This Week in Science

Science  21 Aug 2009:
Vol. 325, Issue 5943, pp. 918
  1. Going Retro


    In a year celebrating Darwin, the question of how new functional genes arise during evolution is of particular interest. Through a multibreed genetic analysis of the domestic dog, Parker et al. (p. 995, published online 16 July; see the Perspective by Kaessmann) find that the short-legged phenotype that characterizes at least 19 common dog breeds, including the corgi, dachshund, and basset hound, is specifically associated with the expression in developing bone of a gene encoding fibroblast growth factor 4 (fgf4), a member of a gene family previously implicated in dwarfism in humans. Interestingly, the culprit fgf4 gene in dogs has the hallmarks of a “retrogene,” a gene that arises when a parental gene is duplicated through an RNA-based copying mechanism.

  2. Bend Me, Stretch Me


    In the push toward flexible electronics, much research has focused on using organic conducting materials, including light-emitting diodes (LEDs), because they are more readily processed using scalable techniques. Park et al. (p. 977) have developed a series of techniques for depositing and assembling inorganic LEDs onto glass, plastic, or rubber. Conventional processing techniques are used to connect the LEDs in order to create flexible, stretchable displays, which, because the active diode material only covers a small part of the substrate, are mostly transparent.

  3. Saturated Magnetism in Photoexcited Nanocrystals

    Switching the magnetic state of semiconductors with either an electric field or by light absorption is a key requirement for spintronics, in which devices are based on electronic spin state rather than charge. In semiconductor nanoparticles doped with magnetic ions, excitons can form a spin state, a magnetic polaron, but often the effect is limited to low temperatures (below 30 kelvin) and does not saturate in the absence of an applied magnetic field. Beaulac et al. (p. 973) report the synthesis of Mn-doped CdSe nanocrystals in which the quantum confinement effects lead to long exciton lifetimes. Photoexcitation results in exchange fields that can exceed 30 Tesla at low temperatures and that persist even up to room temperature in the absence of an applied magnetic field.

  4. Discussing Polymerization Dynamics

    Actin and tubulin represent the major components of microfilaments and microtubules, which are responsible for the cytoskeletal framework of cells. Together with their recently discovered bacterial counterparts, the mechanisms of assembly and disassembly of polymers of these proteins are fundamental to our understanding of cytoplasmic organization and motility. Kueh and Mitchison (p. 960) review mechanisms of cytoskeleton polymerization dynamics and discuss the relative importance of structural plasticity in regulating polymerization.

  5. Toward Detecting Dark Matter Signals

    Dark matter, whose presence has been inferred indirectly through its gravitational effects on luminous matter, is thought to hold galaxies together and may form most of the matter in the universe. Although its nature is still unknown, theoretical models predict that when certain dark matter particles collide, they annihilate, producing gamma rays. Using a numerical simulation of a galaxy the size of the Milky Way, Kuhlen et al. (p. 970, published online 16 July) investigated the potential signals from dark matter in our galaxy, for the case in which the annihilation rate is boosted by an attractive force between incident dark matter particles. If this enhancement effect is valid, hundreds of dark matter clumps in our galaxy should be detected by the Fermi Gamma-ray Space Telescope.

  6. Solar Chronometer

    The use of the short-lived radioactive isotope 26Al as a precise chronometer of early solar system processes relies on the assumption that it was uniformly distributed in the initial solar accretion disk. Villeneuve et al. (p. 985; see the Perspective by Davis) validate this assumption on the basis of high-precision isotopic analyses of primitive meteoritic materials. Furthermore, chondrules—constituents of the most common type of meteorites and among the first materials to have formed in the solar system—formed episodically over a period of more than one million years.

  7. Crustal Details Revealed

    In seismic tomography, a large collection of data representing paths through Earth are inverted to provide an analysis of variation of density in which errors are minimized. Typically, the inversion starts with a simple layered model of the tomographic region. Tape et al. (p. 988) show how, starting with a three-dimensional model, based on synthetic seismograms, an improved iterative inversion approach can lead to a much more detailed view of a region. Using the rich data for Southern California, the model reveals details of the geologic history of the crust in this region.

  8. Attacks Wasps

    The bacterium Hamiltonella defensa infects aphids and carries virulence determinants from a bacteriophage virus. Oliver et al. (p. 992) have now found that the toxin-bearing phage does not harm the aphid host of the bacterium, but targets the larvae of parasitoid wasps that infest the aphids. If an aphid population is not infested by wasps, the allied bacteriophage is shed by the bacterium, presumably because there is a cost to carrying it. If the wasps then resume their predation, the aphids are no longer protected and succumb to the parasitoid.

  9. Blast-Resistant Rice

    The durability of disease resistance for an agricultural crop reflects the extent to which the defense stands up to evolutionary dodges on the part of the pathogen. Pi21, which is a quantitative trait locus (QTL) of rice, contributes to a particularly durable resistance to a fungal rice blast disease: Rice plants carrying the resistant allele have been in cultivation for more than a century, and yet the pathogen has been unable to find a way through the defense. Fukuoka et al. (p. 998; see the news story by Normile) have now cloned the responsible Pi21 QTL allele and were able to separate Pi21 resistance from tightly linked reductions in grain quality, paving the way for more widespread use of this allele in rice breeding.

  10. Protein Synthesis Initiation Complex


    The final step in the initiation phase of protein synthesis is the formation of the first peptide bond, which requires initiator transfer RNA (tRNA) to be bound at the ribosomal P site. Elongation factor P (EF-P) is a protein conserved in all eubacteria that stimulates this initial bond formation. Insight into how this is achieved comes from a structure of Thermus thermophilus 70S ribosome bound to EF-P, initiator tRNA, and a short piece of messenger RNA presented by Blaha et al. (p. 966). EF-P binds between the P and E sites and facilitates proper positioning of initiator tRNA in the P site. A similar mechanism is likely to apply to structurally homologous initiation factors in archea and eukarya.

  11. T Follicular Helper Cell Differentiation

    When B cells respond to an infection, they often require help from CD4+ T cells to mount a proper response. It is thought that a subset of CD4+ effector T cells, called T follicular helper cells (TFH), performs this function. Several subsets of effector CD4+ T cells arise, depending on the type of infection, which have distinct transcriptional programs driving their differentiation. Whether this is also the case for TFH cells has not been clear (see the Perspective by Awasthi and Kuchroo). Nurieva et al. (p. 1001, published online 23 July) and Johnston et al. (p. 1006; published online 16 July) now demonstrate that the transcription factor Bcl6 is both necessary and sufficient for TFH differentiation and subsequent B cell–mediated immunity, suggesting that it is a master regulator of this lineage. Johnston et al. also show that expression of Bcl6 and the transcription factor, Blimp-1, are reciprocally regulated in TFH cells and that, when ectopically expressed, Blimp-1 inhibits TFH development.

  12. Translational Rearrangements

    Conformational changes in the ribosome are required to translocate messenger RNA and transfer RNA (tRNA) during protein biosynthesis. For example, after peptide bond formation, rotation of the large and small subunits results in a hybrid state of tRNA binding—tRNAs are bound respectively in the aminoacyl-tRNA (A) and peptidyl-tRNA (P) sites in the small subunit, but in the P and exit-tRNA (E) sites on the large subunit. Zhang et al. (p. 1014) now describe x-ray structures of the intact Escherichia coli ribosome, either in the apo form or with one or two anticodon stem-loop tRNA mimics bound, which show intermediate states of intersubunit rotation. The structures provide insight into how the interface between the large and small subunits rearranges in discrete steps to reach the hybrid state.

  13. Making Memories Last

    How can memory traces persist over days or weeks, despite the short-lived nature and rapid turnover of their molecular substrates? It has recently been reported that, in order to persist, an otherwise rapidly forgotten long-term memory requires BDNF (brain-derived neurotrophic factor) expression in the hippocampus 12 hours post training. Rossato et al. (p. 1017) now show that this mechanism is gated into action by activation of the ventral tegmental area acting upon dopamine D1 receptors in the hippocampus. Time-limited N-methyl-d-aspartate receptor–dependent activity in the ventral tegmental area–hippocampal circuitry underlies the delayed increase in BDNF levels in the hippocampus 12 hours after inhibitory avoidance, a hippocampus-dependent form of learning.

  14. Silver Emission Follows Golden Rule

    Electrons emitted from the tip of a scanning tunneling microscope (STM) can be used in certain cases to excite optical emission from surface structure. Such methods can be used to characterize the “particle-in-a-box” states created in chains of varying lengths of metal atoms on surfaces. Chen et al. (p. 981) were able to spatially resolve photon emission for silver chains up to 10 atoms long, assembled on a nickel-aluminum alloy surface. The emission maxima correlated with the nodes seen in the derivative of STM current with voltage. This correlation follows from Fermi's golden rule, which connects the initial and final states of the radiation transition through the momentum operator and leads to a better understanding of the light-emission process.

  15. APC Transporter Structure

    Amino acid, polyamine, and organocation (APC) transporters that move a range of organic molecules across the cell membrane are important in many cellular processes. Shaffer et al. (p. 1010; published online 16 July) report the crystal structure of apoApcT, a proton-dependent APC transporter. The structure shows similarity to the sodium-coupled amino acid transporter LeuT and has an amino group from a lysine in an equivalent position to a sodium ion in LeuT. This suggests common principles between proton- and sodium-coupled transporters.