This Week in Science

Science  13 Jul 2001:
Vol. 293, Issue 5528, pp. 169
  1. Single-Atom Delivery on Demand

    The manipulation of single-quantum objects is a key requirement for the engineering of microscopic quantum systems. Applications such as single-atom micromasers (microwave lasers), triggered single-photon sources, or deterministic entanglement of atoms all require the ability to deliver single atoms on demand to a desired location. The trapping and manipulation of atoms as neutral species is more challenging than for charged ions because of the weak interaction of neutral species with electromagnetic fields. Kuhr et al. (p. 278) have overcome these difficulties by combining magneto-optical trapping and optical-trapping techniques. They demonstrate the precision transport of single neutral cesium atoms over distances of 1 centimeter and the ejection of single atoms into free flight.

  2. Found in the Ozone

    Recombination of oxygen atoms and oxygen molecules in the upper atmosphere can form ozone, and this process leads to enrichments in the isotopes 17O and 18O by approximately the same amount above their natural abundances. This effect is puzzling, because standard chemical wisdom would predict the enrichment of 18O should be about twice as great as that of 17O. Under other conditions, however, extremely large mass-dependent fractionation has been observed. Gao and Marcus (p. 259; see the Perspective by Thiemens) present a theoretical description of the transition state dynamics for this reaction that can explain why both mass-independent and mass-dependent oxygen isotope fractionation can be observed for ozone synthesis under different conditions.

  3. A Trick Shot in Quantum Billiards

    When a cloud of atoms is cooled and held in an optical trap and then allowed to cool again, the trajectories of the atoms will depend on the geometry of the confining walls imposed by light beams. Most trajectories will be chaotic, but some geometries exist in which the scattering processes satisfy momentum-selection rules that give rise to stable trajectories. Steck et al. (p. 274; see the Perspective by Habib), using a system in which the atoms are confined in an optical standing wave and using a velocity-selection technique, show that for special regions of momentum space, termed “islands of stability,” the atoms can tunnel between one stable momentum state and its symmetric opposite. Time-slice measurements show that the atoms oscillate between the two stable momentum states.

  4. Faster Route to Fatty Acids

    In eukaryotic cells, the synthesis of polyunsaturated fatty acids (PUFAs), lipids that are critical for generating cellular membranes and signaling molecules, requires numerous steps by several enzymes to generate carbon-carbon double bonds. Metz et al. (p. 290) show that in certain marine prokaryotes and protists, a more simplified scheme is at work. These microrganisms can produce PUFAs through the action of a single enzyme called polyketide synthase. These enzymes may prove useful in the synthesis of new anitbiotics.

  5. Boron in Magmas

    Subduction of a crustal plate leads to the formation of a volcanic arc, but the processes that generate the volcanoes' magmas remain poorly understood. Rose et al. (p. 281) used the concentration and the isotopic characteristics of boron from melt inclusions trapped in olivine phenocrysts in basaltic lavas from Mount Shasta, California, to evaluate the melt process. The melt inclusions contain a low concentration of isotopically light boron. Dehydration of the subducting slab would reduce the amount of boron left in the slab residue. This residue may be re-equilibrated with the surrounding rocks and melted to produce the Mt. Shasta lavas.

  6. Parkinson's Disease Comes Together

    The cause of most cases of Parkinson's disease, a neurodegenerative disorder, is not known, but in a few families that show clear inheritance of the disease, a mutation in one of three genes results in symptoms. Shimura et al. (p. 263; see the Perspective by Haass and Kahle) show that the products of two of those genes, parkin and α-synuclein, functionally interact in the cell. Parkin, a ubiquitin ligase that tags other proteins for degradation, favors a substrate that is a p21 isoform of α-synuclein. When mutated, parkin cannot bind to or add ubiquitin to the synuclein isoform, and so the isoform builds up in the cell. The authors suggest that this overabundance of synuclein isoform causes the Parkinsonian degeneration.

  7. Squeezing Superconductivity into Boron

    Although theory has suggested that boron should superconduct at high pressure, conductivity measurements are difficult to make at very high pressures and cryogenic temperatures. Eremets et al. (p. 272; see the Perspective by Geballe) show that boron does indeed transform into a superconductor with a transition temperature of 6 K at a pressure of 175 gigapascals and 11.2 K at 250 gigapascals. A phase transition that changes the bonding in boron might accompany the onset of the superconducting transition.

  8. Sending Messenger RNA Messages

    Certain plant viruses propagate their infection via the intercellular transport of the viral RNA molecule. Kim et al. (p. 287) now show that endogenous messenger RNA (mRNA) molecules in noninfected plants not only travel between cells, but also may execute their developmental functions in cells far removed from those in which the RNAs were transcribed. Grafts between tomato plants with normal and mutant leaf shapes show that mRNA from the mutated gene responsible for the altered leaf shape can travel from the mutant graft stock into the wild-type graft scion and alter leaf shape in the otherwise genotypically wild-type scion. The promotion of mRNA from intracellular to intercellular information conduit complicates views of developmental regulation.

  9. Cryptic Damage

    Injury to the gastrointestinal (GI) tract limits the use of radiation for the treatment of cancer. According to the prevailing hypothesis, this so-called “GI syndrome” arises because radiation therapy directly damages the epithelial stem cells within the intestinal crypts of Lieberkühn and causes their clonogenic death. Paris et al. (p. 293; see the Perspective by Folkman and Camphausen) argue that the actual target of radiation damage is the intestinal microvascular endothelial cells. Using mouse models, they found that pharmacological inhibition of GI endothelial apoptosis prevented radiation-induced crypt damage, organ failure, and GI death. Thus, it may be possible to reduce radiation damage to the bowel by administering endothelial survival factors such as basic fibroblast growth factor.

  10. Divergent Interferon Signaling

    Two tools used by the immune system to fight viruses and bacteria are the interferons IFNα/β and IFNγ. These cytokines induce the transcription of antiviral and antibacterial response genes via the recruitment of signal transducer and activator of transcription (STAT)-1 proteins. In a study of a naturally occurring mutations, Dupuis et al. (p. 300) show that STAT-1 may direct immunity to viruses and bacteria through distinct pathways. A mutation that diminished phosphorylation of a specific tyrosine residue acted in a dominant-negative fashion to prevent the nuclear translocation of homotypic STAT-1 transcriptional complexes. In turn, this mutation correlated with a severe impairment of immune responses of individuals carrying the mutation to nonvirulent forms of mycobacteria. Remarkably, the same mutation did not affect viral immune responses and correlated with relatively normal nuclear translocation of a second heterotrimeric complex involving STAT-1.

  11. Viruses Get Nervous

    Infection with certain neurotrophic viruses can cause chronic encephalitis and progressive neurologic disease. Although B cells have been shown to clear alphavirus from neurons, less information is available on mechanisms by which T cells participate in viral clearance. In a study of a Sindbis virus infection model, Binder and Griffin (p. 303), observed that mice lacking the capacity to mount a B cell response could nevertheless clear the virus from neurons within the spinal cord and brain stem but not from cortical neurons. This T cell dependent clearance was recapitulated in the absence of both B and T cells by using recombinant viruses that express interferon γ. Distinct mechanisms appear to be important in the selective immune clearance of virus from populations of neurons in the central nervous system.

  12. Pain, Emotion, and Engogenous Opioids

    The μ-opioid receptors are implicated not only in the action of opiate drugs but also help mediate stress and pain. In a positron emission tomography study, Zubieta et al. (p. 311) investigated normal human volunteers during episodes of experimentally induced and controlled pain. They detected significant endogenous opioid release and interaction with μ-opioid receptors in the contralateral thalamus, hypothalamus, and insula, in the ipsilateral amygdala, and bilaterally in the cingulate cortex and prefrontal cortex. They also found direct evidence for the role of μ-opioid receptors in the regulation of the individual subjective experience of pain. Thus, the endogenous opioid system, through the activation of μ-opioid receptors in specific brain regions, is involved in the attenuation of sensory and pain-specific emotional responses.

  13. Modeling Melting Ice

    Meltwater injection from continental ice sheets modified the surface temperature and salinity of large regions of the ocean. Meltwater may have also affected climate by shutting down North Atlantic deepwater formation, which would have interrupted thermohaline circulation and the transfer of heat from low to high latitudes. Clark et al. (p. 283) propose a general mechanism to link melting of the Laurentide ice sheet and high-amplitude millennial-scale climate variations during the last deglaciation. By combining reconstructions of meltwater routing events with proxy records of climate, they extended in more general terms a scheme that was generally thought unique to the cooling of the Younger Dryas.

  14. Extending the Range of TiO2

    Titanium dioxide (TiO2) can be used to catalyze the photodegradation of organic molecules, and new approaches to environmental cleaning have been based on the use of irradiated TiO2. These reactions are most efficient under ultraviolet radiation, but there is great interest in extending the band gap of TiO2 into the visible range. Asahi et al. (p. 269) report that substitutional doping of TiO2 with nitrogen atoms improves the reactivity of TiO2 films toward organic molecules and enhances their hydrophilicity.

  15. Three Counts and Then You're a Knockout

    Receptor protein-tyrosine kinases (PTKs) are essential components of the signaling pathways required for cell-cell regulation. Many such pathways have been identified in various cell systems, but there has been little information on receptor PTKs in antigen-presenting cells (APC) of the immune system. Lu and Lemke (p. 306; see the Perspective by Schwartzberg) have identified three members of the same family of PTKs—Tyro, Axl and Mer—that are involved in regulating the function of APCs, including dendritic cells and macrophages. In mice genetically deficient in one, two, or three of these PTKs, an autoimmune phenotype developed with an increasing level of severity that correlated with an elevated activation status of APCs. These results suggest a direct link between this family of PTKs and the normal regulation of immune responses by APC.

  16. Frugal Scavengers

    The elements used by organisms to synthesize amino acids vary in availability and must be funneled through several metabolic pathways. Baudouin-Cornu et al. (p. 297) have used the large protein sequence data sets available for the eukaryote Sacccharomyces cerevisiae and the bacterium Escherichia coli to analyze the atomic composition of proteins involved in sulfur and carbon metabolism. In both organisms, sulfur-and carbon-assimilatory proteins are depleted in sulfur and carbon, respectively. Under conditions of elemental scarcity, proteins involved in assimilation of that element apparently evolve to optimize their own metabolic costs.