This Week in Science

Science  14 Nov 2008:
Vol. 322, Issue 5904, pp. 1021
  1. How Stilbene Twists


    Over the last decade, ultrafast vibrational spectroscopy has offered detailed glimpses into how molecules rearrange upon excitation. However, the available information tends to be confined to the small segment of the molecule that is most actively changing. Takeuchi et al. (p. 1073; see the Perspective by Blank) used a coherent Raman technique to track vibrations more globally across the framework of stilbene during its photoinduced cis-to-trans isomerization about the central C=C double bond. Tracking the steady frequency shift of a skeletal vibration and then modeling the process theoretically produced a thorough picture of the order in which different portions of the molecule move, starting with a lengthening of the double bond and extending to the twisting of pendant H atoms out of plane.

  2. Homo erectus Hips


    Human pelvic morphology is central to the understanding of obstetrics, sexual dimorphism, and neonatal brain size and patterns of brain growth, as well as the evolution of body form and its relation to locomotor refinements and adaptation to tropical environments. However, sufficiently preserved pelvic bones are very rare in the fossil record, and only few such hominid fossils are known from the entire Plio-Pleistocene record of Africa. Simpson et al. (p. 1089) have successfully recovered, and restored a near-complete adult female Homo erectus pelvis from Gona, Afar, Ethiopia, dated to ∼ 0.9 to 1.4 million years ago. The H. erectus pelvis was much more australopithecine-like than hitherto thought, and allowed a neonate brain size 25 to 30% larger than earlier estimates, suggesting that H. erectus lacked a fully human-like phase of infant dependency. Additionally, H. erectus did not have the tall narrow body form of modern humans adapted to tropical, semi-arid environments or hips adapted for long distance running, previously thought to characterize H. erectus.

  3. Oxygen Torn Apart

    When molecules are photoionized with excess energy, they can relax by ejecting a second electron. However, photoionized oxygen has appeared to relax in this way only after the nuclei spread a substantial distance apart. By pairing recently developed attosecond x-ray pulse generation techniques with precise ion imaging, Sandhu et al. (p. 1081) uncover the detailed dynamics underlying this behavior. The initially generated O2+ ion quickly drops below the energy threshold for formation of the dication, but, as the nuclei repel one another, the dication becomes accessible at a separation distance of roughly 30 angstroms. At that stage, a transient state (a Feshbach resonance) can be observed, which persists due to spin-orbit coupling before eventually decaying through further ionization or radiative relaxation.

  4. Quantifying Global Photosynthesis

    The utility of climate models rests in part on how well the uptake of CO2 by plants, i.e., photosynthesis, can be represented, because carbon and climate are so inextricably entwined. However, how much photosynthesis actually occurs on a global scale is very difficult to measure, because the available techniques are either indirect, or direct but not amenable to large-scale application. Campbell et al. (p. 1085) show using measurements made during the North American growing season that carbonyl sulfide, COS, is a good surrogate for CO2, and that the quantitative relationship between the two that has been measured in the laboratory also extends to the bulk atmosphere. Thus, the measurement of vertical atmospheric concentration gradients of COS should reflect how much photosynthesis is occurring over continents during the growing season.

  5. Digitizing Development

    Current microscopes provide neither the speed nor the low phototoxicity required for recordings of entire embryos over long periods of time, which would be required to reconstruct a complete picture of vertebrate development. Keller et al. (p. 1065, published online 9 October; see 10 October news story by Vogel; cover) developed digital scanned laser light sheet fluorescence microscopy that overcomes these limitations and delivers quantitative information for entire zebrafish embryos at subcellular resolution. The data provide a developmental blueprint of a vertebrate species and simultaneously track about 20,000 cells up to a stage in which major organs show function.

  6. Disordering of Surface Tiles

    Molecular networks on surfaces could provide a readily interrogated model for understanding the structural basis of glasses, but often the interactions between molecules lead to well-ordered arrays. Blunt et al. (p. 1077) used scanning tunneling microscopy to investigate an intermediate case between crystals and glasses in which an organic molecule (p-terphenyl-3,5,3′,5′-tetracarboxylic acid) absorbed on graphite locally organizes into rhombus tiles. The tiles have a nonperiodic arrangement and are not ordered translationally. Networks formed as junctions of three to six molecules with hexagonal symmetry, and triangular defects could form and move through the network causing reordering oh the local arrangement.

  7. Putting the Brakes on Inflammation

    Numerous adhesion receptors of the selectin, integrin, or immunoglobulin family promote inflammatory cell recruitment. In contrast, inhibitors of the leukocyte adhesion cascade are not well known. Now Choi et al. (p. 1101) have characterized developmental endothelial locus-1 (Del-1) as an endogenous inhibitor of the leukocyte adhesion cascade. Del-1, which is an endothelially expressed, secreted molecule, is a ligand of the major leukocyte adhesion receptor LFA-1. Soluble Del-1 inhibited neutrophil adhesion under both static and physiologic flow conditions. Endothelial Del-1 deficiency promoted increased leukocyte adhesion, and mice lacking Del-1 displayed significantly higher neutrophil accumulation during lung inflammation, which was reversed in Del-1/LFA-1 double deficient mice. Thus Del-1 interacts with LFA-1 preventing inflammatory cell recruitment.

  8. Ubiquitin's Pup(py)?

    Ubiquitin is a universal modifier used by eukaryotes to tag proteins for degradation. Now Pearce et al. (p. 1104, published online 2 October; see the Perspective by Mukherjee and Orth) describe a ubiquitin-like protein system they call Pup in prokaryotes. Pup appears to be required for protein degradation by the Mycobacterium tuberculosis (Mtb) proteasome. Because proteasome function is essential for the virulence of Mtb, the Pup conjugation pathway could potentially be targeted for the development of antituberculosis drugs.

  9. From Flower to Flower


    Although horizontal gene transfer has been extensively studied in bacteria, its role in the evolution of multicellular plants and animals has been explored little. M. Kim et al. (p. 1116) analyze a key morphological and ecological trait transferred naturally between two higher eukaryotic species, flowers of the genus Senecio. The transfer involves introgression of a cluster of regulatory genes that control flower morphology. The genes are expressed in the outer regions of the developing flower where they promote the production of asymmetric florets with large petals, yielding a daisy-like head that confers higher levels of outcrossing. Thus, regulatory genes can allow a key morphological and ecological trait to be gained, lost, and regained during evolution, providing a more dynamic view of evolutionary change than the traditional one which considers each lineage as evolving independently.

  10. Sole Food

    Termites have a formidable capacity for digesting dead wood. Consequently they have become major pests destroying man-made structures around the world. The biochemical talents required for digesting wood are much sought after by humans for processing biofuels, but because wood is an unbalanced foodstuff and lacks nitrogen, no simple solution is available. Termites owe their success to arrays of symbiotic microorganisms possessing complementary metabolisms. Hongoh et al. (p. 1108) have sequenced the genome of a dominant bacterial symbiont living within a dominant protozoan that lives in termite guts. The sequence reveals genes that allow the bacterium to fix atmospheric nitrogen, to recycle nitrogen from waste nitrogen products from its protozoan host, and to make amino acids for its own and both its host's and its host's host's use. The energy required for nitrogen fixation is considerable and the bacterium obtains this not only from hydrogen produced during nitrogen fixation but also from the anaerobic fermentation of sugars released from cellulose by the protozoan.

  11. To Be or Not to Be?

    The tiny pores, or stomata, that open and close on a leaf's surface allow for the exchange of gases according to the needs of the plant's physiology. The number of stomata formed during development is a result of competing signals that activate or repress pore formation. Lampard et al. (p. 1113) now show how these competing signals converge so that their inputs result in one question: Will there or won't there be a stoma placed here? The transcription factor SPEECHLESS, which activates stomatal development programs, can be phosphorylated by certain mitogen-activated protein kinsases (MAPKs), a group of kinases that, among myriad other functions, repress formation of stomata. All the phosphorylation sites are contained within the 93-amino acid MAPK target domain of SPEECHLESS, which thus integrates positive and negative signals.

  12. Growing Crystals

    One route to growing nanoscale materials is through vapor-liquid-solid growth, where vapors of one or more materials are deposited onto a substrate where they form liquid droplets. When supersaturation occurs, crystals nucleate and grow from the liquid. B. J. Kim et al. (p. 1070) studied the formation of silicon particles growing from a gold-silicon droplet and tracked the kinetics of nucleation processes using transmission electron microscopy. Nucleation rates did not depend on the supersaturation for particles down to 12 nanometers in diameter.

  13. Resolving Reverse Transcriptase's Repertoire

    Before human immunodeficiency virus (HIV) can integrate into host cell DNA, the single-stranded viral RNA must be converted into double-stranded DNA. This requires several activities, including DNA polymerization, RNA cleavage, strand transfer, and strand displacement synthesis. All are catalyzed by the multifunctional HIV reverse transcriptase (RT) enzyme, making it a primary target for HIV drug therapy. To gain insight into how the enzyme switches between different functions, Liu et al. (p. 1092; see the Perspective by Sarafianos and Arnold) have used fluorescence resonance energy transfer to monitor the interaction of individual HIV-1 RT molecules with nucleic acid substrates. RT slides over long distances on nucleic acid duplexes, shuttling back and forth between opposite termini. In addition, it can spontaneously flip between opposite binding orientations that support different activities. The enzyme can thus rapidly sample different sites and access the binding configurations required to function.

  14. Cross-Presentation and Tumor Rejection

    Cross-presentation is the pathway by which an antigen produced in a virally infected somatic cell can end up being presented by the major histocompatibility complex class I molecules of a professional antigen-presenting cell, such as a dendritic cell. Hildner et al. (p. 1097) have produced a mouse line that selectively lacks the development of the CD8α+ subset of conventional dendritic cells and analyzed the in vivo responses to model antigens, viral pathogens, and rejection of tumors. The findings confirm previous speculations that this dendritic cell subset is important for CD8+ T cell responses to virus and show that the rejection of tumors is also dependent on these particular dendritic cells.

  15. Missing Features

    Oceanic cyanobacteria are the oxygen-producing, carbon- and nitrogen-fixing engines of our planet. Because oxygen is toxic for nitrogenase, nitrogen fixation is a bit problematic to fit into this metabolic mix. Multicellular cyanobacteria like Trichodesmium have evolved a range of temporal and compartmental strategies to protect their nitrogenase enzymes from being poisoned. A newly discovered and abundant group of unicellular cyanobacteria has dispensed with carbon fixation and oxygen production altogether and so it can run nitrogenase without hindrance. Zehr et al. (p. 1110) have extracted this group by cell sorting and, by genome analysis, show that indeed it lacks the genes both for photosystem II and for carbon fixation via the reductive pentose phosphate cycle. This finding makes it necessary to reevaluate current models of nitrogen and carbon cycling on Earth.