This Week in Science

Science  11 Nov 2005:
Vol. 310, Issue 5750, pp. 939
  1. Bigger in the Middle

    CREDIT: JOHANNESSEN ET AL.

    Rapid thinning is now occurring along the perimeter of the Greenland Ice Sheet, but the response of the interior has been more difficult to determine precisely. Johannessen et al. (p. 1013, published online 20 October) have compiled a vast set of ice sheet elevations (45 million points) from satellite observations from 1992 to 2003. The expansive interior of the ice sheet is increasing in thickness by an average of around 5 centimeters per year, driven mostly by increasing rates of snow accumulation. The authors suggest that this growth is the result of the North Atlantic Oscillation on winter precipitation. This effect must be considered carefully when predicting ice sheet mass balance changes, because the behavior of the North Atlantic Oscillation is also thought to depend on global warming.

  2. Oxo Above, Sulfur Below

    CREDIT: BUKOWSKI ET AL.

    Sulfur coordination from cysteine to iron likely affects the selectivity of hydrocarbon oxidation by cytochrome P450 enzymes. However, small model compounds that could offer more details on the reaction mechanism have been hard to construct, because without the protein scaffold, sulfur ligands are unstable in an oxidizing environment. Bukowski et al. (p. 1000, published online 27 October) have prepared an iron complex with a modified cyclotetradecane ligand, which like heme has four coordinating nitrogen atoms, but also bears a pendant thiolate group rigidly positioned near the metal. Mössbauer and x-ray absorption spectroscopy confirmed that this molecule can form an Fe=O bond at low temperature, while retaining the coordinated sulfur opposite the oxo group. The sulfur-bound iron oxo favored one-electron over two-electron oxidation chemistry relative to an analogous compound in which the sulfur ligand was absent.

  3. Looking Through HOOPs

    The molecular trigger for visual response is a light-induced cis-to-trans isomerization in the retinal chromophore of rhodopsin that occurs in less than 1 picosecond. Kukura et al. (p. 1006; see the Perspective by Champion) have used femtosecond-stimulated Raman spectroscopy to discern which atoms move when in this process. Their technique offers sufficient simultaneous time and frequency resolution to monitor the coherent spectral features due to hydrogen out-of-plane (HOOP) bending motions around the isomerizing alkene group. By modeling the data, they find evidence for a pathway of rapid (<200 femtoseconds) electronic relaxation, followed by twisting of a distorted retinal backbone to the relaxed trans structure over the ensuing 800 femtoseconds.

  4. Transitional Forcing

    During the mid-Pleistocene, the characteristic length of glacial cycles changed from 41,000 to 100,000 years. There has been much speculation about what might have caused that transition, and about the respective roles of high-latitude and low-latitude processes. Medina-Elizalde and Lea (p. 1009, published online 13 October) reconstructed a history of sea surface temperature (SST) in the western equatorial Pacific warm pool for the time interval between 1.3 million and 450,000 years before the present. The cyclicity of SST variability shifted from 41,000 to 100,000 years at the mid-Pleistocene transition, and throughout this transition, changes in tropical SSTs preceded changes in ice volume. The authors conclude that atmospheric greenhouse forcing was the cause of the switch in climate periodicities at this time.

  5. Climate Change and Ancient Plant Ranges

    Using a plant fossil assemblage from Wyoming, Wing et al. (p. 993; see the cover) show that global warming at the Paleocene-Eocene boundary (55.8 million years ago) caused rapid change in the geographic ranges of plant species. These range shifts were similar in rate and magnitude to climate-induced change in more recent, postglacial floras. Such short-term ecological change (<10,000 years) has seldom been shown in deep-time records because it is difficult to resolve transient events. The assemblage shows “individualistic” response of species to climate change (similar to conclusions from studies of quaternary pollen records), and that the “stasis” in species composition seen in deep-time records can mask dramatic, geologically short-lived events.

  6. Piece by Piece

    The electrochemically driven assembly of oligomers from different thiophene monomers on an iodine-covered gold surface has been visualized by Sakaguchi et al. (p. 1002) with the scanning tunneling microscope (STM). The polymers are grown on the surface from the monomers in solution by applying voltage pulses to the substrate. The homopolymers formed from 3-octyloxo-4-methylthiophene have a lower energy gap and show broader features in the STM images than do those from 3-octyl-4-methylthiophene. In this way, the different types of copolymer strands formed at the surface can be distinguished.

  7. Dueling Hunger Hormones?

    Ghrelin, a circulating peptide hormone produced in the stomach, has attracted much attention because of its stimulatory effect on food intake, but the effect of ghrelin may represent only half of the story. Using a bioinformatics approach, Zhang et al. (p. 996; see the Perspective by Nogueiras and Tschöp) show that ghrelin encodes a second peptide hormone that is processed from the same protein precursor as ghrelin. In rodents, a synthetic version of this hormone, obestatin, has the opposite physiological effect as ghrelin—it suppresses food intake. Obestatin mediates its actions through an orphan G protein-coupled receptor, GPR39, which shares sequences with, but is distinct from, the receptor targeted by ghrelin.

  8. Modeling Complexity

    Simple models are often insufficient for predicting or explaining complex systems such as ecosystems or financial markets, but complex, mechanistic models can be difficult to test and cannot be fully analyzed mathematically. Grimm et al. (p. 987) review several recent advances in simulation modeling in an approach they call pattern-oriented modeling, a general strategy for designing and developing explanatory models of complex systems. Pattern-oriented modeling can predict multiple observed ecological patterns at different levels of organization. This approach can be used to distinguish among alternative model structures, to focus on the most important parameters, and to simplify models when possible.

  9. Mainly a Cultural Legacy

    Neither archaeological nor modern DNA sequence data have resolved whether modern Europeans are descended from paleolithic communities inhabiting the continent for 40,000 years, or from Neolithic farmers who arrived in Europe after the end of the most recent glaciation 10,000 years ago. Haak et al. (p. 1016; see the news story by Balter) present mitochondrial DNA sequence data derived from 7500-year-old Neolithic human remains excavated from sites in Central Europe to explore the extent to which early farmers generated the present-day genetic profile of Europe. The presence of sequences now rare in modern Europeans suggests that early Neolithic farmers have left little genetic legacy, and that their impact was largely cultural.

  10. Targeting TNF-α Interactions

    The proinflammatory cytokine, tumor necrosis factor-α (TNF-α), plays a role in diseases such as rheumatoid arthritis, Crohn's disease, and psoriasis. TNF-α forms a homotrimer that binds to the TNF receptor to activate inflammatory responses. Although antibodies against TNF-α or soluble versions of the receptor are therapeutically effective, rationally designed small-molecule drugs that target protein-protein interactions would be useful. He et al. (p. 1022) report on a small-molecule inhibitor that functions by dissociating the TNF-α trimer. The inhibitor binds to the intact biologically active trimer, accelerates subunit dissociation, and forms a complex with a dimer of TNF-α subunits.

  11. Detailed View of the HIV Spike

    CREDIT: HUANG ET AL.

    The human immunodeficiency virus (HIV) envelope spike contains three gp120 glycoproteins that promote viral entry into cells. Structures of gp120 unliganded and bound to CD4 receptor have provided important insights but have lacked the immunodominant third variable region (V3) critical for coreceptor binding. Huang et al. (p. 1025) determined the structure of V3 in the context of an HIV-1 gp120 core complexed to the CD4 receptor and to the X5 antibody at 3.5 angstrom resolution. The structure provides a rationale for how V3 can serve its dual roles in neutralization and HIV entry.

  12. A Time to Grow, A Time to Crop

    Barley is a very adaptable grain crop that can be grown from the Arctic Circle to subequatorial near desert regions. Part of barley's success derives from its diverse strains that have various responses to changes in photoperiod. Turner et al. (p. 1031) have now identified the Ppd-H1 gene of barley and find that it participates in the coordinate regulation of flowering by circadian clocks and seasonal photoperiod. A spring variety of barley shows reduced photoperiod response caused by a mutation in this gene that delays its flowering. Instead, the plant accumulates the vegetative mass required to produce more grain.

  13. Real-Time Photosystem Analysis

    During photosynthesis, O2 is evolved at a tetra-manganese complex bound to the proteins of photosystem II. In the classic model for O2 evolution, the S-cycle, the manganese complex cycles through five oxidation states, but only four intermediates have been identified experimentally (S0 through S3). Dioxygen is formed during the transition from S3 to S0, but the expected S4 intermediate in this transition has been elusive. Haumann et al. (p. 1019; see the Perspective by Penner-Hahn and Yocum) have used time-resolved x-ray experiments to monitor photosynthetic O2 production in real time. They identify the S4 intermediate and, in contrast to previous proposals, find that it is formed by a deprotonation process rather than by electron transfer.

  14. A Model Forest of Carbon Storage

    The carbon storage potential of vegetation is critical for accurate modeling of climate warming. Bunker et al. (p. 1029, published online 20 October) provide evidence that tree species composition governs the potential for tropical forests to store or sequester atmospheric carbon. The study uses a computer simulation of the loss of different trait-based suites of tree species from a Panamanian rain forest to reveal how species composition affects carbon storage patterns. Different extinction scenarios resulted in a wide range of consequences for carbon storage, and there was a general increase in variability of carbon stocks as species losses progressed. Management options that favor higher functional diversity would tend to increase predictability of carbon stocks.