This Week in Science

Science  26 Mar 1999:
Vol. 283, Issue 5410, pp. 1977
  1. Keeping Secrets

    Secure cryptography is possible if the key to unlocking the code can be kept secret. Quantum key distribution at first was thought to solve this problem because eavesdropping on a quantum signal would change the states in a detectable manner, but it was later shown that undetectable eavesdropping was indeed possible. Lo and Chau (p. 2050) now show that a scheme offering unconditional security could in principle be constructed, given the availability of quantum computers. Although it may seem paradoxical, reduction of the quantum scheme to a classical one can make key distribution arbitrarily secure, even if the eavesdropper helps in distributing the elements of the quantum key.

  2. Metals Sliced Thin

    Until recently, it had been accepted that when charge carriers such as electrons or holes were confined to two dimensions, the resulting materials would be insulators, but experiments on high-quality, two-dimensional (2D) systems have revealed unexpected metallic behavior. Papadakis et al. (p. 2056) controlled the extent of the spin splitting (the lifting of spin degeneracy) in high-quality gallium arsenide 2D hole systems and found a relation between the temperature dependence of the resistivity and the spin splitting of the charge carriers. These results should help constrain any successful theory that attempts to describe this metallic behavior in reduced dimensions.

  3. Pressuring Polymer Topology

    Synthesis of highly branched polymers, such as the highly regular dendrimers or the more random hyperbranched polymers, often require many synthetic steps or require synthesis of specific reagent monomers that make these potentially interesting materials expensive. Guan et al. (p. 2059) now show that the simple ethylene molecule can be used to make hyperbranched polymers with a palladium-α-diimine catalyst. At high ethylene pressure, linear polyethylene with some branching is produced, but at lower pressures the catalyst “walks” along the growing chain and creates numerous additional branch points.

  4. Shaking Sediments

    Prediction of the ground motion that results from large earthquakes is critical for the engineering of structures. Regions of alluvial fill, such as in the Los Angeles Basin, have been particularly difficult to model because of the nonlinear amplification caused by sediments—for example, the amplification of aftershocks of the 1994 Northridge earthquake was twice that of the main shock. Such nonlinear responses need not be an intrinsic material effect. O'Connell (p. 2045; see the cover and the Perspective by Frankel) found that a three-dimensional, finite difference model that includes a linear response from soil sites, along with random variations in velocity, can reproduce the apparent nonlinearities and observed ground motions for the Northridge earthquake. He cautions that seismologists should avoid using only one-dimensional, layered velocity structures or including nonlinear soil responses in their models because these approximations can lead to inaccurate ground-motion predictions.

  5. Aimed Right at Us


    Gamma-ray bursts are brief emissions (seconds to minutes) of high-energy photons that are thought to be associated with some unobserved stellar explosion. Only recently have astronomers had the tools, such as the x-ray satellite BeppoSAX and the Compton Gamma-Ray Observatory, to pinpoint the location of these bursts and observe their decay of the optical afterglow over days and months at numerous wavelengths. Three reports indicate that the steep decline of the recent bright gamma-ray burst GRB 990123 is consistent with beaming of the emission in our direction, as opposed to isotropic emission (see the news story by Schilling). Starting with observations in Beijing about 8.5 hours after the burst and ending with observations in Spain on 18 February, Castro-Tirado et al. (p. 2069) determined that the decay in the intensity of the emission from the afterglow may be due to a collimated jet pointing directly at us. Hjorth et al. (p. 2073) used the Nordic Optical Telescope (NOT) on the Canary Islands to measure an upper limit of linear light polarization of 2.3% from the afterglow, which is consistent with emission from a jet, and Andersen et al. (p. 2075) obtained an optical spectrum of the afterglow with NOT that is also consistent with a jet and that places an upper limit on the redshift of 2.05 for the distance of the burst from Earth. Although a collimated jet is not a unique explanation for these observations, it provides a plausible solution to the quandry of a highly energetic source at such large cosmological distances.

  6. Attenuating Arctic Ozone

    The springtime ozone losses in the Antarctic and the Arctic polar stratosphere have largely been attributed to increases in halogens from chlorofluorocarbons (CFCs) reaching the stratosphere. Worldwide emission restrictions on CFCs are beginning to show an effect, and thus reductions in ozone destruction are expected. However, another culprit in ozone destruction has been identified: Removal of active nitrogen from the gas phase through aerosol-particle sedimentation (denitrification) can substantially increase ozone losses. Waibel et al. (p. 2064) show that this loss is particularly significant in the Arctic, where future stratospheric cooling may enhance denitrification and ozone losses despite halogen reductions.

  7. Trapped in Ice

    Spectra of the icy surface of the jovian satellite Europa obtained during the Galileo mission have revealed the presence of sulfur dioxide and hydrated minerals. Carlson et al. (p. 2062) have now matched a previously unidentified peak in Europa's infrared spectrum with a peak in a laboratory spectrum caused by the presence of hydrogen peroxide in water ice. They infer that this trace of hydrogen peroxide (0.13%) is produced by intense plasma irradiation of the Europan surface by the jovian magnetospheric.

  8. A Heartfelt Role

    Signaling by transforming growth factor-β (TGF-β) is a conserved pathway that controls cell growth and differentiation. Two TGF-β receptors, TBRI and TBRII, function in this signaling pathway. TGF-β binds to TBRI, which in turn phosphorylates TBRII for subsequent activation of Smad transcription factors. A third receptor, TBRIII, has been identified, but its role in TGF-β signaling was uncertain because TBRIII lacks a recognizable signaling domain. Brown et al. (p. 2080) used explanted chick atrioventricular cushion to examine the role of TBRIII during cardiac development and found that TBRIII is necessary for epithelial-mesenchymal transformation and mesenchymal cell migration. A mechanism for TBRIII action in TBRI-TBRII signaling is proposed.

  9. Breaking and Entry

    Upon contacting a host cell, the bacterium Salmonella typhimurium uses a specialized secretion system to translocate several bacterial effector proteins into the host. These proteins then somehow trigger rearrangements in the actin cytoskeleton and cell membrane that facilitate bacterial uptake. Zhou et al. (p. 2092) show that one of these translocated proteins, SipA, is an actin binding protein that functions by stabilizing actin filaments. This stabilizing activity results in spatial restriction of the host cell membrane structures that are involved in bacterial entry.

  10. Transfer RNA Target

    One of the common side effects of a regimen of antibiotics is diarrhea caused by the disturbance of intestinal flora that otherwise live unnoticed. Colicins are bacterial proteins that are secreted under stressful conditions and they act through a variety of pathways to kill other strains of Escherichia coli. Ogawa et al. (p. 2097) describe the mechanism by which colicin E5 inhibits protein synthesis. Unlike other members of the colicin E family that appear to cleave ribosomal RNA, E5 hydrolyzes transfer RNAs (for Tyr, His, Asn, and Asp) that contain the modified base queuine at the wobble position of their anticodons.

  11. Enzymes in Action

    Understanding of complex cell biological processes such as transmission of signals through networks of biochemical pathways would be facilitated by the ability to measure the local activity of regulatory proteins. Ng et al. (p. 2085) describe a method that allows analysis of localized enzymatic activity of protein kinase Cα (PKCα) in live or fixed cells. An autophosphorylation site was identified on PKCα and used as a marker of the activated enzyme. A specific antibody to this site and another monoclonal antibody to PKCα were labeled with fluorophores, and their proximity was measured by monitoring fluorescence resonance energy transfer. This protocol enabled imaging of active PKCα in live or fixed cells.

  12. Cytoskeletal Dynamics

    Changes in cell morphology and cell movement are mediated by remodeling of the actin cytoskeleton. The small guanosine triphosphatases (GTPases) Rac and Cdc42 regulate the actin cytoskeleton through stimulation of the protein kinase PAK (p-21-activated protein kinase). Sanders et al. (p. 2083; see the Perspective by Burridge) show that PAK1 then acts to phosphorylate myosin light chain kinase (MLCK) and decreases the activity of that enzyme. Subsequent decreases in phosphorylation of myosin light chain apparently lead to decreased interaction of myosin II with actin and contribute to cell spreading induced by Rac. These results help explain the opposite effects of the small GTPase Rho, which leads to decreased phosphorylation of MLCK.

  13. Keystone Collapse

    The classic example of a “keystone species” —one that controls the structure and function of a community or ecosystem—is the sea star, Pisaster ochraceus, which preys on mussels on the Northwest Pacific shoreline. In the presence of the predator, there is a rich assemblage of algae and invertebrates; in its absence, the intertidal zone shifts to a monoculture of mussels. Sanford (p. 2095) now shows that small temperature changes exert a strong effect in this species. Both in field studies during cold water upwelling associated with El Niño and in lab experiments, decreases in water temperature of around 3° Celsius dramatically affected sea star predation.

  14. A Mixed Picture for Manganite Magnetism

    The positively doped (hole-doped) manganese perovskites have attracted interest because of their large magnetoresistive effects, the change of their resistance when they encounter a magnetic field. However, a number of experimental aspects of these materials, such as unusual insulating ferromagnetic states and the complexity of their low-temperature magnetic phases, have been puzzling. Moreo et al. (p. 2034) present the results of theoretical calculations, which show that strong electron-electron correlations lead to microphase separation of clusters that have stronger and weaker doping levels than the average value for the material. Charge interactions limit this effect, so macroscopic phase separation is prevented.

  15. Ocean Currents

    Global ocean circulation reflects in part the cooling and increasing salinity in surface waters, which sink to form deep water. The boundary between low-density surface waters and high-density deep waters is known as the pycnocline (a gradient in density). A variety of processes control the global position of this boundary and thus potentially affect ocean circulation. Gnanadesikan, (p. 2077) in developing a theory for controls on the structure of the pycnocline, shows that processes in the Southern Ocean, particularly winds, are critical for determining ocean circulation.

  16. Controlling β-catenin

    The β-catenin protein functions in regulation of transcription during development and mutations that disrupt regulation of the activity of β-catenin are found frequently in certain human cancers. In a yeast two-hybrid screen, Seeling et al. (p. 2089) found that the regulatory subunit of protein phosphatase 2A (PP2A) (B56) associated with the APC protein, a component of a complex that inhibits β-catenin function by keeping β-catenin in the cytoplasm and promoting its degradation. Overexpression of B56 in mammalian cells reduced the abundance of β-catenin and inhibited β-catenin-dependent transcription. Along with other evidence that subunits of PP2A might be associated with complexes containing β-catenin in mammalian cells, the results indicate that PP2A might contribute to regulation of β-catenin.

  17. Species Abundance Across Spatial Scales

    W. E. Kunin (Reports, 4 Sept. p. 1513) developed “a scale-independent measurement of species abundance” with the use of “a commonly available device: the distributional ‘dotmap.’” He found that species abundance could be “estimated accurately even at scales finer than those used to parameterize the model….”

    J. C. Finlayson took data from his studies “of birds in the southern Iberian Peninsula” and “tested the predictive power of coarse- and medium-scale abundance to fine-scale abundance” with the use of Kunin's linear extrapolation method. Finlayson concludes that Kunin's method “appears to have wider taxonomic application [and] opens a window for the use of the now plentiful bird atlas data….” The full text of these comments can be seen at

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