This Week in Science

Science  07 May 2010:
Vol. 328, Issue 5979, pp. 663
  1. In Living Color

    CREDIT: EDITH WIDDER

    Light produced using the chemical process of bioluminescence spans the entire range of the visible spectrum. Bioluminescence has evolved independently several times in the tree of life. However, the majority of bioluminescent organisms reside in the open ocean, where their bioluminescence helps species in over 700 genera evade predators, attract mates, and find food. Widder (p. 704) reviews recent advances in understanding the evolution and distribution of bioluminescence in marine systems.

  2. Kissing Cousins

    Neandertals, our closest relatives, ranged across Europe and Southwest Asia before their extinction approximately 30,000 years ago. Green et al. (p. 710) report a draft sequence of the Neandertal genome, created from three individuals, and compare it with genomes of five modern humans. The results suggest that ancient genomes of human relatives can be recovered with acceptably low contamination from modern human DNA. Because ancient DNA can be contaminated with microbial DNA, Burbano et al. (p. 723) developed a target sequence capture approach to obtain 14 kilobases of Neandertal DNA from a fairly poorly preserved sample with a high microbial load. A number of genomic regions and genes were revealed as candidates for positive selection early in modern human history. The genomic data suggest that Neandertals mixed with modern human ancestors some 120,000 years ago, leaving traces of Neandertal DNA in contemporary humans.

  3. Centaurus A Gamma-Ray Emissions

    The nearest radio galaxy, Centaurus A, has been studied for many years at all wavelengths. Centaurus A exhibits two lobes of radio emissions that lie either side of the galaxy. The emissions extend over very large distances from the central source, which is thought to be powered by an accreting black hole with a mass around 100 million times that of the Sun. Using the Fermi Large Area Telescope, Abdo et al. (p. 725, published online 1 April) now report the detection of gamma-ray emissions emanating from the radio lobes of Centaurus A. Electrons with energies of 0.1 to 1 teraelectron volts are present in the lobes and are either accelerated in situ or transported efficiently from near the central source. The lobe pressure is comparable to that of the surrounding thermal gas, suggesting that the lobes may be affecting their surroundings.

  4. Patterning a Molecular Glass

    Lithographic patterning for device fabrication is usually based on initiating polymerization reactions with photons or electrons in a molecular resist. However, patterning can be achieved by mechanically removing a hard resist with scanning probe microscopy tips, but in many cases the resolution is low and excess material is left on the surface. Pires et al. (p. 732, published online 22 April) found that thin films of organic molecules could form glasses through weak interactions and be patterned to tens of nanometers with a heated scanning probe tip. These patterns could be transferred to other substrates or sculpted into three-dimensional shapes by successive rounds of patterning.

  5. Mixing the Magma Ocean

    Molten silicate makes up the majority of the magma we see spewing out of volcanoes, yet the mantle from which these melts originate is largely solid. The high pressures and temperatures at which these melts exist make interrogating their physical properties difficult. Karki and Stixrude (p. 740) used high-powered computational methods to calculate the viscosity profiles of one of the more abundant silicate melt compositions. The addition of water to the melt lowers the viscosity to the point that large mineral grains can sink. Because Earth may have been nearly all liquid in its earliest stages of formation, a deep magma ocean with this viscosity could have controlled the temperature of Earth's early surface.

  6. Dust to Dust

    Interplanetary dust particles are thought to sample the most primitive materials in the solar system. Because of their large deuterium enrichments, they are thought to have formed in interstellar molecular clouds—the birthplaces of stars—and to predate the solar system. Duprat et al. (p. 742; see Perspective by Nittler) describe two large interplanetary dust particles collected from Antarctic snow. The particles contain large zones of organic matter with deuterium excesses 10 to 30 times the terrestrial value. Because the organic matter is associated with crystalline silicates similar to those formed within the solar accretion disk, it is expected that the particles themselves formed in the Sun's protoplanetary disk, contradicting the idea that all organics with deuterium excesses are of interstellar origin.

  7. Elementary Steps in Electromigration

    CREDIT: TAO ET AL.

    Electrical current in small metal wires slowly changes their structure. Features such as surface islands can appear to move against the electron flow for metal ions moving along the terrace, or with it if electrons transfer momentum to neutral ions moving around the edge. Tao et al. (p. 736) present evidence for step-edge diffusion of silver islands, 2 to 50 nanometers in diameter, undergoing electromigration. Forces on the edge atoms at kink sites, where the step changes direction, were unusually large, in part because of the locally higher electron density.

  8. Dangerous Dengue Provocation

    One problem with dengue virus is that one infection does not protect against a subsequent infection; secondary infections can result in the severe immunopathology of dengue hemorrhagic fever. Dejnirattisai et al. (p. 745) derived a panel of monoclonal antibodies specific for dengue viruses. These antibodies were mainly directed against the dengue virus precursor membrane protein (prM), and most cross-reacted with all four dengue serotypes. The antibodies were not capable of fully neutralizing the virus, but instead promoted immune responses over a wide range of concentrations. During virus production and virion assembly, maturation of prm is often incomplete, and, consequently, a major part of the host's natural antibody response recognizes a component that is present in variable numbers on the virion. Thus, rather than resulting in complete neutralization, the antibody response promotes virus infection of cells that carry receptors for antibodies.

  9. Tolerating Tumors

    Successful tumor growth depends on the ability of the tumor to escape detection by the immune system. Human cancers that express the chemokine receptor CCR7 are associated with tumor metastasis and poor prognosis, suggesting that CCR7-dependent signaling might lead to an immunotolerant tumor microenvironment. Shields et al. (p. 749; published online 25 March; see the Perspective by Zindl and Chaplin) studied a mouse melanoma model in which the tumors expressed varying amounts of the CCR7 ligand, CCL21. Tumors expressing CCL21 exhibited more aggressive growth and attracted a class of suppressive, rather than pro-inflammatory, leukocytes. Furthermore, the tumor microenvironment was rich in immunosuppressive cytokines and exhibited lymph node–like features. These features were not present in tumors that expressed low amounts of CCL21. Thus, tumor CCL21 expression promotes an immunotolerant tumor microenvironment, which is permissive for tumor growth and spread.

  10. A Trick of the Tail

    CREDIT: LEE ET AL.

    The synaptic vesicle protein, synaptotagmin 1 (Syt1), acts as the main Ca2+-dependent switch for neurotransmitter release. In vitro studies of the truncated Syt1, which lacks the transmembrane domain, have unveiled the fusion-triggering mechanism of Syt1. However, in vitro approaches using the full-length, membrane-anchored Syt1 have not only failed to recapitulate Ca2+-triggered membrane fusion, but could even inhibit vesicle fusion. In contrast, the membrane anchor is conserved across the Syt family, suggesting a critical functional role for the membrane anchor. Now, using a single vesicle fusion assay, H.-K. Lee et al. (p. 760) show that the membrane anchor is indeed essential for Syt1 to induce physiological rates of Ca2+-induced vesicle fusion on a 100-millisecond time scale.

  11. Age-Old Problem

    With the increase in human life span, there is an associated increase in incidence of age-associated cognitive decline, which causes a huge emotional and economic burden. However, the mechanisms underlying age-associated memory impairment are poorly understood. Now, Peleg et al. (p. 753; see the Perspective by Sweatt) have found that the memory disturbances in the aging mouse brain are associated with specific changes in learning-induced histone acetylation, which interferes with the hippocampal gene-expression program. Restoration of dynamic histone acetylation reinstated cognitive function in the aging mouse.

  12. Target Acquisition

    The proper localization of proteins to the correct intracellular destinations is essential for the structure and function of all cells. Most membrane and secretory proteins are targeted to membranes by virtue of a signal sequence that is recognized by signal recognition particle (SRP) as the protein is being translated, forming a complex that docks with target membranes bearing the SRP receptor. Now, Zhang et al. (p. 757) have found, using cell-free bacterial extracts, that the initial binding of cargo by SRP is not sufficient to discriminate against all the incorrect cargos. Instead, a series of fidelity checkpoints during subsequent steps of the protein-targeting and translocation pathway help reject incorrect cargos.

  13. Determining the Crossover Point

    The equation of state of an equilibrated system at zero temperature relates its pressure to other macroscopic parameters (such as the chemical potential) and can be used to deduce all relevant thermodynamic properties. For a quantum interacting gas, both the measurement and the theoretical derivation of the equation of state have been challenging. Now, Navon et al. (p. 729, published online 15 April) have used a two-component ultracold Fermi gas of lithium atoms with tunable interactions to quantify the corrections to the mean-field predictions for the equation of state in the crossover between Bose-Einstein condensation and Bardeen-Cooper-Schrieffer limits at near-zero temperature. The polaron mass in the spin-imbalanced gas was also measured. The results agree with known beyond-the-mean-field corrections and present a challenge to future theoretical efforts.

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