This Week in Science

Science  25 Feb 2000:
Vol. 287, Issue 5457, pp. 1361
  1. Atom Pathways

    A form of optical microscopy has been developed to visualize the paths of individual cesium atoms in the gas phase. Hood et al. (p. 1447) trapped cesium atoms between two mirrored surfaces by using a probe laser beam. The combination of a single atom and a single photon from the laser field created a bound state analogous to the formation of a molecule. Variations in the probe beam's transmission corresponded to changes in the trajectory of the atom; the authors were able to reconstruct most of these atom trajectories with an inversion algorithm.

  2. Keeps Rolling and Rolling...

    If an ordinary metal wire is stretched or rolled many times, it will usually harden (and eventually break) because dislocations of atoms in the crystalline lattice build up at grain boundaries. If these dislocations could be made to diffuse faster, the metal would remain plastic. Because the diffusion rates increase as grain size decreases, metals with nanometer-scale crystallites should exhibit “superplasticity” and remain ductile even after much processing. Lu et al. (p. 1463) describe an electrochemical deposition process for copper that creates such a nanocrystallite material. Samples can be cold-rolled to more than 50 times their original length and still remain ductile.

  3. Magnetic Logic

    Quantum cellular automata (QCA) is a proposed architecture for future microelectronics in which the quantum mechanical characteristics of the switching elements are used to perform the logical operations. However, most implementations to date have been restricted to very low temperatures because they have relied upon the small electrostatic forces between a pair of single electrons. Cowburn and Welland (p. 1466) introduce and demonstrate an architecture based on magnetism (MQCA), in which the magnetization of series of magnetic disks in response to an external field depends on the magnetization of a single control disk. The magnetic switching is not restricted to low temperatures and can operate at room temperature.

  4. Close-Up of a Catalyst Reaction

    Industrial heterogeneous catalytic processes usually proceed at high pressures, whereas laboratory surface science tools often require vacuum conditions. This “pressure gap” is quite significant, because the catalytic activity of a surface often changes significantly with pressure. Over et al. (p. 1474; see the Perspective by Knözinger) study one such reaction, the oxidation of carbon monoxide on ruthenium, which proceeds very efficiently at high pressure but not at low pressure. An oxygen-rich overlayer on the surface may be responsible for the catalytic activity, but detailed evidence has been lacking. The authors investigated such an oxygen-rich surface phase using low-energy electron diffraction and scanning tunneling microscopy. These results, along with those from density functional calculations, show that this oxygen-rich overlayer is actually a surface oxide, RuO2. The binding strengths of CO and O atoms are more comparable on the oxide than on the metal, which facilitates the reaction.

  5. Immunization Against Stroke and Epilepsy

    The N-methyl-D-aspartate (NMDA) receptor is important in brain plasticity and development and may also be involved in the pathology that results from neurological disorders. During et al. (p. 1453; see the news story by Helmuth) have found that immunization of mice with a DNA vaccine encoding the NR1 subunit of the NMDA receptor could induce antibody-mediated, but not cellular, immunity in rats. This treatment induced protective effects against kainate-induced seizures (a model for temporal lobe epilepsy) and endothelin-1-induced middle cerebral artery occlusion (a model for stroke). No effects on movement or behavior were observed. Although long-term effects of such immunization are not yet known, it has the potential of having fewer side effects than pharmacological approaches with NMDA receptor antagonists.

  6. Holding Back a Death Signal

    In the worm Caenorhabditis elegans, certain genes are critical for the timed death of a cell during development. How the protein products from these genes regulate one another in vivo is not clear. Chen et al. (p. 1485) report that the pro-death protein CED-4 is translocated from the mitochondria (where it colocalizes with the anti-death protein CED-9) to the perinuclear membrane upon receipt of a death signal, such as expression of the pro-death protein EGL-1. The movement of CED-4 to the perinuclear membrane was not dependent upon caspases and was a prerequisite for normal death, which suggests that the role of CED-9 may be to sequester CED-4 at the mitochondria until the appropriate developmental time for cell death.

  7. Fomenting Resistance

    The evolution of antibiotic resistance in bacteria can be costly to the microbe in terms of biological fitness. These fitness costs can determine whether a resistant genotype spreads through a population. However, bacteria also have the capacity for mutation to compensate for these costs, and these compensatory mutations can take a variety of forms. Bjorkman et al. (p. 1479; see the Perspective by Bull and Levin) examine the conditions under which such mutations are selected in Salmonella. The form and frequency of compensatory mutations differed, depending on the environment of the bacteria, in this case a host organism versus laboratory culture medium. They suggest that the management of antibiotic resistance will be hampered if it relies solely on data gathered in vitro.

  8. Protecting the Nest

    Behavioral ecologists studying birds have long been interested in the evolution of patterns of parental care and clutch size. In a large, long-term comparative study of bird species in Argentina and Arizona, Martin et al. (p. 1482) provide evidence that nest predation is also an important factor shaping the evolution of parental care difference (measured as nest visiting frequency and food delivery rate) among bird species. Both in North and South American sites, clutch size is positively correlated with food delivery rate. However, they found no relation between nest predation and the pronounced latitudinal gradient in clutch size (small in the South but large in the North), an enigma that still requires a solution.

  9. Staying in the Swim

    Mammalian spermatogenesis depends on continual replenishment of differentiating spermatids from a pool of spermatogonial stem cells. Meng et al. (p. 1489) now show that glial cell line-derived neurotrophic factor (GDNF), known for its importance in directing development of neuronal cells and kidney morphogenesis, also regulates spermatogenesis. In transgenic mice, oversupply of GDNF limits differentiation of spermatogonia, whereas undersupply of GDNF results in early depletion of these stem cells. These results may offer insight into some mechanisms of male infertility.

  10. Preventing Jekyll's Conversion to Hyde

    The transmissible spongiform encephalopathies (BSE in cows and Creutzfeldt-Jakob disease in humans) are fatal, neurodegenerative diseases for which there is no treatment. It is believed that a critical event in pathogenesis is the conversion of the normal form of a cellular prion protein (PrP-sen) into an abnormal protease-resistant form (PrP-res). Transgenic mice that overexpress the hamster version of normal PrP-sen usually die within 80 to 100 days when infected with hamster scrapie. Priola et al. (p. 1503) show that treating these infected transgenic mice with tetrapyrrole compounds (effective at preventing the conversion of PrP-sen to PrP-res in vitro) starting on the first day of infection resulted in greatly increased survival times. Certain tetrapyrrole compounds may prove valuable for the prophylactic treatment of the transmissible spongiform encephalopathies.

  11. Unlocking Viral Latency

    Herpes simplex virus can maintain itself in a latent state and then become reactivated to start active replication and induce diseases of the eye and genital tract. This process depends on transcription of the viral latency-associated transcript gene LAT. Perng et al. (p. 1500) have determined that LAT-containing viruses block apoptosis in rabbit trigeminal ganglia, and thus promote survival of infected neurons. Similar results were obtained when cultured cells were infected with a plasmid containing LAT.

  12. Mechanically Hard Magnets

    Hard materials such as ceramics are ideal for applications in rocket nozzles and automobile parts. There is now interest in combining the attributes of wear and high-temperature resistance with magnetic properties for application in antistatic films and new recording media. However, the problem in the past has been in the actual synthesis process, where it has been difficult to produce materials that have good mechanical properties, appreciable yield, and tunable magnetic properties. Now, MacLachlan et al. (p. 1460) show that by combining ring-opening-polymerization with a carefully chosen organometallic polymer precursor, a mechanically hard, lightweight ceramic whose magnetic properties can be produced can be tuned from paramagnetic to ferromagnetic.

  13. When the Bubble Bursts...

    When a bubble of air rises to the top of a viscous liquid and bursts, the hole grows slowly because of the high viscosity. As the air escapes, the decrease in pressure inside the bubble can no longer sustain the weight of the liquid. The sheet making up the bubble collapses under gravity and begins to buckle, or ripple, as it becomes flatter while maintaining its area. Da Silveira et al. (p. 1468) present a model of this rippling effect based on competitive contributions of pressure, bending forces, and gravity and show that the number of ripples in the collapsing sheet can be predictable.

  14. Supercritical Control of Nanowire Growth

    Crystalline nanowires of semiconductor materials should exhibit unusual electronic and optical properties that arise through quantum confinement of charge carriers. One synthetic route to forming semiconductor nanowires is to use nanocrystals to catalyze and direct their growth from reactants in solution. Holmes et al. (p. 1471) have taken this approach a step further for silicon nanowires by using a supercritical solvent to speed diffusion of the reactants and uniformly sized gold nanoparticles that constrained the wire diameters to between 4 and 5 nanometers. Different orientations of crystalline nanowires, which exhibited differing optical properties, could be obtained by changing the reaction pressure.

  15. Genetic Ties in Silk

    Spider silks are a model system for studying the evolution of protein families. Spiders can synthesize at least nine different types of silk, each characterized by a small set of repeated amino acid sequence motifs. How have such highly repetitive long proteins evolved? Hayashi and Lewis (p. 1477) have performed an extensive molecular analysis of the flagelliform silk genes of two closely related spiders and show that both the introns and exons have undergone concerted evolution. Nevertheless, the highly repetitive architecture of the coding regions within the genes prevents complete homogenization within these regions. [See the news story by Stokstad.]

  16. General Acid-Base Catalysis in RNA Enzymes

    Enzyme catalysis of reactions often involves partial deprotonation of attacking groups and partial protonation of leaving groups as mechanisms used to lower the activation energy barrier. In order to operate within a narrow physiological pH, protein enzymes have developed the capacity to use amino acid side chains, such the imidazole moiety of histidine, to provide or abstract protons. Nakano et al. (p. 1493) describe detailed studies of the reaction catalyzed by hepatitis delta virus ribozyme, an RNA enzyme. Building on earlier work identifying a cytosine as the critical active site nucleotide, they show that it serves directly as a general acid and a metal-bound hydroxide serves as a general base. This work expands the known repertoire of catalytic capabilities for RNA and suggests how a transition from RNA-mediated chemistry to protein-mediated biochemistry might have occurred.

  17. An Upsetting Protein

    The cause of peptic ulcers and stomach cancer has been associated with invasion by the bacterium Helicobacter pylori, which uses a type IV secretion system to effect its pathogenicity. Odenbreit et al. (p. 1497) now show that during infection by H. pylori, the CagA protein is translocated from the bacterium to the host cell, and this protein is apparently then phosphorylated within the host by a yet-unidentified protein tyrosine kinase. Further knowledge into the functioning of the microbe's pathogenic mechanism may help lead to future therapeutic treatments.

  18. Was that a Right or a Left?

    Why is it that we are more likely to mix up lateral (left-right) than vertical (upside down) mirror images? Rollenhagen and Olson (p. 1506) have detected nerve cell activities in the brain that could explain this widely observed phenomenon. Neurons in an area in the inferotemporal cortex of the brain, which in primates is critically involved in the recognition of objects, showed a much more similar pattern of firing when confronted with an object and its lateral mirror image than with its horizontal mirror image. These results show that a behavioral phenomenon is reflected in neuronal activity in a precisely located region in the central nervous system.

  19. Caspase Phosphorylation, Cell Death, and Species Variability

    Cardone et al. (Reports, 13 November 1998, p. 1318) showed that phosphorylation by the kinase Akt prevents the activation of pro-caspase-9, thereby demonstrating that caspases, which are key players in programmed cell death, can be directly regulated by protein phosphorylation. Rodriguez et al., however, note that “despite the close similarity between human and mouse proteins,” mouse caspase-9 has no Akt phosphorylation sites, and that such sites are also absent from dog kidney cells. “Either it must be assumed that Akt regulates apoptosis differently in mice and humans,” they conclude, “or the notion that caspase-9 is regulated by Akt phosphorylation has to be re-examined.” Reed et al. respond that the absence of an Akt phosphorylation site in “shorter-lived lower mammals … suggests that this phosphorylation site evolved relatively recently in humans.” The original work of Cardone et al., they stress, was performed in human cell lines, and the work thus remains potentially relevant to human disease, especially “pathologies (such as cancer or neurodegenerative diseases) where cell accumulation or cell death occurs.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/287/5457/1363a

  20. The Nature of Ant Colony Success

    Cole and Wiernasz (Reports, 6 Aug. 1999, p. 891) found that harvester-ant colonies with high genetic diversity enjoyed strong survival advantages relative to lower diversity colonies, and tied that fitness advantage to polyandry—multiple matings by a single queen in the colony. Fjerdingstad and Keller observe that polygyny (multiple queens per nest) could also lower within-colony relatedness and “has been shown to be associated with greater colony size and higher productivity in several ant species.” Cole and Wiernasz respond that there is little evidence that multiple queens are common in the genus or species studied and that a variety of arguments suggest that polygyny is an unlikely explanation for their results. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/287/5457/1363b