This Week in Science

Science  28 Jan 2005:
Vol. 307, Issue 5709, pp. 477
  1. Bumping into Your Partner-to-Be


    Pluto and Charon orbit the Sun on an inclined and eccentric path that comes inside that of Neptune from time to time. Although the Pluto-Charon system is small (both objects are smaller than Earth's Moon) and has an unusual orbit, it shares some similarities with the Earth-Moon system, particularly a high angular momentum. Canup (p. 546) modeled the formation of Pluto-Charon with collisions between 1000-kilometer objects and found that Charon is likely to have formed as an intact satellite from a grazing collision between undifferentiated bodies. These simulations suggest that many satellites might form by collisions, and that collisions in the Kuiper Belt may have been relatively common in the early solar system.

  2. Images by Connecting with Dots

    Chemical dyes have long been used to enhance contrast in biological imaging, but are limited by their rapid bleaching and a lack of tunability in their emission spectrum. Semiconductor nanoparticles, also known as quantum dots, show exceptional stability and can be easily tuned by changing the particle size and chemistry. However, to achieve widespread use, a number of techniques have been developed to synthesize, solubilize, and functionalize the particles for use in vitro and in vivo. Michalet et al. (p. 538; see the cover) review the state of the art in making and applying quantum dots to biological problems.

  3. Electrons Light the Darkness

    After molecules absorb light, they must somehow dissipate the excess energy. In fluorescence, much of this energy is simply re-emitted as photons. Alternatively, the molecules may undergo a range of radiationless processes, such as bond stretching and breaking, or changes in electronic structure, and the intermediates formed are termed “dark” states. Srinivasan et al. (p. 558) have used time-resolved electron diffraction to probe the structures of transient dark states that form after photoexcitation of benzaldehyde and several pyridine derivatives. In benzaldehyde, competing bond scission and singlet-to-triplet conversion pathways can be distinguished. The pyridine study reveals temporary C-N bond cleavage in pyridine and methylpyridine relaxation, in contrast with energy transfer to delocalized vibrations in dimethylpyridine.

  4. Recovering Rates


    One goal in catalysis is to reproduce the rate of a complex reaction from quantum chemical calculations. Honkala et al.. (p. 555) calculated the rate of ammonia synthesis over nanosized ruthenium particles using density functional theory, with the only experimental input being the particle size distribution, from which the authors estimated the number of step sites that favor the rate-limiting N2 dissociation step. Their rates agree with experimental data for ammonia synthesis over Ru nanoparticles supported on magnesium aluminum spinel to within factors of 3 to 20 for temperatures between 320° and 440°C. Compensation effects between changes in activation energies and surface coverages help to make the final rates less sensitive to absolute errors in the several individual elementary reaction steps.

  5. Martian Glow

    The ultraviolet and infrared spectrometer (SPICAM) on the Mars Express spacecraft has detected a 20-kilometer-thick spherical shell of nitrous oxide (NO) at an altitude of about 70 kilometers in the martian atmosphere. Bertaux et al. (p. 566) found that the concentration of NO is highest (and the emission is brightest) in the south polar winter night. The emission, called night glow, occurs in a region not illuminated by the Sun, so the NO must have formed from O2, CO2, and N2 that had photodissociated on the sunlit side. The N and O atoms were transported to the night side and formed NO at lower altitudes. The night glow allows scientists to track the atmospheric circulation pattern and understand climate change on Mars.

  6. Farming-Biodiversity Face-Off

    Farming is already the single greatest threat to the persistence of the other species on the planet. This threat is set to grow as human demand for food rises two- to threefold during the next 50 years. Two main solutions to the problems of coexistence of biodiversity and increased demand for agricultural products have been proposed: wildlife-friendly farming, and land-sparing permitted by high-yield farming. Green et al. (p. 550, published online 23 December 2004) show that the best type of farming for species persistence depends on the demand for agricultural products, and on the shapes of functions relating the population densities of different species on farmland to agricultural yield. Data for a range of taxa in developing countries suggest that high-yield farming may allow more species to persist.

  7. Bat Origins and Diversity

    Bats represent one-fifth of extant mammalian diversity, yet they are the least represented mammalian order within the fossil record. Where fossils do exist, they are often only single teeth, making it difficult to identify species and decipher the chronology of divergence events and biogeography. Teeling et al.. (p. 580; see the Perspective by Simmons) have sequenced multiple genes from representatives of all bat families and have derived a phylogeny that divides the order Chiroptera into two suborders and four superfamilies, as well as resolving the phylogenetic position of Craseonycteris thonglongai, the smallest mammal in the world. The four microbat lineages originated 52 to 50 million years ago at the peak of Tertiary insect diversity in Laurasia. The analysis also suggests that as much as 61% of the bat fossil record may be missing.

  8. Let ZmEA Guide You


    In flowering plants, the immotile sperm cells are transported toward the egg through the pollen tube, which navigates its own path from the pollen grain to the female gametophyte. Márton et al.. (p. 573) have now identified one of the signals likely to guide the pollen tube. Studying maize, a small egg apparatus- derived protein, ZmEA1, was found to light up the way for the final steps of pollen tube guidance. The ZmEA1 gene was expressed in the egg apparatus, and the protein acted as a short-range signal required for successful fertilization. After fertilization, ZmEA1 expression was rapidly down-regulated.

  9. Seeds Great and Small

    Seed mass varies across more than ten orders of magnitude, from the dustlike seeds of orchids to the 20-kilogram double coconut. Using data from nearly 13,000 plant species, Moles et al. (p. 576) trace the history of the evolutionary radiation of seed mass, from the emergence of angiosperms to the present day. They identify the largest divergence events and those that contributed most to the spread of seed mass across present-day species. Large divergences in seed size were more consistently associated with divergences in plant-growth form than with divergences in dispersal syndrome or in latitude.

  10. The Rewards of a Stand-Up Routine

    Although purposeful exercise like jogging is well known to reduce one's risk of obesity, it is less clear whether interindividual differences in posture and body movement during the routines of daily life have an impact on body weight. Using exquisitely sensitive technology, Levine et al.. (p. 584; see the Perspective by Ravussin) continuously measured the posture and body position of 20 volunteers during a 10-day period. All volunteers were self-proclaimed “couch potatoes,” but 10 were lean and 10 were mildly obese. On average, the lean individuals stood up and moved around 2 hours longer each day than the overweight individuals, reinforcing the notion that even simple changes in behavior, such as sitting less each day, may help to prevent weight gain.

  11. The Nuts and Bolts of an Autoimmune Disorder

    Autoimmune disorders, such as systemic lupus erythematosus (SLE), arise from a breakdown of immune tolerance to the body's own constituents, and represent the culmination of multiple environmental and genetic influences. Nevertheless, it is likely that specific regulatory pathways of the immune system are perturbed. McGaha et al.. (p. 590) studied genetically distinct strains of mice that share a susceptibility to developing SLE and that also express reduced levels of a particular inhibitory antibody-binding receptor. Engineering bone marrow from these animals to express the receptor gene prevented disease by partially restoring levels of the receptor on B cells. Thus, even in the context of multiple contributing factors, the modulation of a single regulatory pathway can be sufficient to dictate the course of autoimmune pathology.

  12. Autophagy and Antigen Presentation

    One-third of all eluted major histocompatibilty complex (MHC) class II natural ligands are derived from endogenous cytosolic or nuclear proteins, but the underlying pathway has been difficult to pinpoint. EBNA1, the dominant CD4+ T cell antigen of the human oncogenic Epstein-Barr virus (EBV), is the sole viral antigen present in all EBV-associated malignancies. Paludan et al.. (p. 593, published online 9 December 2004) describe how autophagy, a process by which the cell degrades defunct cytosolic components in times of stress, leads to MHC class II processing and presentation of endogenous EBNA1. The viral protein was imported into lysosomes by autophagy, where a subset of lysosomal proteases was responsible for EBNA1 degradation. Furthermore, inhibition of autophagy decreased target recognition by EBNA1-specific CD4+ T cell clones.

  13. Photoelectron Spectroscopy of Salt Solutions

    Studies of the reactivity of aerosol salt-water droplets, as well as molecular dynamics (MD) simulations, indicate that anion concentrations are enhanced at the water liquid-vapor interfaces. Spectroscopic probes such as vibrational sum-frequency generation can quantify differences between the near-surface region and the bulk but tend to integrate the entire near-surface composition. Ghosal et al. (p. 563) performed x-ray photoelectron spectroscopy experiments, which are often limited to high-vacuum conditions, on KBr and KI solutions formed on salt crystals as a function of water vapor pressure up to and at the deliquescence point. They could vary the photoelectron kinetic energy by changing the incident photon energy, and in this manner they profiled the ion concentrations as a function of depth from the interface. They observed enhanced concentrations of anions at the interface, and greater enhancements for the more polarizable I anion. These enhancements appear to exceed even those predicted by MD simulations.

  14. Designer Hydrogen Membranes

    When used as a chemical feedstock or fuel, hydrogen gas often needs to be purified. Metal membranes such as those based on palladium work well, but are prone to poisoning by impurities such as sulfur. While palladium alloys are able to resist poisoning while still maintaining high flux and selectivity, designing alloy systems is costly and time consuming. Kamakoti et al.. (p. 569) use ab initio calculations and coarse-grained modeling to predict hydrogen flux behavior for a series of alloys. The only input parameters for the models are the bulk crystal structures. For a series of palladium copper alloys, they can predict the fluxes for both body- and face-centered cubic structures, as confirmed by experimental measurements on thick foils.

  15. Watching a DNA Motor in Action

    DNA translocases are adenosine triphosphate (ATP)-dependent molecular motors that rapidly move DNA during processes such as chromosome partitioning, DNA recombination, and DNA export. FtsK is a membrane-bound translocase that coordinates chromosome segregation with cell division. In particular, it is involved in resolving circular DNA dimers that occur during replication, and to do this, it must move toward recombination sites termed dif. Pease et al.. (p. 586) directly measured the movement of FtsK along DNA by single-molecule methods. FtsK moved at 5 kilobases per second over long distances toward dif against forces up to 60 piconewtons, but could locally reverse direction without dissociation. FtsK appears to be composed of two motors, only one of which is active at any one time.

  16. How Not to Differentiate a Neuron

    What do neuronal precursor cells and non-neuronal cells have in common? Neither expresses the genes typical of differentiated neurons. Yeo et al.. (p. 596) now show what else is in common: expression of certain SCP phosphatases. The SCP phosphatases serve to repress expression of neuronal genes, and perhaps stabilizing non-neuronal differentiation programs or keeping progenitor cells in check as they wait for their call to action. Inhibition of the SCP phosphatase activity increased neuronal differentiation. It appears that SCP phosphatase activity acts globally to silence neuronal genes in neuronal precursors and non-neuronal cells.

  17. Not So Different After All

    Melanopsin is an atypical opsin protein required in vertebrates to mediate nonvisual responses to light, including regulation of circadian behavior and pupil constriction. Panda et al.. (p. 600) report that when expressed in Xenopus oocytes, melanopsin activates heterotrimeric G protein signaling pathways characteristic of invertebrate opsins in response to light. Melanopsin could also active mammalian TRPC channels, whose paralogs in Drosophila serve as phototransduction channels. Thus, an invertebrate signaling system appears to be preserved in the mammalian retina.