This Week in Science

Science  23 Mar 2001:
Vol. 291, Issue 5512, pp. 2271
  1. Squeezing Condensates

    Measurements in quantum mechanics are limited by the Heisenberg uncertainty relations, but these limits hold as a time average. It is possible to “squeeze” quantum-mechanical states so that the uncertainty limit for an observable, such as position, is beaten periodically. Quantum optics has created squeezed states for photons, and because the de Broglie wavelengths associated with atoms are much shorter than those of photons, the creation of squeezed atomic states should have an impact on areas such as high-resolution interferometry. Orzel et al. (p. 2386; see the news story by Voss and the related Science Express reports by Abo-Shaeer et al. and Robert et al.) show that the number states of atoms in a Bose-Einstein condensate can be squeezed, thus greatly reducing the variance in the number of atoms trapped in a particular well.

  2. Catching Fennoscandia on the Rebound

    Fennoscandia has been rising since the retreat of a massive glacier at the end of the Pleistocene. Although there are decades' worth of tide gauge records for the coast of Sweden and Finland, it has been difficult to extract an estimate of sea level rise by subtracting the ground surface uplift. Milne et al. (p. 2381) completed a 7-year global positioning system (GPS) campaign and combined these land-surface deformation measurements with the tide gauge records to derive a more accurate regional sea level rise of 2.1 plusmn; 0.3 millimeters per year. This value is consistent with the global average and relevant to models of global warming. They also derived a refined viscosity for the mantle and an elastic thickness of the lithosphere, which are fundamental inputs for mantle convection and crustal flexure models.

  3. Tiny Breaths

    Marine biological control of the flux of CO2 between the atmosphere and upper ocean is mediated by a combination of carbon uptake by phytoplankton photosynthesis, remineralization of organic carbon back to CO2 (community respiration), and export of biogenic carbon to deeper waters. These processes also help control the distribution of biogenic carbon in the ocean. Rivkin and Legendre (p. 2398) show that there is an inverse relation between temperature and bacterial growth efficiency and that bacterial respiration accounts for nearly all of the community respiration. Besides their ecological implications, these results suggest that increases in sea surface temperatures will increase the proportion of the assimilated carbon that is remineralized and thus decrease the solubility of CO2. These effects would then create a positive feedback between atmospheric CO2 and temperature.

  4. Potential for Turing Patterns

    Chemical waves and patterns observed in solution for certain reactions can be explained by a mechanism proposed in the early 1950s by Turing. A critical ingredient for pattern formation in solution is that one species must diffuse much more rapidly than the others—a somewhat difficult requirement for small molecules. Recent theoretical work has suggested that reactions at electrodes could also exhibit Turing-type patterns if the electrode potential took the place of the rapidly diffusing species and if field-induced migration replaced diffusion. Li et al. (p. 2395) now experimentally verify this theory for the reduction of periodate on gold electrodes in the presence of camphor, an organic molecule that can condense on the electrode at certain potentials and inhibit reduction. Such pattern formation should be observable in many electrochemical systems and could operate at biological membranes that exhibit potential gradients.

  5. The Details of Taking a Hit

    When a polycrystalline material is deformed, the individual grains change shape through the movement of dislocations and through grain rotation. Although many theories have been developed to model this process, experimental data for the actual grain evolution have been harder to obtain. Margulies et al. (p. 2392; see the Perspective by Heidelbach) used focused hard x-rays to map out the evolution of several grains in an aluminum sample undergoing tensile deformation. The results are inconsistent with the two classical models, those of Taylor and of Sachs, but provide needed inputs for more detailed numerical models.

  6. Turning On (and Off) a Nanocrystal

    Quantum-confinement effects in semiconductor nanocrystals should make the electronic and optical properties of these materials more sensitive to the injection of extra charge carriers compared to the corresponding bulk materials. Wang et al. (p. 2390) demonstrate how electrochemical injection of extra carriers in CdSe nanocrystals quenched their photoluminescence. The photoluminescence could be recovered by reversing the bias, thus illustrating the potential of such an effect for optoelectronic applications.

  7. Turning Over a New Leaf

    In plant development, the growing shoot changes from a juvenile, or reproductively incompetent state, to an adult state. Mechanisms governing this change were examined by Berardini et al. (p. 2405), who found that a protein termed squint, a chaperone-like protein, was involved. In mutants lacking squint protein, leaves exhibited adult properties early, but flowering time was unaffected. Surprisingly, the mutant plants expressed elevated levels of another heat shock chaperone protein, Hsp90, but the response of seedlings to heat shock was not affected.

  8. Molecular Ruler or Measuring Cup?

    At the base of the bacterial flagellum, there is a hook structure onto which the flagellin subunits are assembled. The total length of the flagellum itself is highly variable, but the hook structure is remarkably homogenous in its size. In examining the mechanism for this relative constancy, Makishima et al. (p. 2411) found that rather than using a molecular ruler to determine hook length, Salmonella appears to regulate the amount of subunits secreted for hook assembly.

  9. Having Trained Reserves in Place

    Long after T cells first deal with a pathogen, small numbers persist so that immunological memory is not lost in the event of reinfection. Using fluorescently tagged antigenic tetramers, Masopust et al. (p. 2413; see the Perspective by MacKay and Andrian) tracked memory CD8+ T cells and observed a marked preference of these cells for a range of nonlymphoid tissues, including the lung, liver, kidney, and gut. T cells from each site could respond immediately to antigen by producing interferon-γ and by killing target cells. Because most infections occur at such peripheral sites, these results emphasize the importance of having a ready supply of memory T cells that can be delivered to wherever the immunological action might be.

  10. Learning and Relearning

    Experimental extinction, the disappearance of a conditioned behavior after repetitive trials in the absence of the unconditioned stimulus, is not simply a kind of forgetting but rather a new learning phenomenon. To address the cellular mechanisms underlying this phenomenon, Berman et al. (p. 2417) studied conditioned taste aversion in the rat insular cortex. Both acquisition and extinction of conditioned taste aversion were impaired by inhibition of protein synthesis and involved β-adrenergic receptor activation. Acquisition, but not extinction, was decreased by antagonists of muscarinic receptors and N-methyl-Δ-aspartate receptors and also by mitogen-activated protein kinase inhibitors. There are thus clear differences between memory encoding and consolidation during retrieval.

  11. Making the Transfer

    The origin and movement of neurotrophic factors such as brain-derived neurotrophic factor (BDNF) between nerve cells has been controversial. Kohara et al. (p. 2419) used BDNF tagged with green fluorescent protein to follow anterograde synaptic transport from axons of neurons to postsynaptic cells. This synaptic transport of the growth factor is absent when neuronal activity was blocked by tetrodotoxin and was enhanced after reduction of inhibition. These results constitute direct evidence for an anterograde, trans-synaptic transport of BDNF.

  12. Blocking a Survival Pathway

    In several neurodegenerative diseases, proteins that bear expanded polyglutamine repeats aggregate, a process that somehow leads to the demise of neurons. Nucifora et al. (p. 2423) studied the interaction of aberrant versions of huntingtin (which is affected in Huntington's disease) and atrophin (which is affected in dentatorubral and pallidoluysian atrophy) containing expanded polyglutamine repeats. These aberrant proteins avidly bound to the transcriptional coactivator CREB-binding protein (CBP) and prevented the transcription of target genes known to be crucial for neuronal survival.

  13. Exploiting a Preexisting Condition

    Phosphorylation is usually assumed to regulate signal transduction by triggering a conformational switch. Volkman et al. (p. 2429; see the Perspective by Buck and Rosen) show that in the signaling protein NtrC, activation by phosphorylation involves stabilization of a preexisting conformation. Nuclear magnetic resonance measurements of backbone dynamics show that both active and inactive conformations are populated in unphosphorylated NtrC, with the inactive form favored. Phosphorylation shifts the equilibrium far toward the active conformation so that conformational exchange virtually disappears. Thus, allosteric regulation of single domains may be important in signal transduction.

  14. Crystal Dissolution in Detail

    Understanding crystal dissolution is needed to follow fluid-rock reactions in many practical environmental and engineering problems, such as weathering of rock and cement, nuclear waste remediation, and corrosion. Lasaga and Luttge (p. 2400) have used microscopic observations of dissolution, Monte Carlo simulations of etch pit formation, and empirical bulk dissolution rates to derive a dissolution rate law that is consistent with data for several minerals. The rate law predicts that the slower rates of dissolution measured in the field arise from metastable phases that hinder the reactions and that the process is nonlinear even as equilibrium conditions are approached.

  15. Following Protein Activation in Real Time

    Fluorescence resonance energy transfer (FRET) is emerging as a powerful tool that allows real-time monitoring of protein interactions in living cells. Janetopoulos et al. (p. 2408) added fluorescent tags to the Gα2 and Gβ subunits of heterotrimeric guanine nucleotide binding proteins (G proteins) in Dictostelium discoideum. The G protein-coupled receptors are a very large family of proteins that mediate the action of hormones, neurotransmitters, and other signaling molecules. In this case, the authors treated cells with a chemoattractant and then observed the dissociation of the active G protein subunits by FRET. During continuous stimulation, the physiological response to the chomoattractant subsided, but the G protein remained activated (as measured by dissociation of the heterotrimer). The authors suggest that signal adaptation must occur downstream of the activated G protein. The method is expected to be extensible to mammalian cells and should enable substantial improvement in our quantitative understanding of mechanisms of G protein signaling.

  16. Noble Gases in Mantle Plumes

    Drawing on experiments that showed, in mantle and plume-derived rocks, neon isotope ratios indistinguishable from those of meteorites, Trieloff et al. (Reports, 12 May 2000, p. 1036) argued that Earth's solar-type rare gas inventory likely was acquired “during accretion from small planetesimals previously irradiated by solar wind” rather than directly from the solar nebula. Ballentine et al. suggest that the signal found by Trieloff et al. may reflect limitations of the ball mill crushing technique they employed, as other release techniques have revealed higher neon isotope ratios, and that the data reported by Trieloff et al. do not show an expected correlation with argon isotope data. Trieloff et al. respond that the high neon isotope ratios cited by Ballentine et al. represent “the high-value tail of a statistical distribution,” the Gaussian center of which agrees well with their reported value, and that the argon isotope data can be reconciled with their neon data when all available popping rock data are considered. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/291/5512/2269a

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