This Week in Science

Science  17 Sep 2004:
Vol. 305, Issue 5691, pp. 1673
  1. Standing CO2 on Its End

    CREDIT: CASTRO-RODRIGUEZ ET AL.

    Understanding how plants reduce CO2 to sugars, and facilitating attempts to mimic this chemistry, requires better insight into the specific binding geometry of CO2 at metal centers. Synthetic chemists studying the problem usually start with metal complexes that coordinate CO2 through the C atom, with one or both O atoms bent away from the metal. Castro-Rodriguez et al. (p. 1757) have prepared a U complex in which coordinated CO2 remains linear and binds end-on to the metal through a single O atom. X-ray crystallography verified this unusual bonding geometry.

  2. Sugar in Two Steps

    Hexose sugars are naturally abundant, but it is often useful to modify their structures for chemical and biochemical studies. Standard synthetic routes tend to be long and tedious and require multiple protection steps. Northrup and MacMillan (p. 1752, published online 12 August 2004) now describe a reaction sequence for generating the sugars from achiral aldehyde precursors in just two steps, thereby offering a convenient means of preparing diverse structural variants. In the first step, α-oxyaldehydes are dimerized with l-proline as the only source of asymmetry throughout the sequence. In the second step, an aldol addition-cyclization step is controlled by variation of solvent and Lewis acid to afford any of three stereoisomeric products (glucose, mannose, or allose), all in high yield and stereochemical purity.

  3. Disilyne Debut

    Double and triple bonds are common in compounds of the first-row elements carbon, nitrogen, and oxygen. In contrast, the heavier main group congeners tend to form single-bonded networks instead, because repulsion by inner-shell electrons keeps the atoms too far apart for π-bonding. Sekiguchi et al. (p. 1755; see the Perspective by West) have managed to push two Si atoms close enough together to form a Si-Si triple bond. They reduced a brominated precursor in which the Si atoms bear very bulky side groups that help destabilize more conventional bonding options. X-ray crystallography revealed a bent geometry consistent with theoretical predictions that the silicon orbitals do not hybridize like those of carbon do in rigidly linear alkynes.

  4. Damage-Free Dating

    Many geologic boundaries reflect dramatic changes in species abundances or mark the origination of species. Thus, the accurate determination of their ages is essential for defining the pace of evolution. One of the best dating methods, based on the decay of U isotopes to Pb can be problematic if damaged parts of zircons, the primary uranium-bearing mineral, lose radiogenic Pb or incorporate older cores. Mundil et al. (p. 1760, see the News story by Kerr) used a recent method that strips out these damaged areas to refine the age of the end-Permian extinction and Permo-Triassic boundary. Their data on a sequence of ashes in two localities place the extinction at 252.6 million years ago, about 1 million years older than previously determined. The results support the conclusion that the extinction occurred within the limit of the method, just a few hundred thousand years.

  5. Early Oxygen History

    Measurements of the three stable isotopes of oxygen in primitive meteorites that formed in the solar nebula indicate that the nebular gas had an initial enrichment in 16O that was quickly depleted. Observations of molecular clouds indicate that ultraviolet radiation selectively dissociates C17O and C18O, but not C16O, which leaves the atomic oxygen gas in the interior of the cloud depleted in 16O. Yurimoto and Kuramoto (p. 1763; see the Perspective by Yin) have developed a model to explain the meteoritical data using the astronomical observations. The oxygen isotopic differences developed in the molecular cloud via photodissociation. When the cloud collapsed into the solar nebula disk, the isotopic differences were transported to the inner disk by icy dust grains that evaporated when they neared the Sun.

  6. Why the Ice?

    The large, permanent ice sheets that presently occupy Antarctica began to form around 14 million years ago, when Earth entered a phase of global cooling. However, the climate processes that produced these changes, as well as the temporal relation between ice sheet growth and cooling, have remained obscure. Shevenell et al. (p. 1766) analyzed Mg/Ca ratios (a proxy for temperature), oxygen isotopes (which record a combination of temperature and seawater oxygen isotopic composition), and carbon isotopes (a proxy for atmospheric CO2 concentrations) of benthic foraminifera from Southern Hemisphere marine sediments with ages between 15 and 13.2 million years. Deep-ocean cooling began roughly 60,000 years before ice sheet growth, and both of these processes happened during a period of atmospheric CO2 increase. These findings suggest that factors other than radiative forcing, such as ocean heat transport, were key elements of this climate transition.

  7. Two Membranes, Two Fusion Mechanisms

    CREDIT: MEEUSEN ET AL.

    Mitochondria, the powerhouses of the cell, are surrounded by a double membrane. Within the cell, mitochondria continually fuse with one another, but the mechanism by which their two membranes can faithfully fuse remains obscure. Meeusen et al. (p. 1747, published online 5 August 2004; see the Perspective by Pfanner et al.) now present a cell-free assay that reconstitutes efficient mitochondrial fusion in vitro. In the assay, the fusion of the outer and inner mitochondrial membranes can be individually scrutinized, and the two fusion events can be mechanistically distinguished.

  8. Lasting Legacy of Formative Years

    Development and disease susceptibility are not purely a function of genotype— environment plays a large part in shaping an organism and in its demise. Furthermore, the environment begins having its effect at the earliest of stages of development, during periconception, fetal, and infant stages. The concept of developmental origins of disease has gained credence through epidemiological and clinical studies. Gluckman and Hanson (p. 1733) review fundamental observations, discuss mechanisms of action, and discuss the concept of developmental origins of disease from an evolutionary perspective. Finch and Crimmins (p. 1736) suggest that exposure to infection and other environmental sources of inflammation during infancy and childhood leave a long- lasting imprint on morbidity and life expectancy in old age.

  9. Eosinophil Effects in Mouse Models of Asthma

    CREDIT: LEE ET AL.

    An assortment of leukocyte subsets are recruited to the lung during an asthmatic episode and accompany immediate changes to the mucosal lining, as well as long-term airway remodeling. Eosinophils are dominant among these infiltrating cells, but their presence has, so far, been linked only indirectly with disease (see the Perspective by Wills-Karp and Karp). Lee et al. (p. 1773) used a mouse model in which cell lineage-specific deletion of eosinophils could be achieved. In these animals, challenge with an allergen normally able to elicit a robust asthma-like response failed to generate significant pulmonary dysfunction or mucus accumulation. In a different eosinophil-deficient mouse line generated by Humbles et al. (p. 1776), these acute aspects were not significantly affected, but over the long term, these mice were protected from peribronchiolar collagen deposition and increases in airway smooth-muscle mass.

  10. Dissecting the Evolution of a Sign Language

    Human languages are digital in the sense that they are formed from discrete units. Is the brain predisposed toward dealing with sounds, words, and phrases, or are the existing languages that we learn simply structured discretely? Senghas et al. (p. 1779; see the Perspective by Siegal) offer evidence in support of the former view, drawing upon a population of deaf individuals in Nicaragua who have developed a new sign language. Descriptions of complex motion events are segmented into separate gestures representing the manner of movement (such as rolling) as well as path (such as downward).

  11. How Sweet Is Your Tomato?

    Quantitative traits suggest an underlying complexity of metabolism because gradations of a particular phenotypic trait make themselves apparent. The sweetness of tomatoes, particularly those tomatoes used for making ketchup, is one such trait. Fridman et al. (p. 1786) now analyze near-isogenic lines to identify the particular point mutation in an invertase enzyme that is responsible for gradations of sweetness in tomatoes. Unlike many other quantitative traits, which are often the summed result of several mutations, this sweetness gene acts on its own.

  12. Shark Heavy Chain Structure Revealed

    Although cartilaginous fish diverged from other jawed vertebrates about 500 million years ago, they possess components of the vertebrate adaptive immune system. Besides conventional heavy and light chain antibodies, cartilaginous fish also contain antibodies comprising a single heavy chain (IgNAR). Stanfield et al. (p. 1770, published online 19 August 2004) determined a 1.45 angstrom resolution structure of a nurse shark IgNAR variable domain bound to lysozyme. Unlike camelid single-domain antibodies, the IgNAR lacks one of the three complementarity-determining regions found in conventional antibody-variable Ig domains. Nevertheless, it binds lysozyme with nanomolar affinity and has a binding interface comparable in size to conventional antibodies.

  13. From Gene Expression Patterns to Function

    One goal of microarray is to map patterns of linked gene expression with function. Ao et al. (p. 1743) used an algorithm to establish clusters of genes expressed in different pharyngeal cell types to identify five cis-regulatory elements with biological activity within the foregut of Caenorhabditis elegans. The protein DAF-12 binds one of these elements to modulate the response to food availability. A large data set of putative foregut genes is processed to identify a transcription factor and its binding site and to show physiological conditions under which this factor works.

  14. Moving Forward with Actin

    When cells move on a substrate, their leading edge protrudes forming a lamellipodium. Actin assembly is known to play a key role in promoting cell motility. Ponti et al. (p. 1782) examined the detailed dynamics of actin at the leading edge using fluorescent speckle microscopy. They observed two independent populations of actin with overlapping distributions that possessed distinctive dynamic properties and that appeared to promote different aspects of leading edge motility.