This Week in Science

Science  16 Dec 2011:
Vol. 334, Issue 6062, pp. 1471
  1. Human Touch


    When considering the impact of industrial processes on the environment, it is far easier to grasp direct examples—like huge piles of mining waste or runoff streaming into a river—than indirect influences far away from the source. By analyzing the isotopic signature of sediment cores from several pristine lakes in the Northern Hemisphere, Holtgrieve et al. (p. 1545; see the Perspective by Elser) demonstrate that human activity began adding substantial quantities of reactive nitrogen by the late 19th century, before fertilizer production, a known source of reactive nitrogen, became prevalent. These elevated levels may have been from burning coal residues and, although low, as compared with direct runoff, may have been substantial enough with influence biogeochemical cycles and rates of primary productivity far outside the direct footprint of early industrial activities.

  2. Cause and Effect

    In 1961, Harvard evolutionary biologist Ernst Mayr published a Science article titled “Cause and Effect in Biology,” making the distinction between the immediate (ontogenetic) causes of a trait (“how does it work?”) and the historical (phylogenetic) influence of natural selection (“why is it there?”). Fifty years later, Laland et al. (p. 1512) review the paper's subsequent impact and suggest that several current debates in biology (for example, over evolution and development, niche construction, human cooperation, and the evolution of language) are linked by a common axis of acceptance or rejection of Mayr's model of causation.

  3. Feeling No Pain

    Naked mole-rats live with up to 80 nest mates in deep underground tunnels with little oxygen exchange and high levels of CO2. Possibly because of this environment, naked mole-rats display remarkable acid insensitivity, despite containing the same acid sensors within their pain receptors as do other vertebrates. Smith et al. (p. 1557) found that a specific variant in the sodium channel of naked mole-rat pain receptors was potently blocked by protons, thus inhibiting an action potential, rather than exciting one.

  4. Zeolites Straight Up

    Porous zeolite membranes are useful for molecular separations, but some degree of channel misalignment during membrane synthesis can reduce their efficiency. Pham et al. (p. 1533) found that precise control over the temperature and composition of the gel used as a growth medium can ensure uniform channel orientation in two classes of zeolite films. The key was to restrict growth to steady elaboration of a monolayer of seed crystals on a substrate, while avoiding nucleation of new crystals in the bulk medium that might adhere to the substrate in a skewed orientation.

  5. Eating Right for Immunity


    Environmental cues shape the development of immune tissues and also regulate immune responses. Kiss et al. (p. 1561, published online 27 October) now show that natural ligands of the aryl hydrocarbon receptor (AhR) are important for the postnatal development of specialized immune structures in the gut called intestinal lymphoid follicles and for protective responses against the enteric bacterium Citrobacter rodentium. AhR ligands in the diet regulated the size of the innate lymphocyte pools, and mice fed diets that lacked AhR ligands exhibited impaired intestinal lymphoid follicle development.

  6. Autophagy and Tumor Cell Clearance

    The process of autophagy, through which cells can digest their own components, has complicated, sometimes contradictory, effects in cancer cells. Whereas loss of autophagy can lead to genomic instability and favor generation of cancer cells, maintained or enhanced autophagy can help cancer cells survive in a stressful environment. Michaud et al. (p. 1573; see the Perspective by Amaravadi) found that in mice autophagy could also have a strong influence on the response of the immune system to tumor cells dying in response to chemotherapy. Autophagy caused release of adenosine triphosphate from such cells, which helped to recruit immune cells that contributed to cancer cell clearance.

  7. A Balanced Approach

    The balance of excitatory and inhibitory inputs onto neurons ensures the stability of cortical networks and shapes their response to sensory stimulation. Vogels et al. (p. 1569, published online 10 November; see the Perspective by Ernst and Pawelzik) found that a self-organized balance of excitation and inhibition can arise from plasticity at inhibitory synapses. In this framework, a memory would substantiate itself in strengthened excitatory synapses that “bind” groups of neurons into Hebbian assemblies to allow the complete recall of a memory and in a mirror pattern of inhibitory synapses that keep the memory silent when recall is undesired.

  8. Two for One

    Conventional solar cells create a charge carrier each time a photon of light at or above a certain energy threshold is absorbed. Any energy embedded in short-wavelength photons that exceeds the threshold is largely wasted. Semonin et al. (p. 1530) now demonstrate a nanoparticulate lead selenide solar cell that channels a fraction of the excess energy into current. Chan et al. (p. 1541) explored the fundamental dynamics of multiple carrier generation in an organic (rather than inorganic) absorber and characterized a short-lived state that bridges the high-energy single carrier state and two discrete lower-energy carrier states.

  9. Improved Data Analysis

    Finding possible relations among variables in huge data sets is difficult. Ideally, the analysis should combine generality—finding all possible relations, not just linear ones—and equitability—rating those relations that have similar noise even if the functions are different. Reshef et al. (p. 1518; see the Perspective by Speed) developed an approach that meets these challenges based on fitting data relations between two variables with grids with increasing resolution and evaluating these. When compared to other approaches, ranging from standard linear regression to a variety of other more flexible statistics, the new measure—the maximum information coefficient—revealed more possible relationships, for example, in world-health data, baseball statistics, yeast gene expression data, and the ecology of gut microbiota.

  10. Close-Up of the Eukaryotic Ribosome

    High-resolution structures of bacterial ribosomes have given a detailed view of the conserved ribosome core. Recently, the architecture of eukaryotic ribosome has started to come into focus. Now, Ben-Shem et al. (p. 1524, published online 17 November; see the cover) significantly sharpen the view by providing a 3.0 angstrom resolution structure of a complete eukaryotic ribosome. The structure provides a molecular description of eukaryote-specific elements of the ribosome and their interaction with the conserved ribosomal core and provides a basis for understanding eukaryotic ribosomal function.

  11. Tracking Unstable Partners

    It has become relatively straightforward, using modern spectroscopic techniques, to probe reactions between one stable and one unstable partner—for instance, a reaction between methane (which can be stored for years and supplied copiously) and O atoms (which must be generated transiently). Daranlot et al. (p. 1538; see the Perspective by Sims) now report low-temperature rate measurements of a reaction, implicated in the chemistry of interstellar clouds, in which both partners are unstable: transformation of N atoms and OH radicals to NO radicals and H atoms. The study required precision coupling of microwave discharge and gas-phase beam techniques and was further facilitated by accompanying high-level theoretical calculations.

  12. Cooperation and Cheaters

    The social amoeba Dictyostelium discoideum produces a sort of multicellular body upon aggregation of freely living cells. Individual amoeba group together, first forming a “slug” which then differentiates into a fruiting body and stalk: Only individuals in the fruiting body get to reproduce, leaving the stalk residents to sacrifice themselves. This system allows for the investigation of the significance of relatedness for conflict resolution in clonal systems. Kuzdzal-Fick et al. (p. 1548) now demonstrate that in D. discoideum, lowering the relatedness between individuals in the aggregating slug leads to an increase in “cheating” mutants, which avoid residence in the stalk, thereby increasing their own reproduction to the detriment of noncheaters.

  13. Complete at Last

    For the past 44 years, based on the failure to detect an enzyme activity or gene sequence, cyanobacteria have been considered to have an incomplete tricarboxylic acid cycle (TCA) cycle, which explained why most of these organisms are obligate autotrophs. No 2-oxoglutarate dehydrogenase activity has been detected and no sequences for this enzyme were found in the cyanobacterial genomes. Zhang and Bryant (p. 1551; see the Perspective by Meeks) overturn this dogma with their discovery of functionally equivalent alternative enzymes in many cyanobacteria and anaerobes. Two enzymes, 2-oxoglutarate decarboxylase and succinic semialdehyde dehydrogenase, together close the TCA cycle by converting 2-oxoglutarate directly to succinate. Furthermore, homologs of the genes encoding these two enzymes are found in many bacteria that lack 2-oxoglutarate dehydrogenase.

  14. I'm Starving!

    The intravacuolar pathogen Legionella pneumophila injects the AnkB F-box effector into the host amoeba or human cell. Host-mediated farnesylation then anchors AnkB into the outer leaflet of the Legionella-containing vacuole (LCV) membrane, which is required for intravacuolar proliferation of L. pneumophila and for its virulence in vivo. Price et al. (p. 1553, published online 17 November) now show that AnkB promotes preferential docking of Lys48-linked polyubiquitinated host proteins to the LCV, which is followed by their proteasomal degradation. This generates an intracellular source of amino acids, which suppress an amino acid starvation response by L. pneumophila, and allows intravacuolar proliferation. Supplementation of amino acids completely rescued an ankB null mutant from its severe intracellular growth defect and suppressed its amino acid starvation response in amoeba and human cells.

  15. Matrix Machinery Mutants

    The synaptic cytomatrix is a poorly understood meshwork of proteins in the presynaptic active zone that ensures that synaptic vesicles, SNARES, and calcium channels are properly organized for efficient synaptic vesicle release during neurotransmission. Liu et al. (p. 1565) found that Drosophila Rab-interacting mole cule–binding protein (DRBP) is part of the active-zone cytomatrix core. DRBP was required for normal organization of calcium channels, and, in its absence, synaptic transmission was seriously impaired. Stoichiometric amounts of DRBP were needed for proper active-zone ultrastructure and function. Thus, DRBP is required for the coupling of voltage-gated calcium channels and the fusion machinery of synaptic vesicles.

  16. Power in Numbers

    Flocks of birds and schools of fish often move together with fantastic coordination. These groups, however, are composed of individuals, each of which can make its own decisions. How can so many individuals come together to make a single collective decision? Past work has suggested that a strongly opinionated minority of individuals, both in animals and humans, might be able to exert disproportional pressure on the decision-making process. Applying theory and experimentation to the shiner, a schooling fish, Couzin et al. (p. 1578; see the Perspective by West and Bergstrom) confirmed that a small informed minority can dictate group decisions. However, the presence of uninformed individuals limited the influence of the minority opinion.