13. Subdued Supernova

    Energetic supernovae, produced as a massive star collapses to form a black hole, generate strong gamma-ray bursts; explosions of smaller stars typically produce weaker x-rays. In January, a new supernova was detected following a weak x-ray burst. Mazzali et al. (p. 1185, published online 24 July) show that the evolution of the event was different from previous supernova. Initially, the star's light curve resembled those of the more energetic supernova, but the density of the outer explosion dropped revealing helium lines, seen in less energetic explosions. Thus, this example may link previous observations and models. The exploding star had an initial mass 30 times that of our Sun, and about one-fourth of its material was ejected by the event.

  14. VDAC-1 Structure in Solution

    In eukaryotes, a voltage-dependent ion channel (VDAC) facilitates diffusion of metabolites across the mitochondrial outer membrane. Closure of the human isoform, VDAC-1, leads to mitochondrial apoptosis. Hiller et al. (p. 1206) have determined the nuclear magnetic resonance solution structure of VDAC-1 in detergent micelles. Whereas known β-barrel structures have an even number of strands, VDAC-1 has a 19-stranded barrel with parallel pairing of the first and last strands. Chemical shift data was used to map interaction sites for the substrates NADH and cholesterol, and for the antiapoptotic protein Bcl-xL that opens the channel and thus inhibits apoptosis. The structure provides a framework for reanalysis of existing biochemical and biophysical data.

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