This Week in Science

Science  22 Aug 2003:
Vol. 301, Issue 5636, pp. 1013
  1. Developing a Firm Position

    In the classical world, the position of an object is well defined, but in quantum mechanics, increasing the precision of position measurement comes at the expense of losing information about the particle's momentum. This difference is well accepted, but it is still important to understand how larger objects that are made up on quantum-mechanical particles can develop their “classical” properties. Taking an approach that views position as a relative measurement, Rau et al. (p. 1081) present a thought experiment that shows how two particles, when subjected to a series of scattering interactions in which they become progressively more entangled, should localize in space with a well-defined separation. The proposal suggests that localization and the emergence of the classical world can be understood in terms of entanglement.

  2. Hawaiian Drift

    The Hawaiian-Emperor volcanic chain has been thought to track the motion of the Pacific plate over a static hotspot plume that causes the volcanism. Tarduno et al. (p. 1064; see the Perspective by Stock) determined the age and paleolatitude of the Koko, Nintoko, and Detroit seamounts along the oldest part of the chain. Modeling this data with large-scale mantle flow indicates that the plume moved southward rapidly (40 millimeters per year) between 81 and 47 million years ago. Thus, models of plate motion, true polar wander, and mantle convection that rely on a fixed hotspot reference frame may need revision.

  3. Spin Transfer over Molecular Bridges

    Quantum dots are attractive for spin-based quantum information processing because their spins can be fairly well protected from the dissipative effects of the environment and can thus have relatively long spin lifetimes. Most of the techniques for coupling quantum dots have been limited to ones that work at cryogenic temperatures or that use fabrication techniques that are not readily scalable. Ouyang and Awschalom (p. 1074; see the 1 August news story by Service) report on a self-assembly technique in which layers of quantum dots are coupled by conjugated molecules. Spins excited in one quantum dot were coherently transferred over the molecular bridges. This hybrid of molecular and spin electronics establishes a promising route toward a scalable architecture for coherent quantum information processing.

  4. Preemptive Strikes Against Gene Expression

    RNA interference (RNAi) defends an organism's genome against invading nucleic acids by recognizing and destroying aberrant (probably double-stranded) RNA molecules that can be generated by viruses, transposons, and transgenes. RNAi is also involved in the formation of heterochromatin at certain repeated sequences in the genome of fission yeast. Schramke and Allshire (p. 1069; see the Perspective by Matzke and Matzke), also working in fission yeast, now show that the RNAi-driven formation of heterochromatin can work in trans and can shut down gene expression from a normally active euchromatic locus. A subset of meiotically activated genes are repressed in nondividing vegetative cells in an RNAi-dependent manner that involves the formation of heterochromatin and requires the nearby long terminal repeat (LTR) sequences of transposons.

  5. Probing Phonons in Plutonium

    Toxicity, radioactivity, strong electronic correlations, and difficulties in growing crystals of appreciable size for analysis have conspired to make plutonium (Pu) a rather challenging material to study. Despite these technical obstacles, Wong et al. (p. 1078; see the Perspective by Lander) present results of an inelastic x-ray scattering study used to probe the lattice vibrations (phonon structure) of δ-Pu, the face-centered cubic form, that was stabilized by alloying with a small percentage of gallium. The results provide an experimental test for the recent theoretical approaches used to describe Pu.

  6. Carbonates on Mars

    Carbonate minerals form most commonly in sedimentary and hydrothermal environments on Earth, where they precipitate in water-rich conditions and ultimately extract carbon dioxide (CO2) from the atmosphere. Martian meteorites contain carbonates, which suggests not only the presence of water but also a sink for CO2 on Mars. Previous remote sensing and in situ measurements of the martian surface have not detected carbonates. Bandfield et al. (p. 1084; see the news story by Kerr) now report spectroscopic evidence for small concentrations of about 3 weight % of magnesium-rich carbonate (magnesite) in the martian dust with data from Mars Global Surveyor. Although the concentrations are small, the spectra suggest a relatively wide distribution, which would make magnesite a potentially important sink for CO2 from the martian atmosphere.

  7. Stop-and-Go Sliding

    The West Antarctica Ice Sheet contains enough water to raise sea level by 6 meters if it were to melt completely, so there is great interest in determining if its flow may respond to ongoing global warming. Bindschadler et al. (p. 1087) report that a large portion of the West Antarctica Ice Sheet experiences rapid variations of its movement in response to ocean tide changes of 1 meter or less. The transition between a stationary state and one in which the ice slides at a speed of more than 1 meter per hour happens in only minutes. These changes are consistent with a model in which the ice flow is forced from upstream and resisted variably downstream according to the changing ocean tide. This staccatto motion shows just how sensitive the ice sheet is to subglacial conditions and to tidal changes in sea level.

  8. Keeping Transcription Going

    In order to initiate transcription successfully, the packaged chromatin must be rearranged to allow access to the transcription machinery. How chromatin packaging is remodeled during the elongation phase of transcription is less well understood. Presumably DNA must be unwound from nucleosomes ahead of the transcribing RNA polymerase and replaced after the enzyme has passed by. Three reports address how elongating RNA polymerase II negotiates chromatin (see the Perspective by Svejstrup). Belotserkovskaya et al. (p. 1090) show how a protein complex in yeast that facilitates chromatin transcription, the FACT complex, destabilizes and reassembles nucleasomes during elongation. Kaplan (p. 1096) show that another yeast protein, the transcription elongation factor Spt6, represses aberrant transcription initiation from cryptic promoters by maintaining normal chromatin structure during elongation. Saunders et al. (p. 1094) find that the FACT complex and Spt6 are also associated with actively transcribed genes on Drosophila chromosomes.

  9. Immune Evasion

    Helicobacter pylori, which infects roughly half of the world population, can cause chronic and persistent infections of the stomach that can eventually lead to chronic gastritis, gastric and duodenal ulceration, and even malignancies. Gebert et al. (p. 1099) now show how H. pylori can suppress the activation and proliferation of T lymphocytes. Vacuolating cytotoxin (VacA) is secreted by the bacteria and targets the T cell receptor signaling pathway. VacA interferes with the Ca2+−dependent phosphatase calcineurin, which blocks nuclear translocation of the transcription factor NFAT. NFAT coordinates the expression of genes involved in inducing inflammation and controlling an efficient immune response. This strategy of immune suppression in the virulence of H. pylori could have implications for other chronically persisting bacterial pathogens as well.

  10. Vulnerable Memory Traces

    As we are all painfully aware, memories can be stored and can also be lost. In animals, conditioned-response behaviors have been used to study the robustness of long-term memories and their sensitivity to amnestic drugs. What are the mechanisms underlying experimental extinction—the decline of conditioned responses in the absence of a reinforcer? Eisenberg et al. (p. 1102) show that in Medaka fish and in rats conditioned to respond to two different stimuli, the memory that is “dominant” after a reactivation experience, as measured by its ability to control behavior, is vulnerable to the effects of amnestic treatment.

  11. Enough Is Enough

    Animal studies have indicated that the amygdala and the orbitofrontal cortex play an essential role in associative learning. In a functional magnetic resonance imaging study, Gottfried et al. (p. 1104) taught subjects to associate a picture with the smell of vanilla or peanut butter. When subjects were tested again after eating vanilla ice cream or peanut butter sandwiches, their brain activity, upon seeing the picture associated with the respective smell, decreased. In contrast, neural activity in the amygdala and orbitofrontal cortex did not change when presented with the picture associated with the other food. Thus, the perceived desirability of the expected reward affected the strength of association.

  12. Overcoming Negatives in Auxin Regulation

    The plant hormone auxin is a key regulator of growth and development. Zhao et al. (p. 1107) identified a compound, sirtinol, that stimulates biological responses similar to those initiated by auxin. They then screened for mutants of Arabidopsis that did not respond to sirtinol and identified a mutant they called sir1 that encodes a protein with similarity to a ubiquitin-activating E1-like protein. This result may help explain how auxin can rapidly degrade negative regulators as part of its mechanism of action.

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