This Week in Science

Science  16 Jun 2000:
Vol. 288, Issue 5473, pp. 1929
  1. Stretching Extreme Matter

    Most materials become narrower in cross section when stretched, but some materials, such as reentrant foams, have the unusual property of becoming wider when stretched. In an anisotropic material, expansion in one direction may be accompanied by contraction in another, and the process usually leads to overall densification. Baughman et al. (p. 2018; see the Perspective by Lakes) now show that such materials can also be incompressible, that is, show almost no change in volume upon stretching. They predict such unusual behavior for materials with very high density, such as neutron star crusts, or very low density, such as ion plasmas. Experimental data demonstrate the validity of these predictions for ion plasmas.

  2. Quiescent Accretion

    The formation of accretion disks around a massive object requires the dissipation of angular momentum of the gas and therefore some process that gives the gas viscosity. For close binary systems where the disks are weakly ionized and quiescent, the process of accretion does not follow the standard model of self-sustained magnetohydrodynamical turbulence. Menou (p. 2022) shows that an anticorrelation between the recurrence of outbursts and the mass ratio of the two objects leads to spiral waves, or to shock waves tidally induced by the companion, that ensures accretion during quiescent periods.

  3. Estimating Oceanic Productivity

    Approximately half of the photosynthetic production of O2 takes place in the ocean. In addition to producing O2, this process also consumes atmospheric CO2 and is important in evaluating the effects of fossil-fuel burning. Luz and Barkan (p. 2028; see the Perspective by Bender) have developed a new technique for estimating oceanic production that takes advantage of the ability of marine biological activity to erase an isotopic anomaly in 17O that is introduced into O2 by the mass-independent fractionation that occurs in the stratosphere. This technique allows a time-integrated value of production to be calculated.

  4. Entangled One-by-One

    The development of quantum computers will depend on the ability to control the entanglement the quantum states of the information elements, the qubits. Many-body entanglement (to date, four qubits) has been an all-or-nothing process, and so addressing and interrogating individual qubits would be difficult because of the close proximity of the qubits to each other. Rauschenbeutel et al. (p. 2024) describe a process in which the qubits can be entangled step-by-step and are spatially separated (on the order of a centimeter). The technique provides a powerful route for many-bodied engineered entanglement.

  5. Cooler Fuel Cells

    Most fuel cells run on hydrogen, which would likely be obtained by reforming hydrocarbon fuels at high temperatures. Another approach is to react light hydrocarbons such as methane, ethane, or propane directly with air. In these fuel cells, the electrolyte is a ceramic that becomes ionically conducting only at high temperatures (typically above 550°C). Hibino et al. (p. 2031; see the news story by Service) now report that lower operating temperatures (as low as 350°C) can be achieved by using samarium-doped ceria as the electrolyte. In this single-chamber cell design, fuel and air enter the electrolyte together, and current is drawn from the electrodes that bracket both sides of the electrolyte.

  6. Reverse Genetics in Drosophila

    T. H. Morgan's choice of Drosophila a century ago for studying the mechanisms of hereditary has resulted in the lowly fruit fly becoming one of the model organisms of biology. Drosophila has played a critical role in revolutions in genetics and molecular biology—the announcement in 1981 that the P transposable element could be used to create transgenic fruit flies also assured Drosophila a prominent place in genome research. Despite this utility and convenience, a couple of very important tools—homologous recombination and targeted mutagenesis—have been missing from the fruit fly molecular tool kit that yeast and mouse researchers take for granted. Rong and Golic (p. 2013; see the Perspective by Engels) now report a method of gene targeting by homologous recombination in the fruit fly that may also find use in other organisms.

  7. Forming Three-Layered Membranes

    Complexes of cationic lipids with DNA, which have found application in gene transfer, can exhibit intricate structures that result from complex interactions between these oppositely charged species. Wong et al. (p. 2035) have extended this approach by replacing DNA with another polyelectrolyte, filamentous actin (F-actin) of the cytoskeleton, which has a lower surface charge density. What results is a network of micrometer-scale tubules. An analysis of x-ray scattering and freeze-fracture electron microscopy data reveals that F-actin self-assembles into two-dimensional parallel arrays on both sides of the lipid bilayers and wraps to form ribbon-like tubules; these tubules can then be surrounded by similar layers separated by water layers that form through osmotic swelling. These results suggest that many unusual morphologies can be accessed by “frustrated” assembly in which charge interactions cannot be easily accommodated by simple structures.

  8. Bad Weather for Birds

    An understanding of the factors that affect the population dynamics of migratory birds, many of which are declining in abundance, requires long-term observation at both breeding and overwintering localities. Sillett et al. (p. 2040; see the Perspective by Sæther) have used a 13-year data set to show that populations of a migratory songbird are susceptible over a range of spatial and temporal scales to shifts in global climate patterns. Regional climate patterns like El Niño can affect a species on its breeding grounds as well as its wintering grounds; all demographic parameters (fecundity, survival, and juvenile recruitment) were lower than normal in El Niño years.

  9. Small Molecule Switches

    The ability to switch macromolecular interactions on or off would give control over many cellular events. Guo et al. (p. 2042) have used a small molecule to modulate the interaction between human growth hormone and its receptor. They introduced a mutation into each of the proteins to create a cavity at the interaction interface. They then screened a library of indole analogs and selected a ligand that increased the affinity of the mutant hormone for its receptor more than 1000-fold. Cell proliferation and JAK2 phosphorylation assays showed that the ligand switched on the growth hormone signaling pathway in cells containing the mutant hormone and receptor.

  10. Know Thyself

    The immune system attacks foreign cells and viruses when they lack “markers of self.” Oldenborg et al. (p. 2051; see the news story by Hagmann) describe a marker of self on the red blood cell that is distinct from the well-known major histocompatibility antigens. This protein, CD47, must be present on red blood cells for them to avoid destruction by a class of splenic macrophages, the red pulp macrophages.

  11. Growing But Not Dividing

    The molecular mechanisms by which cell growth is coordinated with proliferation are not well understood. Volarevi et al. (p. 2045) show that the two processes are separated in mice in which the gene encoding ribosomal protein S6 is conditionally deleted during adulthood. In cells from such animals, 40S ribosomal subunits (of which the S6 protein is a component) were absent. However, liver cells in such animals showed normal growth when nutrients were restored to fasted animals. In contrast, liver cells from animals lacking S6 did not proliferate after partial hepatectomy as do cells from normal animals, apparently because they did not increase the expression of cyclin E. A checkpoint mechanism may exist that halts cell proliferation if deficiencies in the ribosomal translational machinery are detected.

  12. Folding Single RNA Molecules

    Less is understood about RNA folding versus protein folding, in part because less is known about RNA secondary structure elements and in part because of stringent technical challenges. Zhuang et al. (p. 2048) describe results from observations of single RNA molecules, in this case the group I ribozyme from Tetrahymena. From histograms of the changes in fluorescence of single molecules, they derive the docking and undocking rate constants for the substrate and ribozyme active site. They also uncover a new pathway for folding that occurs too rapidly to have been observed in previous ensemble measurements.

  13. Basaltic Asteroid in the Outer Belt

    Some asteroids are thought to represent fragments of a larger, differentiated parent body that was disrupted early in the evolution of the solar system. However, only one large asteroid, 4 Vesta, has been observed to have a basaltic surface composition and thus be a potential differentiated parent body. Lazzaro et al. (p. 2033) obtained spectra of asteroid 1459 Magnya in the outer main belt, which shows that it has a basaltic surface as well. Orbital dynamic simulations suggest that Magnya may represent an isolated fragment from a parent body disruption. The asteroid has survived so long only because it has found a stable orbit in the midst of several destabilizing resonances. Magnya is so far from Vesta that the two are probably not from the same parent body, and Magnya is probably not a good source for basaltic meteorites found on Earth.

  14. Seeing in 3D

    Object recognition is performed in area TE of the inferior temporal cortex of the brain, but very little is known about any functional specialization within this large cortical area. Janssen et al. (p. 2054) report that neurons in one subregion of TE show a much higher selectivity for the three-dimensional (3D) surface structure of objects than neurons in a different subregion. In agreement with earlier morphological studies, this finding provides a functional basis for subdividing the inferior temporal cortex into separate regions with distinct specializations and shows that the 3D shape is treated as a special object property by the visual system.

  15. Recombination in Human Mitochondrial DNA

    Awadalla et al. (Reports, 24 Dec., p. 2524) found a decline in linkage disequilibrium (LD) with distance in human and chimpanzee mitochondrial DNA (mtDNA) that they held is consistent with genetic recombination. Four groups dispute that conclusion on a number of grounds. Kivisild and Villems question the quality of the data used, and suggest that some of the observations “can be more plausibly interpreted phylogenetically.” Jorde and Bamshad, along with Kumar et al., argue that the measure of LD used by Awadalla et al., r2, is highly sensitive to allele frequency variation, and note that reexamining the data using another LD measure, D', yields no evidence for a significant relationship between LD and distance. Kumar et al. also assert that a phylogenetic analysis of the data supports “the lack of recombination in human mtDNA, rather than providing evidence for it.” And Parsons and Irwin, focusing on the human mtDNA control region (CR), find “no indication of a negative correlation between LD and distance that would signal the action of recombination.”

    Awadalla et al. argue that neither random nor systematic sequencing errors are likely to influence their analysis, and show that even when several data specifically cited as suspect by Kivisild and Villems are removed from the analysis, “the correlation between LD and distance remains negative.” They present several lines of evidence that “r2 should have more power to detect recombination than D',” and note that the phylogenetic analysis presented by Kumar et al. may rest on a logical error, because their phylogenetic tree implicitly assumes that recombination does not occur. Finally, the absence of evidence of an LD-distance correlation in the CR may not argue against recombination, according to Awadalla et al., as “many sequences that have undergone recombination do not show a significant decline in LD with distance.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/288/5473/1931a