COVER Wind turbines near the Great Wall in Shanxi, China. The Chinese government has aggressively fostered wind power development and tripled its target for the year 2020 to 100 gigawatts of installed capacity. Combining assimilated meteorology with current turbine technologies and concession policies, McElroy et al. (page 1378) report on the total wind power potential of China and its prospects for reducing China’s CO2 emissions. Photo: Haiying Chen
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This Week in Science
Highlights of Research in This Issue
Volume 325, Number 5946, Issue of 11 September 2009
©2009 by The American Association for the Advancement of Science.
Assessing the Arctic
The Arctic is experiencing some of the most rapid climate change currently under way across the globe, but consequent ecological responses have not been widely reported. At the close of the Fourth International Polar Year, Post et al. (p. 1355) review observations on ecological impacts in this sensitive region. The widespread changes occurring in terrestrial, freshwater, and marine systems, presage changes at lower latitudes that will affect natural resources, food production, and future climate buffering.
Efficient Carbon Nanotube Photodiodes
A single photon absorbed in a single-walled carbon nanotube device can generate multiple unbound particles carrying an electric charge. Gabor et al. (p. 1367) report that in such a device at low temperatures, excitation with light of increasing energy leads to well-defined stepwise increases in current. Interestingly, because of the unique band structure of carbon nanotubes, this behavior is analogous to particle-antiparticle creation commonly observed in high-energy particle physics. These observations point to the promise of investigations in other nanoscale carbon systems, such as graphene, and could lead to numerous applications, including highly sensitive photon detection and ultra-efficient photovoltaics.
Mapping Out an Entropic Landscape
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Quantum critical points are continuous phase transitions occurring near absolute zero displaying interesting properties that may have valuable applications. Experimental thermodynamic information on quantum critical systems is sparse, partly because the better known systems are tuned using hydrostatic pressure, and thermodynamic measurements are difficult to perform under such conditions. Rost et al. (p. 1360, published online 6 August 2009; see the Perspective by Fisk) built and calibrated bespoke apparatus for a thermodynamic study of strontium ruthenate using a magnetic field as the tuning parameter. The specific heat and magnetocaloric measuremements were combined to map an “entropy landscape” of the quantum criticality and phase formation. This technique should offer a way to probe other materials to reveal unusual properties, such as novel metallic states and superconductivity.
Himalayan-Tibetan Underplate
The Himalayas formed from the collision of India with Eurasia beginning about 50 million years ago, but the fate and position of the subducted Indian crust was not well defined until the Hi-CLIMB seismic experiment was initiated. The centerpiece of the project is an 800-kilometer-long, closely spaced, linear array of broadband seismographs, extending from the Ganges lowland, across the Himalayas, and onto the central Tibetan plateau. Náb
lek et al. (p. 1371) present images of the crust and upper mantle of the Southern Tibetan plateau underthrust northward by the Indian plate, in which they trace the base of the Indian plate to 31°N. The character of the crust-mantle interface in this region suggests that the Indian crust is at least partly decoupled from the mantle beneath.
It’s a Gas
Many antibiotics, including beta-lactams, aminoglycosides, and quinolones, kill bacteria (at least in part) by oxidative stress. Gusarov et al. (p. 1380) show that nitric oxide (NO) produced by bacterial NO synthases (bNOS) protects bacteria, including Staphylococcus aureus and Bacillus anthracis, against toxic agents they may encounter in the soil or in host organisms. Thus, bNOS activity is specifically induced in response to antibiotics and, in turn, activates the expression of another key antioxidant enzyme: superoxide dismutase. Hence, NO-mediated antibiotic resistance not only operates by direct chemical modification of toxic molecules, but also alleviates oxidative stress caused by naturally occurring antibiotics.
Blowing Away Coal
China is the world’s largest carbon dioxide producer and the world’s second-largest producer of electrical power, 80% of which it generates by burning coal. An affordable, carbon-free source of electrical power generation would thus constitute an important way for China to reduce its CO2 emissions and other environmental impacts of fossil-fuel burning. McElroy et al. (p. 1378, see the cover) show that there is enough wind in China to generate electricity to supply the nation’s entire projected demand for 2030 (about twice what is used now) at reasonable prices per kilowatt-hour.
Making Mother of Pearl
Nacre is an iridescent layer of calcium carbonate lining the inside of shells of marine mollusks and is commonly known as “mother of pearl.” It is composed of layers of uniformly oriented crystals of aragonite (a metastable form of calcium carbonate) separated by layers of organic matrix. How the ordered structure of aragonite layers is achieved has been unclear. Suzuki et al. (p. 1388, published online 13 August 2009; see the Perspective by Kröger) identified two acidic matrix proteins (Pif 97 and Pif 80) that regulate nacre formation in the Japanese pearl oyster. The proteins appear to form a complex in which Pif 80 binds to aragonite and Pif 97 binds to other macromolecules in the organic matrix.
Cycling Around
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Water vapor is the most important greenhouse gas, and clouds are one of the most important components of climate, but the global hydrological cycle is still poorly-enough understood that the atmospheric cycling of water and cloud formation are inadequately represented in global climate models. As the transformation from liquid into vapor tends to deplete water of the isotope deuterium, Frankenberg et al. (p. 1374) were able to use satellite measurements of global “heavy” water abundances to provide a deeper understanding of atmospheric water dynamics.
MITE-y Jumps
Tranposable elements are genetic elements ubiquitous to most genomes, and their expansion, loss, and potential movement are major components of genome evolution. A type of noncoding transposable element in plants, known as a MITE (minature inverted repeat transposable element), shows evidence of recent expansion within specific rice genomes. However, these elements lack the necessary transposase to help them move. Yang et al. (p. 1391; see the Perspective by González and Petrov) found that the expansion of MITEs occurs because they exploit the activity of other, unrelated, transposons and consequently identified the molecular determinants that allowed mobilization. Surprisingly, the element coding for the transposase repressed its own movement unless the MITE was present. These findings may explain why some genetic elements can replicate within the genome and reach high copy numbers while others do not.
Seeing Double
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A particular set of ubiquitous small (nucleolar) ribonucleoproteins are important for optimal ribosome function and protein synthesis. Bleichert et al. (p. 1384) used electron microscopy and single-particle analysis to investigate the structure of an archaeal version that contains the small RNA (sRNA) and all the associated core proteins. Unexpectedly, this ribonucleoprotein is a homodimer, formed of two sRNAs and four copies of each of the core proteins. This dimer is likely to be the enzymatically active form, as mutations disrupting di-sRNP formation inhibited activity.
Maintaining Germline Stem Cells
Spermatogonial stem cell pools in postnatal testes have to be maintained to continuously generate spermatozoa. It has been difficult to identify these stem cells in vivo, because of their small numbers and lack of appropriate molecular markers, but now Sada et al. (p. 1394) show that the RNA-binding protein NANOS2 is expressed in a small subset of spermatogonia that behave as self-renewing stem cells in intact testes. By a combinatorial use of loss- and gain-of-function studies, NANOS2 was found to be essential for the maintenance of the immature state of spermatogonial stem cells by supporting their self-renewing properties and by suppressing differentiation.
Making Myelin
The myelin sheath insulates neurons and facilitates rapid axonal conduction, and its disruption is characteristic of some neurological disorders, such as multiple sclerosis. Axonal signals stimulate Schwann cells to form myelin in peripheral nerves, but the mechanism is not completely known. By characterizing a mutation identified in zebrafish, Monk et al. (p. 1402; see the Perspective by Meijer) show that Gpr126, a member of the G protein–coupled receptor superfamily, is essential for myelin formation. It appears that Gpr126 acts as a receptor for axonal signals to elevate cAMP levels in Schwann cells and trigger myelination.
Minimizing Brain Energy Consumption
How much energy is actually required to generate neuronal activity and information processing? By combining direct recordings at physiological temperatures from mossy fiber axons in rat brain slices with modeling and simulation approaches, Alle et al. (p. 1405; see the Perspective by Magistretti) found that the regenerative action potentials in nonmyelinated axons of mammalian hippocampus are remarkably energy efficient. The data indicate a surprisingly minor contribution of action potentials to the entire energy expenditure of neural information processing.
A Lower Tunnel
Among the peculiarities inherent in quantum mechanics is the ability of particles to tunnel through barriers that they lack the energy to surmount classically, as happens during radioactive decay. Strong laser fields can liberate electrons in this way from atoms and molecules. Akagi et al. (p. 1364) elegantly confirm that tunneling is not limited to the highest-energy electrons in a system by mapping the energy and momentum of both the ejected electron and positive ion produced when an intense laser pulse impinges on hydrogen chloride. When the molecule adopts specific orientations relative to the laser field, tunneling occurs from lower-lying states, as well as the highest-energy occupied orbital. This raises the possibility of tunneling microscopy capable of imaging the electronic structure of single molecules.
Crystal Clear
Guanine nucleotide exchange factors stimulate exchange of guanine diphosphate (GDP) for guanine triphosphate (GTP) and activate small guanosine triphosphatases (GTPases) required for the regulation of many biological processes. Yang et al. (p. 1398) provide a detailed picture of the complete catalytic cycle by which a guanine nucleotide exchange factor, DOCK9, activates the small GTPase, Cdc42. A small region of DOCK9 appears to sense whether GTP or GDP is bound to Cdc42 in a mechanism that is distinct from that observed for other GTPases.
Editors’ Choice
Volume 325, Number 5946, Issue of 11 September 2009
©2009 by The American Association for the Advancement of Science.
Biotechnology:
Verkehrsknotenpunkt in Miniature
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Engineering microbes so that they produce desired molecules, such as the isoprenoid precursor mevalonic acid, can have the unhealthy side effect of rerouting the host’s metabolism into a suboptimal configuration. Heterologous enzymes, if expressed at high levels, may rob the cell of essential metabolites or, conversely, may introduce toxic intermediates. One conceptual way around this problem would be to centralize the synthetic machinery, reducing the transit times of intermediates as they travel from one enzyme to the next, increasing local substrate concentrations, and decreasing exposure to the elements.
In this vein, Dueber et al. have developed a modular system for constructing intracellular chemical factories. They fabricated joints by adding one of three well-characterized protein-protein interaction motifs—a GTPase binding domain, a Src homology domain, or a PDZ domain—to the N terminus of the downstream biosynthetic enzyme and the corresponding peptide ligand to the C terminus of the upstream enzyme. Varying the number of peptide ligands added and, independently, the number of peptide-binding domains attached generated a range of enhanced titers over that observed when uncatenated enzymes were used; one such combination yielded 5 mM mevalonate, an increase of almost two orders of magnitude. The advantage of building a self-contained enzyme complex, akin to the fatty acid synthases, is that the orthogonal production of chemicals imposes a tolerable cost on the host organism.
Nat. Biotechnol. 27, 753 (2009).
Microbiology:
Lying Dangerously Dormant
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It was Louis Pasteur who first thought earthworms might play a part in aiding the persistence of anthrax in the soil. He did not follow up on this hunch, but observations on the sporadic occurrence of anthrax spores, coupled with the virus-infested nature of Bacillus anthracis and its close relatives, suggested to Schuch and Fischetti that this dangerous pathogen may do more in the soil than simply lie in wait, encased in a resistant coat. In a set of extensive analyses, B. anthracis was found to harbor not only its own distinctive virulence plasmids but also to act as host for several lysogenic bacteriophages.
Lysogeny was found to alter the capacity of the bacterium to sporulate, to form biofilm exopolysaccharide, to reproduce vegetatively, and to colonize earthworm guts. Phage-encoded sigma factors transcriptionally activated bacterial loci to switch on these phenotypes, enabling B. anthracis and its cousins to live in the soil in a variety of modes, and not just as dormant spores.
PLoS ONE 4, e6532 (2009).
Climate Science:
Leafing Through Time
Geol. Soc. Am. Bull. 121, 1441 (2009)
Chemistry:
Acid-Driven Delivery
J. Am. Chem. Soc. 131, 10.1021/ja9010157 (2009).
Chemistry:
A Distinct Take on Flex Fuel
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Technological miniaturization is rapidly reducing the macroscopic surface footprints needed for applications ranging from electronic circuits to chemical reactors. In this landscape, concomitant miniaturization of power sources such as batteries and fuel cells would be helpful to render chip systems self-sufficient, eliminating the need for connections to bulky external sources. Tominaka et al. present progress toward development of an efficient fuel cell roughly the size of a postage stamp, fabricated on flexible plastic to reduce the cost and brittleness associated with a silicon-based prototype. The device was prepared through a combination of hot embossing, photolithography, and electrodeposition, and operated with methanol fuel and ambient oxygen. The authors propose to improve the observed low power density by increasing the relative surface areas of the catalysts.
Energy Environ. Sci. 2, 10.1039/b915389f (2009).
Climate Science:
Believable Fingerprints
Proc. Natl. Acad. Sci. U.S.A. 106, 10.1073/pnas.0901736106 (2009).
Cancer:
Green Surgery
Proc. Natl. Acad. Sci. U.S.A. 106, 14514 (2009).
标签：科学杂志
