This Week in Science

Science  31 Aug 2012:
Vol. 337, Issue 6098, pp. 1016
  1. Mitochondrial Dynamics

    Mitochondria—the powerhouses of the cell—are autonomous organelles with their own genomes. Within cells, mitochondria are remarkably dynamic, continually moving around the cytoplasm and undergoing fusion and fission reactions. Youle and van der Bliek (p. 1062) review the importance of mitochondrial fusion and fission in cellular responses to stress, interference with which are likely to play an important role in a variety of diseases including Parkinson's disease. In their Perspective, Hoppins and Nunnari explain that the endoplasmic reticulum is an active participant in mitochondrial division and discuss how mitochondrial dynamics and cell death are linked.

  2. Dark Forcing

    Soot, or black carbon, is a ubiquitous atmospheric pollutant whose warming effect might be second only to carbon dioxide. When black carbon is emitted, it combines with other aerosols to form heterogeneous mixtures. Models have predicted that internal mixing of black carbon with other materials can double the amount of radiation absorbed. Cappa et al. (p. 1078) report that in situ measurements of the enhancement of radiation absorption by these mixed black carbon–containing particles in the atmosphere show a much smaller effect. Thus, many climate models may be overestimating the amount of warming caused by black carbon emissions.

  3. Vibrating in a Crowd


    High-vacuum molecular beam studies can probe the roles of specific vibrations and rotations on molecular reactivity with remarkably fine resolution. Glowacki et al. (p. 1066; see the Perspective by Tyndall) now show, through a combination of spectroscopy and theoretical modeling, that oxidation of acetylene under effectively atmospheric conditions proceeds in part through vibrationally excited intermediates prior to collisional randomization.

  4. Pretend Wires

    Cold atomic gases have been successfully used to simulate solid-state phenomena such as quantum criticality. However, simulating mesoscopic electronic transport like that realized in quantum wires is challenging. Brantut et al. (p. 1069, published online 2 August) connected two reservoirs of fermionic 6Li atoms (simulating electrons) with a narrow channel (simulating a wire), created a nonequilibrium situation by applying a magnetic field gradient, and observed the flow through the channel. When the mean-free path of the atoms exceeded the length of the channel, the atomic density in the channel was constant in the central region and only changed at the ends, indicating the presence of contact resistance. The opposite diffusive regime created by imposing a disordered laser potential produced a uniformly varying density inside the channel.

  5. Boundaries on Plasmonic Excitations

    The localization of optical fields within a metal nanostructure can achieve strengths that are orders of magnitude greater than that of the incident field. This focusing and enhancement of the optical field maybe useful in sensing, nonlinear optics, and optical scattering applications. In probing the properties of metallic nanoparticles, Ciracì et al. (p. 1072; see the cover) show that the enhancement is limited by the electronic response of the metal, which has implications for the ultimate performance of nanophotonic systems.

  6. Salty Origins of Fresh Water

    Cloud droplets above the Amazonian rain forest form mostly around organic aerosols, but the source of the aerosols has been a mystery. Pöhlker et al. (p. 1075) report that particles rich in potassium salts emitted by Amazonian vegetation can act as the seeds for the growth of organic aerosol particles that function as condensation nuclei for water droplets. These specks of biogenic salts provide a surface for the condensation of low- or semi-volatile organic compounds formed by the atmospheric oxidation of isoprene and terpenes, molecules produced in great abundance by many kinds of Amazonian plants.

  7. Curls Beget More Curls

    Cucumber tendrils reach up to find an attachment, and then coil to shorten and drag the plant up toward the sunlight. Gerbode et al. (p. 1087) analyzed the biomechanics of cucumber tendril coiling. The process depends on a thin layer of cells within the tendril that becomes lignified during the coiling process. A construct of pre-strained silicon sheets, fabric ribbon, and copper wire reproduced the coiling functions in abiotic materials. Physical and mathematical models explained the peculiar response by which the cucumber tendril initially overwinds when pulled further.

  8. Skin Specifics

    Much of the recent attention paid to the trillions of bacteria that colonize our bodies has been given to the bacteria that reside in the gut. Naik et al. (p. 1115, published online 26 July) report that colonization of the skin with commensal bacteria is important for tuning effector T cell responses in the skin and for protective immunity against cutaneous infection with the parasite Leishmania major in mice. In contrast, selective depletion of the gut microbiota, which plays an important role in modulating immune responses in the gut, had no impact on T cell responses in the skin.

  9. Natural Selection at Work


    Catching the evolution of a novel function and determining its selective parameters in nature remains an extremely difficult task. Prasad et al. (p. 1081) have undertaken this quest documenting the molecular basis of a natural allelic polymorphism and its effects on herbivory and survival in the Arabidopsis relative, Boechera stricta, living in the Rocky Mountains.

  10. A Fungal Culprit to Carbon Loss

    In some ecosystems, such as in the layer of soil containing plant roots, fungi, and bacteria, increased levels of CO2 should stimulate more efficient aboveground photosynthesis, which in turn should promote increased sequestration of organic carbon in soil through the protective action of arbuscular mycorrhizal fungi. However, in a series of field and microcosm experiments performed under elevated levels of CO2 thought to be consistent with future emissions scenarios, Cheng et al. (p. 1084; see the Perspective by Kowalchuk) observed that these fungi actually promote degradation of soil organic carbon, releasing more CO2 in the process.

  11. Epithelial Defense Force

    The nature of the cells that maintain and heal the epithelium lining the esophagus has been controversial. Doupé et al. (p. 1091, published online 19 July; see the Perspective by Kushner) show that, unlike many other tissues, mouse esophagus is devoid of slow cycling stem cells. Instead, the epithelium is maintained and repaired by a single population of proliferating cells that can switch rapidly from homeostatic behavior into “repair mode” in the vicinity of a wound.

  12. Modulating the Clock

    Because of the close association of the circadian clock with a wide range of physiological processes, identification of clock-modulating small molecules may prove useful for the treatment of circadian-related disorders, which include circadian sleep disorders, cardiovascular disease, cancer, and metabolic disease. Hirota et al. (p. 1094, published online 12 July) screened for chemical compounds that affected the period of the circadian clock in a human osteosarcoma cell line. A carbazole derivative named KL001 appeared to act by inhibiting proteolytic degradation of the cryptochrome proteins, which in turn caused a lengthening of the circadian period. KL001 also inhibited glucagon-induced gluconeogenesis in primary cultures of mouse hepatocytes.

  13. Keeping DNA Flexible


    The elastic behavior of DNA is important to biological processes that involve DNA bending and looping. However, there has been considerable debate over the flexibility of DNA at lengths below the persistence length (around 150 base pairs). A widely used approximation, the wormlike chain model, predicts stiff short DNA. Vafabakhsh and Ha (p. 1097; see the Perspective by Nelson) directly monitored cyclization of single molecules of DNA, by using a fluorescence assay, and found significant looping, with the looping rate having only weak length dependence between 67 and 105 bp, which is inconsistent with the wormlike chain model. Instead, DNA binding proteins may stabilize transiently bent or looped DNA conformations.

  14. Good Enough Can Be Good Enough

    To begin to understand why some enzymes are promiscuous and have many substrates, whereas others are highly specific, and why some have high activity, whereas others appear not to be optimized, Nam et al. (p. 1101) analyzed metabolic networks in bacteria. Specialist enzymes are essential for life, catalyze a high flux of enzymatic activity, and are more highly regulated. However, not all enzymes appear to be on a track of gradual improvement of specificity and efficiency. Generalist enzymes seem to well serve their own purposes, and their optimization may not justify the evolutionary cost.

  15. From Farm to Clinic?

    Soil organisms have long been assumed to be an important source of antibiotic resistance genes, in part because of antibiotic-treated livestock and in part because of the natural ecology of antibiotic production in the soil. Forsberg et al. (p. 1107) developed a metagenomic protocol to assemble short-read sequence data after antibiotic selection experiments, using 12 different drugs in all antibiotic classes, and compared antibiotic resistance gene sequences between soil bacteria and clinically occurring pathogens. Sixteen sequences, representing seven gene products, were discovered in farmland soil bacteria within long stretches of perfect nucleotide identity with pathogenic proteobacteria.

  16. Ocean Methane

    Is methylphosphonate a natural product? If it is, then its catabolism by microrganisms seeking phosphorus might explain the ubiquity of methane in the oceans. Anaerobic Archaea, so far, are the only significant source of methane known. Thus, Metcalf et al. (p. 1104) looked for evidence of methylphosphonate biochemistry in the widespread archaeon Nitrosopumilus maritimus. The organism can synthesize C-P bonds using phosphoenolpyruvate mutase. By using its gene, ppm, as a marker, a possible phosphonate biosynthesis gene cluster was identified, and adjacent to this a gene for a cupin was found. Together with Fe(II) and O2 the cupin catalyzed the production of methylphosphonate and formate. Homologs of the cupin gene are found in a wide range of common marine microbes, including Pelagibacter ubique and Prochlorococcus.

  17. A Double Escapee

    Toll-like receptors (TLRs)—TLR2 and TLR7—are thought to contribute to the sensing of Gram-positive bacteria like Staphylococcus aureus and Streptococcus pneumoniae by the immune system. Mice deficient in these receptors, however, are still sensitive to infection with these bacteria. Oldenburg et al. (p. 1111, published online 19 July) demonstrate that TLR13 also plays a role in detecting Gram-positive bacteria. TLR13 recognized a conserved region in the peptidyl transferase loop of bacterial 23S ribosomal RNA. Intriguingly, this same sequence is modified by specific methyltransferases that confer resistance to erythromycin. Indeed, erythromycin-resistant bacteria were no longer detectible by TLR13.

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