This Week in Science

Science  04 Apr 2014:
Vol. 344, Issue 6179, pp. 8
  1. Designer Chromosome

    One of the ultimate aims of synthetic biology is to build designer organisms from the ground up. Rapid advances in DNA synthesis has allowed the assembly of complete bacterial genomes. Eukaryotic organisms, with their generally much larger and more complex genomes, present an additional challenge to synthetic biologists. Annaluru et al. (p. 55, published online 27 March) designed a synthetic eukaryotic chromosome based on yeast chromosome III. The designer chromosome, shorn of destabilizing transfer RNA genes and transposons, is ∼14% smaller than its wild-type template and is fully functional with every gene tagged for easy removal.

  2. Designer Embryo


    Numerous signaling pathways have been implicated in controlling early vertebrate embryogenesis. P.-F. Xu et al. (p. 87) identify the minimal set of factors necessary to get uncommitted cells to organize a complete embryo. Two opposing gradients of the growth factors Nodal and Bone Morphogenetic Protein were sufficient to instruct zebrafish embryonic pluripotent cells to organize a complete embryo, not only in vivo but also in vitro. These findings may provide guidance for regenerative medicine studies aimed at constructing tissues and organs in vitro from cultured pluripotent cells.

  3. Deep, Driving Temperatures

    Convection in Earth's mantle is largely controlled by the physical properties of the mantle such as density and viscosity. Because these factors are influenced by both temperature and composition, it has been difficult to ascribe one as the primary control over mantle convection or explain the long-wavelength features associated with mid-ocean ridges. Examining correlations between a global seismic velocity model with constraints on the depth and geochemical signature of mid-ocean ridges, Dalton et al. (p. 80; see the Perspective by Kelley) suggest that large temperature variations extending into the upper mantle explain most of the geophysical and geochemical observations. Moreover, the analysis provides support for deeply rooted mantle plumes as the source of hot spot volcanism.

  4. Engineering Larger DNA Structures

    Several approaches now exist for the self-assembly of DNA into nanostructures. For example, three-arm DNA tripods can be assembled into larger wireframe polyhedra, but for the most complicated shapes, assembly yields can be low, apparently because the flexibility of smaller tripods allows for misassembly. Iinuma et al. (p. 65, published online 13 March) now show that larger, stiffer tripods that have controlled arm lengths and interarm angles can be designed to form a wide variety of open wireframe polyhedra—including tetrahedra, cubes, and hexagonal prisms, with edges 100 nanometers in length.

  5. Inside Enceladus

    Saturn's moon Enceladus has often been the focus of flybys of the Cassini spacecraft. Although small—Enceladus is roughly 10 times smaller than Saturn's largest moon, Titan—Enceladus has shown hints of having a complex internal structure rich in liquid water. Iess et al. (p. 78) used long-range data collected by the Cassini spacecraft to construct a gravity model of Enceladus. The resulting gravity field indicates the presence of a large mass anomaly at its south pole. Calculations of the moment of inertia and hydrostatic equilibrium from the gravity data suggest the presence of a large, regional subsurface ocean 30 to 40 km deep.

  6. Constructed for Deconstruction

    Lignin provides strength to wood but also impedes efficient degradation when wood is used as biofuel. Wilkerson et al. (p. 90) engineered poplar to produce lignin that is more amenable to degradation. From a handful of plants that contain more digestible lignin monomers, Angelica sinensis was selected and its monolignol transferase activities analyzed. The enzyme involved, coniferyl ferulate feruloyl-CoA monolignol transferase, was then expressed in poplar. The resulting poplar trees showed no difference in growth habit under greenhouse conditions, but their lignin showed improved digestibility.

  7. Optogenetics Applied to Motorneuron Control

    Nerves damaged by disease or injury do not always regenerate. In such cases, therapies involving transplanted stem cells show some promise. However, the new neurons derived from transplanted cells cannot communicate with the central control systems that would normally regulate movement. To avoid the need for such communication, in a proof-of-principle study, Bryson et al. (p. 94; see the Perspective by Iyer and Delp) added optogenetic control to differentiation and transplantation of motor neurons. In the mouse, these engineered neurons were able to reestablish connections within a damaged sciatic nerve and, when activated by localized light stimulation, could drive muscle contractions.

  8. Wearable Monitors


    Advances in microelectronics have yielded high-quality devices that allow for intensive signal collection or transmission. S. Xu et al. (p. 70) show how to make a soft wearable system that is constructed like a stretchable circuit board, where the electronic components are bridged electrically by thin, meandering conducting traces that float in a highly visco-elastic polymer. A complete soft circuit capable of multisignal physiological sensing on skin was created, with potential for use in health monitoring or neonatal care.

  9. Migration Monitor


    Seasonal migrations move large numbers of animals across often vast distances. Such movement shifts large amounts of biomass from one region to another, but, perhaps more importantly, moves animals that eat, excrete, and sometimes die in multiple remote systems. Such movements impact the communities, trophic structure, and function of these ecosystems in often underappreciated ways. Bauer and Hoye (10.1126/science.1242552) review migrations across taxa to identify the key ecological roles these long-distance movements play, and the unique threats the animals face in our increasingly modified world.

  10. Completing the Set

    G protein–coupled receptors (GPCRs) are membrane proteins that transduce extracellular signals to activate diverse signaling pathways. Significant insight into GPCR function has come from structures of three of four classes of GPCRs—A, B, and Frizzled. Wu et al. (p. 58, published online 6 March) complete the picture by reporting the structure of metabotropic glutamate receptor 1, a class C GPCR. The structure shows differences in the seven-transmembrane (7TM) domain between class C and other classes; however, the overall fold is preserved. Class C GPCRs are known to form dimers through their extracellular domains; however, the structure suggests additional interactions between the 7TM domains mediated by cholesterol.

  11. Cyanide Hitches a Ride

    Cyanide is a by-product of the biosynthesis of ethylene in plants and it has been somewhat puzzling how the ion is safely removed before it can shut down enzymatic pathways by coordination to catalytic iron centers. A proposed mechanism has implicated the cyanoformate ion—essentially, a weak adduct of cyanide and carbon dioxide—as the initial product, although its lifetime was uncertain. Murphy et al. (p. 75; see the Perspective by Alabugin and Mohamed) crystallized this previously elusive adduct and found that its solution-phase stability varies inversely with the dielectric properties of the medium. The results bolster a picture in which the adduct shuttles the cyanide away from the hydrophobic confines of the enzyme before releasing the cyanide into the more polar aqueous surroundings.

  12. Old Gradients

    The surface ocean temperature gradient between the warmer Western Equatorial Pacific and the cooler Eastern Equatorial Pacific is smaller during El Niño episodes than during neutral periods or during La Niñas. Some reconstructions of Pacific Ocean sea surface temperatures (SST) covering periods before ∼3 million years ago have suggested a permanent El Niño–like state. Zhang et al. (p. 84; see the Perspective by Lea) present data from a biomarker-derived proxy for SST that indicate a sizable east-west gradient has existed for the past 12 million years, contradicting the concept of a permanent El Niño–like state existed.

  13. Backward or Forward

    Although land animals generally walk forward, they readily switch to walking backward if they sense an obstruction or danger in the path ahead. Such a switch is likely to involve a neural signal sent from the brain down to local motor circuits, instructing these motor circuits to alter the phase at which specific leg muscles are activated. Bidaye et al. (p. 97; see the Perspective by Mann) identified such a neuron in Drosophila, which they call MDN (moonwalker descending neuron). Blocking synaptic transmission from MDN inhibited backward walking, and conversely artificially activating MDN caused flies to walk backward.

  14. Dissecting SRP

    In the secretory pathway, inserting transmembrane and secretory proteins into and through hydrophobic cell membranes is facilitated by a highly conserved RNA and protein-containing molecular machine, the signal recognition particle (SRP). Grotwinkel et al. (p. 101) determined the x-ray crystal structures of human SRP RNA (7SL RNA) bound to the RNA-binding domain (RBD) of the protein SRP subunit SRP68, both in the presence and absence of the SRP19 subunit. The 7SL RNA is remodeled by the SRP68-RBD, which bends one domain of the RNA and remodels a loop, exposing two nucleotides, which allow direct interaction with the ribosome. The findings explain how the SRP RNA drives translation elongation arrest, which is required for membrane insertion.