This Week in Science

Science  29 Jul 2005:
Vol. 309, Issue 5735, pp. 665
  1. Extrasolar Olivine


    Meteorites and interplanetary dust particles (IDPs) can contain a few minerals and grains with isotopic compositions distinct from those found in our own solar system. Examples of extrasolar silicate grains have been few, however, in part because silicate grains are also the most common type in meteorites and IDPs. Messenger et al. (p. 737, published online 30 June 2005) have now identified such a grain composed of an aggregate of olivine crystals (an iron-rich silicate) from an IDP that most likely formed in a type II supernova. Surprisingly, it is still crystalline, which implies that this IDP spent only a few million years in the interstellar medium before our solar system formed.

  2. Powerful Gamma Rays from X-ray Binaries

    Active galactic nuclei (AGN), the very bright central regions of galaxies thought to be powered by matter falling into a black hole, are among the most energetic objects in the universe, and often exhibit jets of matter expanding at relativistic velocities. Although a million times less massive, x-ray binaries (a star orbiting a neutron star or black hole) can also show powerful outflows. These objects, called microquasars, appear to be smaller siblings of AGN. Aharonian et al. (p. 746, published online 7 July 2005; see the Perspective by Cui) report the detection of very high energy γ rays from an x-ray binary. Such γ emission is considered a key signature of jets in AGN. These results suggest a possible kinship between these two powerful classes of astrophysical objects.

  3. Quantum Logic Spectrosopy

    Precision spectroscopy of atoms usually involves laser-cooling, initial state preparation via optical pumping, and, after interrogation, internal-state detection of the atom. The atomic species generally used have been those that can be readily laser cooled, interrogated, and detected, but often at the expense of compromising the desirable spectroscopic property of narrow linewidth. Schmidt et al. (p. 749; see the Perspective by Peik) now show that these requirements can be fulfilled by using an auxiliary atomic species and quantum-logic techniques. This approach frees up the choice of the spectroscopy atom, including those whose spectroscopic transitions could serve, for example, as accurate atomic clocks.

  4. The Taste of Things


    Tastes can evoke emotional and behavioral responses and may be compared with memories of past encounters with the same food. Sugita and Shiba (p. 781) used transgenic expression of a transneuronal tracer to delineate the gustatory pathways within the brain of mice. The neuronal circuitries that process and integrate the information concerning the different taste qualities, such as bitter versus sweet, were segregated, which may provide the neuronal bases of taste discrimination, contrastive behavioral responses, and emotional states.

  5. Lining Up Vacancies

    In a number of redox reactions catalyzed by noble metals at high temperatures, cerium oxide (CeO2) is used as a support material because it can release and store oxygen. Esch et al. (p. 752; see the Perspective by Campbell and Peden) examined this process on the (111) surface of a CeO2 crystal via high-resolution scanning tunneling microscopy and density functional calculations. When oxygen is released, the surface localizes the electrons through the reduction of Ce4+ to Ce3+. The vacancies form lines of defects that expose the reduced Ce3+ ions, and these multiple defects also create vacancies in the subsurface layer. The initial formation of these structures demand more reducing equivalents than the desorption of a single O2 molecule can provide, which may account for increased oxygen release when CeO2 is doped with nonreducible Zr4+.

  6. Switching the Channel

    One promising method for building nanoscale devices is to modify structures that nature has already produced. Koçer et al. (p. 755) prepared a photochemically gated valve by modifying the large conductance mechanosensitive channel protein, MscL, found in Escherichia coli cell membranes. The native protein functions as a pressure-relief valve and has a 3-nanometer pore. The authors modified a cysteine residue so that it undergoes charge separation upon ultraviolet irradiation. This charge-separated state permits ion flow through the otherwise hydrophobic channel, as evidenced in single molecule patch-clamp conduction studies.

  7. Designed for Robust Rice Production

    Most agriculturally important traits, like grain number and plant height, are regulated by genes known as quantitative trait loci (QTLs) derived from natural allelic variations. Genetic crosses of existing rice lines allowed Ashikari et al. (p. 741, published online 23 June 2005) to identify several important QTLs involved in rice yield. One of these QTLs was identified as a candidate gene encoding a cytokinin oxidase. The locus was shown to encode a functional enzyme that degrades the hormone cytokinin. With less cytokinin degradation comes greater seed production, but also heavier panicles that are more susceptible to damage in the field. Combining the gene favoring greater grain production with a gene favoring shorter plants generated a significantly improved rice plant.

  8. First Steps On All Fours

    Fossil dinosaur eggs are fairly abundant, but finding embryos within them is rare. Reisz et al. (p. 761; see the news story by Stokstad) now have identified several embryos in eggs from South Africa dating to about 190 million years ago, much older than other dinosaur embryos. These embryos can be assigned to a common prosauropod thought to walk bipedally at times, but their forelimbs indicate that they hatched as quadrupeds. This difference raises the possibility that the later sauropods evolved by preservation of this early developmental state. The features of the hatchlings also suggest that they may have required parental care for some time.

  9. Electron Transfer Structure Revealed


    The last structural frontier in mitochondrial respiratory energetics is complex I. This membrane enzyme is the site where the high-energy electrons of reduced form of nicotinamide adenine dinucleotide (NADH) enter the series of mitochondrial complexes to drive adenosine triphosphate synthesis. The bacterial counterpart is a simpler grouping of 14 subunits, of which seven form the cytoplasmic domain where NADH is oxidized. Hinchliffe and Sazanov (p. 771) have dissociated and crystallized this seven-subunit assembly and determined the relative locations of the nine iron-sulfur clusters that provide an electron transfer pathway, 84 angstroms in length, from the NADH binding site to the proton-pumping domain.

  10. Act On Your Senses

    When a pathogen enters its host, it sets off an intruder alert system that ultimately mobilizes an immune attack force to deal with the offender. Is the host immune system perceived and responded to by the invader, just as a burglar might take evasive action upon hearing an alarm? Wu et al. (p. 774) find that Pseudomonas aeruginosa, a common bacterial pathogen of lung and intestine, does just that. By using a cell surface protein to bind the host cytokine, interferon-γ, the bacterium switches on at least two genes involved in the quorum-sensing system that governs growth and virulence within the host.

  11. Squeezing Through the Pore

    How proteins, which are composed of both hydrophobic and hydrophilic amino acid residues, are translocated across hydrophobic lipid bilayers has been the subject of intense scrutiny. The protective antigen component of anthrax toxin forms a homoheptameric pore in the target cell's endosomal membrane that creates a narrow passageway for the enzymatic components of the toxin to enter the cytosol. Krantz et al. (p. 777; see the Perspective by von Heijne) report that a set of seven closely apposed Phe residues in the aqueous lumen of the protective antigen pore is essential for its ability to translocate the other enzymatic subunits of anthrax toxin across the membrane. The “Φ-clamp” appears to be the major conductance-blocking site for hydrophobic drugs and model cations and may serve a chaperone-like function in protein translocation.

  12. Beyond Pavlov

    It is relatively easy to transfer the physiological response to food (salivation) to a ringing bell when the stimuli are paired repeatedly. It also is possible to extinguish this association (or conditioning) if these stimuli are then presented in an unpaired fashion. Some associations appeared to be prepared or innate; a fearful response is more readily linked to seeing snakes rather than birds and is more difficult to extinguish. Olsson et al. (p. 785; see the Perspective by Öhman) now show that a conditioned fear response to faces from a social group different than one's own is more resistant to extinction than a similarly conditioned fear response to faces from one's own social group. This bias appears to be less in individuals with greater experience of the social out-group.

  13. El Niño Likes It Hot

    A potential analog of the warmer world expected for the near future can be found in the geological record of the early Pliocene, between 4.5 and 3.0 million years ago, when global surface temperatures were about 3°C higher than today. Wara et al. (p. 758, published online 23 June 2005; see the news story by Kerr) examined how these conditions affected a major feature of current climate, El Niño, by reconstructing sea surface temperatures from analysis of Mg/Ca and 18O ratios in drill cores from the western and eastern equatorial Pacific. Their record, which extends back to 5 million years ago, indicates that the temperature difference between the western and eastern regions during the early Pliocene was not nearly as large as it is today. This conclusion, exactly opposite that of another recent study (25 March, p. 1948), indicates that the strong asymmetry of the present ocean was absent and suggests that “hothouse” climates can collapse into a permanent El Niño-like state.

  14. On the Fly

    Active transposable elements (TEs) are generally considered to be deleterious to the genomes in which they reside, by directly jumping into and disrupting gene sequences, promoting ectopic recombination, and so forth. Thus, new TE insertions should for the most part be selected against, unless they happen to confer a selective advantage. Aminetzach et al. (p. 764; see the news story by Pennisi) developed a method for identifying advantageous TE insertions and found a specific Drosophila Doc element that, through the agency of a selective sweep sometime within the last 25 to 240 years, is present at very high frequency worldwide. The TE disrupts a gene that has the characteristics of a choline kinase, suggesting a role in pesticide resistance. Indeed, as predicted, the “mutant” flies displayed substantially lower mortality when exposed to organophosphate pesticides.

  15. Silencing the Extra X

    Animals with different sex chromosomes must ensure the correct gene dosage in cells of both sexes. For mammals this is achieved by down-regulating one of the two X chromosomes in female (XX) cells to match the output of the single X chromosome in male (XY) cells. During this “X-inactivation” a counting mechanism must ensure that only one X chromosome is switched off. Lee (p. 768) used transgenic mouse cells to explore the mechanism of counting. X-chromosome counting elements, likely DNA binding sites, were located within 15 kilobases of the 5′ end of the Tsix gene. Counting and X-chromosome choice both required a blocking factor, which prevents X-inactivation, and a competency factor, which induces X-inactivation.

  16. A Scent to Remember

    Diverse phenomena such as fish homing behavior and neonatal attachment of higher vertebrates are reliant on olfactory cues that affect an animal's behavior, termed olfactory imprinting. Remy and Hobert (p. 787) found this learned olfactory response is also displayed in the genetically amenable model system of Caenorhabditis elegans. The imprint is associated with favorable food availability and is generated at an early stage of nematode development. Egg laying and attraction to an imprinted odorant are increased in mature animals. This olfactory imprinting involves the expression of a chemoreceptor family member within a single interneuron that is downstream of odor sensory neurons.