This Week in Science

Science  23 Dec 2011:
Vol. 334, Issue 6063, pp. 1602
  1. Positively Reactive

    Phase transfer catalysis is a versatile strategy for accelerating reactions between substrates with very different relative polarity. Often a cation adorned with nonpolar hydrocarbon groups is used to draw a reactive anion into solution, where a hydrophobic compound lies in wait to bond with it. Rauniyar et al. (p. 1681; see the Perspective by Sodeoka) present a reverse scenario, in which a negatively charged phosphate ion enhances the solubility of a positively charged fluorinating agent. This chiral phosphate doubles as an asymmetric catalyst, guiding the addition of fluorine to olefins with high selectivity and efficiency.

  2. Making Magnetic Fields


    Like Earth, Mercury has a magnetic field of internal origin, albeit much weaker. Both planets are surrounded by a magnetosphere, produced when the magnetic field interacts with the solar wind—the stream of charged particles generated by the Sun. Using three-dimensional numerical simulations, Heyner et al. (p. 1690) show that the electric currents driven by Mercury's solar wind interaction generate external magnetic fields that diffuse deep into the planet and limit the strength of the fields produced by the dynamo process thought to operate in the planet's liquid iron core. This interplay between the magnetospheric and internal fields may thus provide a solution to the long-standing problem of how an ongoing internal convection-driven dynamo can produce such a weak global magnetic field.

  3. Spinning in Place

    Phosphorescent materials glow for a long time because their emission of light requires a change in spin state. Spin conservation is a pivotal principle in the interpretation of optical absorption and emission spectra of isolated molecular chromophores. Guo et al. (p. 1684) now show that the principle can also account for energy transfer dynamics between chromophores. Two different bimetallic complexes were synthesized that had been designed to facilitate energy transfer between spectrally overlapping donor and acceptor ions. The ligands and donors were the same in both complexes, but the acceptor ions varied in their spin state. Photoexcitation led to energy transfer only in the complex with spin-compatible donor and acceptor sites.

  4. Hedgehogs Man the Gate

    The blood-brain barrier (BBB) is composed of tightly bound endothelial cells and perivascular astrocytes and keeps potentially harmful cells and blood-borne molecules away from the brain. In neurological diseases such as multiple sclerosis, the BBB is breached and inflammatory mediators and cells gain access to the brain and contribute to disease pathology. Alvarez et al. (p. 1727, published online 1 December; see the Perspective by Engelhardt) now demonstrate that sonic hedgehog (Shh) signaling promotes BBB integrity. Pharmacological inhibition of Shh signaling or genetic ablation Shh signaling in endothelial cells increased BBB permeability in mice. In a mouse model of multiple sclerosis, blockade of Shh signaling increased disease severity. Inflammation promoted Shh signaling, and expression of Shh signaling mediators was elevated in multiple sclerosis lesions.

  5. Magnetic Biosynthesis

    Inside the cells of magnetotactic bacteria are specialized compartments of tiny magnetic nanocrystals that allow the cells to navigate along magnetic fields. These particles typically consist of the iron-oxide mineral magnetite, but some bacteria instead construct iron-sulfide greigite particles. By sampling lakes and sediments in the Southwestern USA, Lefevre et al. (p. 1720) isolated several sulfate-reducing members of Deltaproteobacteria which, depending on the growth medium, selectively contained magnetite or greigite. This suggests biomineralization can be controlled by external chemistry. However, one strain contained two separate gene clusters for magnetosome biosynthesis: one that resembles those from bacteria that use magnetite, and another that is more similar to greigite-producing bacteria.

  6. Smooth Hips

    A growing fraction of installed hip replacements are of the metal-on-metal type, in order to avoid the wear and debris problems that arise from devices that use polymer sockets. One might think that a metal-on-metal device would suffer from excessive friction, but somehow within patients a low friction layer forms where the surfaces meet. Liao et al. (p. 1687) examined the surface layers from hip replacements retrieved from patients and found a thin graphitic layer. Because graphite is an excellent solid lubricant, this explains the low friction observed in these types of hip replacements.

  7. Carbon Swings

    Throughout Earth history, there have been several disruptions to the link between oxygen levels in the oceans and the global carbon cycle. Deciphering cause and effect in these situations is often challenging, and involves probing rocks representing billions of years of Earth history for clues. Kump et al. (p. 1694, published online 1 December, see the Perspective by Hayes) present carbon and nitrogen isotope evidence from Paleoproterozic (∼ 2 billion years old) rocks in Russia that indicate a massive transfer a carbon from a reduced reservoir to atmospheric and oceanic carbon dioxide. This probably resulted from the Great Oxidation Event, when oxygen produced by newly evolved oxygenic photosynthesizing microorganisms began accumulating.

  8. Not Enough Bird Feed


    Harvest of forage fish is rapidly increasing. Because these species are prey for many others, reduction in their population sizes is likely to have cascading effects throughout marine ecosystems. Cury et al. (p. 1703) used decades-long data sets across multiple marine systems to quantify the effects of forage fish population size on reproductive success in seabirds, which represent a large group of marine predators. When forage fish populations had fallen below a third of their maximum biomass, reproductive output across multiple bird species was lower and more variable.

  9. No Lamins Required

    Nuclear lamin proteins line the nuclear envelope and promote the physical integrity of the nucleus. Lamins are implicated in numerous processes, including regulation of transcription, DNA replication, and modification of chromatin. Furthermore, a range of diseases are caused by mutations in lamins. Nevertheless, Kim et al. (p. 1706, published online 24 November) describe the relatively mild effects of loss of all three lamin genes in mice. The lamin proteins were not required for formation and proliferation of embryonic stem cells. The mice with no lamins did have abnormalities in organogenesis, particularly in brain development, possibly because of defects in spindle orientation and neuronal migration.

  10. Open and Shut


    Processive replication of chromosomal DNA is facilitated by sliding clamps that encircle DNA and bind to polymerase. Sliding clamps are loaded onto DNA by clamp loaders that are members of the AAA+ superfamily of adenosine triphosphatases (ATPases). Kelch et al. (p. 1675) report structures of an ATP-bound T4 bacteriophage clamp loader bound to a sliding clamp and DNA. The ATP-bound clamp loader traps the clamp in an open conformation so that the complex forms a spiral that matches the helical geometry of DNA. Binding to the DNA target triggers ATP hydrolysis, dissociation of the clamp loader, and release of the closed clamp onto the target DNA.

  11. Computer-Aided Synthetic Biology

    What if computer-aided design enabled synthetic biologists to make gene-regulatory machines in a way analogous to the methods electrical engineers model and design electrical circuits? Carothers et al. (p. 1716) show that such technology may be right around the corner. The authors used the relatively predictable nature of folding, binding, and catalytic activities of RNA molecules (compared to those of proteins) to design gene-expression devices that functioned in bacteria to produce a chemical useful in production of biologically active compounds or industrially useful compounds. Quantitative predictions of nearly 30 such devices expressed in bacteria were achieved.

  12. Folding in Place

    Much of our insight into how proteins fold comes from in vitro studies of full-length proteins or domains. However, in vivo, partially synthesized proteins may begin to fold while they are bound to the ribosome, and the ribosome may influence this folding. Kaiser et al. (p. 1723) performed single-molecule experiments to monitor folding of ribosome-bound polypeptides. For the T4 lysozyme, the ribosome promoted compaction, but slowed tertiary structure formation of the full-length protein and prevented an incomplete polypeptide from misfolding. Thus, the ribosome can contribute to productive protein folding to the native state.

  13. Psychotherapy Plus Prozac

    Clinical experience has consistently shown that a combination of antidepressant medication and psychological treatment works better for mood disorders than either therapy on its own. However, the mechanisms underlying these observations are still not understood. Working in mice, Karpova et al. (p. 1731) investigated the effect of fluoxetine (Prozac) on fear-conditioned memories. Fluoxetine accelerated extinction of fear responses. Furthermore, combination of fluoxetine with extinction training disrupted fear renewal and fear reinstatement. The results suggest that fluoxetine reactivates plasticity within the amygdala, which, in combination with extinction training, can lead to the erasure of conditioned fear responses.

  14. Observing the Lyman-α Line

    The Lyman-alpha (Lyα) line is emitted when there is an electron transition between the second and the first energy levels of neutral hydrogen. It is routinely observed from distant galaxies but it hasn't been measured in our galaxy, the Milky Way, because of a much brighter local source from hydrogen atoms that permanently flow around the Sun. Lallement et al. (p. 1665, published online 1 December; see the Perspective by Linsky) report the detection of Lyα emission from the Milky Way, based on the analysis of data obtained by spectrographs onboard the Voyager spacecraft, now far enough from the Sun to be beyond the local source of Lyα emission. The Galactic emission has its origin mainly in regions surrounding newborn stars.

  15. Master Memory Modulator

    Area CA3 of the hippocampus is required for the fast encoding of contextual memory. However, which CA3-specific molecular pathways regulate contextual memory formation are unclear. Using both loss-of-function and gain-of-function genetic manipulations in mice, Ramamoorthy et al. (p. 1669) discovered that activation of the transcription factor Npas4 in CA3 is central to contextual memory formation. Npas4 directly regulates an activity-dependent transcription program. Its transcriptional targets include several well-characterized immediate early genes (IEGs). Npas4 initiates the expression of these IEGs by recruiting RNA polymerase II to their promoter and enhancer regions.

  16. Embryos Need Not Apply

    The Doustantuo Formation in southern China contains some of the earliest and most well-preserved fossils from the Ediacaran Period (∼ 600 million years ago), the prelude to the Cambrian explosion. This formation contains several types of remarkable fossils, including what are thought to be remnants of early animal embryos with well-preserved nuclei. This classification as animals has been challenged partly because no associated juvenile or adult specimens are found along with the embryos, and no evidence exists for epithelial organization. Using synchrotron-based x-ray tomography, Huldtgren et al. (p. 1696; see the Perspective by Butterfield) probed the shape and internal structure of these fossils and suggest that these fossils may represent remnants of organisms in a group that includes ancestors to protists and animals.

  17. The Elephant's Toe

    Externally, elephant feet seem to resemble a tree trunk, right down to their circular footprints. However, the internal foot structure is actually more tip-toed, somewhere between those of a dog and a deer. What makes this internal structure seem so cylindrical externally is a large fat pad found at the back of the foot. Hutchinson et al. (p. 1699) show that elephants have enlarged sesamoid bones in both their fore- and hindfeet, which function as a sixth supportive “toe.” Development of this structure seems to have occurred as ancestral proboscideans increased in size and moved from an aquatic flat-footed stance to a terrestrial tip-toed stance.

  18. Mical in Actin Regulation

    Cell behavior is controlled by extracellular signals that work through signal transduction pathways to regulate the organization of the actin cytoskeleton. Some of these extrinsic signals positively affect the cytoskeleton and induce actin polymerization, but extrinsic signals that negatively regulate and disassemble actin filaments also exist. A family of multidomain proteins, the MICALs directly associate with Semaphorins, cell-surface receptors involved in negative or repulsive cues. Working with purified proteins and in vivo, Hung et al. (p. 1710, published online 24 November) now find that actin filaments serve as a direct substrate for Mical's enzymatic activity. Mical post translationally alters actin at its methionine 44 residue, disrupting the association between actin monomers and cutting actin filaments. Altering the methionine 44 residue makes actin resistant to Mical-mediated disassembly in vitro and in vivo in Drosophila.

  19. Replication and Mismatch Repair

    DNA mismatch repair (MMR) involves recognition of base mispairing in DNA, caused by DNA replication errors, and targeting of the newly synthesized DNA strand for repair. How does the MMR machinery—involving Msh2-Msh6 and Msh2-Msh3 heterodimers in eukaryotes—recognize the error in the new DNA strand? Hombauer et al. (p. 1713) tested if MMR in eukaryotes is coordinated with DNA replication by restricting the Msh2-Msh6 mispair recognition complex to specific stages of the cell cycle of Saccharomyces cerevisiae. Expression of Msh6 during S phase was required for functional MMR.

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