Editors' Choice

Science  03 Sep 2010:
Vol. 329, Issue 5996, pp. 1129
  1. Planetary Science

    Lunar Exposure

    1. Maria Cruz

    The Lunar Magma Ocean hypothesis postulates that the outer parts of the Moon formed from a global magma ocean that separated and crystallized into a dense olivine- and pyroxene-rich mantle and a buoyant plagioclase-rich crust. The crust has been sampled through rocks collected and returned from the surface of the Moon and through remote sensing, but the mantle, being inaccessible, is still poorly understood. Yamamoto et al. now propose that in some regions of the Moon, meteorite impacts may have penetrated the mantle/crust boundary and exposed olivine-rich material from the upper mantle. Their data, acquired with the visible and near-infrared spectrometer onboard the SELENE/Kaguya satellite, show that olivine-rich rocks on the surface of the Moon are limited to regions around impact craters that are located in areas where the crust is particularly thin. Meteorite impacts would have thus blasted through the thin crust and brought olivine-rich material from the mantle to the surface. It is possible, however, that the Kaguya data are instead sampling troctolites, rocks containing both olivine and plagioclase, which are thought to have formed from molten magma that rose from the mantle into the lower crust and cooled there. According to Lucey, if Kaguya's data are explained by troctolites, then these rocks could have formed from melting induced by impacts that blasted into the regions where the mantle intruded into the crust.

    Nat. Geosci. 3, 533; 517 (2010).

  2. Cell Biology

    Turn On and Stay Put

    1. Helen Pickersgill

    The Ras pathway transduces extracellular signals into intracellular gene expression profiles and regulates physiological processes such as proliferation, differentiation, and survival; defective Ras signaling can cause cancer and developmental disorders. Ras proteins are small GTPases, and their compartmentalization and controlled activation are crucial. Ras proteins are activated by guanine nucleotide exchange factors (GEFs) that put on a GTP and take away a GDP. Previous studies have shown that conjugating the small molecule ubiquitin to Ras can regulate its intracellular localization. Xu et al. have found that the GEF Rabex-5 is also an E3 ubiquitin ligase; it can attach ubiquitin to Ras in vitro, and it regulates Ras localization and activity in mammalian cells. Yan et al. have found that Rabex-5 signaling is essential in Drosophila. The loss of Rabex-5 caused gross developmental defects, including increased size and ectopic wing veins. Because Rabex-5 has both GEF and E3 ligase activity, it may serve to integrate these two regulatory pathways.

    Curr. Biol. 20, 1372; 1378 (2010).

  3. Microbiology

    Monsters in the Mangrove

    1. Caroline Ash

    Ovoid bacteria coating filamentous archaea.

    CREDIT: MULLER ET AL., ENVIRON. MICRO. 12, 2371 (2010)

    Life on Earth has been classified into three domains: the Bacteria and the Archaea are single-celled microbes lacking nuclei, and the nucleated Eukarya possess a combination of archaeal- and bacterial-like genes. There are some interesting exceptions to the general smallness of life (apart from us), and Muller et al. have discovered mats of long (30 mm) white filaments attached to the roots of mangrove trees in Guadaloupe. These filaments are two new species in a recently discovered phylum within the Archaea. One type of filament is coated with a single-cell layer of a γ-proteobacterium, which contains large globules of sulfur. The other, uncoated filament was found only where the gregarious species was abundant. The associated bacteria may be symbionts that scrub out otherwise toxic sulfide oozing up from the mangrove swamp sediment, and this detox process in turn could allow the less abundant archaean to survive. Metabolites might not be the only chemicals shared, and the authors speculate that this could represent an example of an evolutionary step in cellular complexity that might also involve the exchange of genes.

    Environ. Micro. 12, 2371 (2010).

  4. Chemistry

    Easing in Fluorine

    1. Jake Yeston

    Fluorine is proving an increasingly useful substituent in pharmaceutical compounds (both for its intramolecular inductive effects and its intermolecular docking tendencies), but its introduction often remains a synthetic challenge. Tang et al. now show that a silver catalyst can transfer fluorine from a common electrophilic amine-based source to tin-bearing carbons in a wide range of aryl structures under mild heating. The authors had previously demonstrated a similar reaction using excess silver, and then discovered that adding bicarbonate was the key to enabling catalysis: The base intercepts an otherwise disruptive acid by-product. The catalyst tolerates numerous functional groups, as showcased by the authors through fluorination of structurally complex molecules such as strychnine. Preliminary mechanistic studies implicate multiple silver centers in one or more steps of the cycle. At the same time, the catalyst appears to be fully homogeneous, a rare circumstance for silver, which in redox catalysis is more often used heterogeneously.

    J. Am. Chem. Soc. 132, 10.1021/ja105834t (2010).