This Week in Science

Science  13 Aug 1999:
Vol. 285, Issue 5430, pp. 981
  1. Clarifying Population Cycles

    Two reports address the underlying causes of well-known animal population cycles (see the Perspective by Ranta et al.). Turchin et al. (p. 1068) used an elegant combination of theory and experiment to show that predators are implicated in the cycle of the southern pine beetle, whose periodic outbreaks cause immense damage to pine forests in the southern United States and Mexico. From an analysis of 30 years of time-series data on beetle outbreaks, they formulated hypotheses which were then tested in a 5-year replicated experiment in a Louisiana loblolly pine forest using exclusion cages. Records of the population fluctuations of the Canada lynx date to early 1800s. Stenseth et al. (p. 1071) used a new statistical approach that has revealed three structural groupings in the lynx population dynamics that broadly coincide with the east-west geoclimatic zonation across Canada rather than the north-south ecological zonation. This result suggests a role for the North Atlantic Oscillation in determining these regional patterns.

  2. In the Loop

    Quantum computers offer the possibility of performing massively parallel and complex calculations that classical computers cannot do. Much of the work at present is concentrated on the theoretical development of the components that will make up the quantum computer—qubits. Mooij et al. (p. 1036) introduce a design for a solid state qubit based on a superconducting loop with several Josephson junctions. In contrast to other solid state implementations, this approach would work in the magnetic regime and should be robust against external electrical noise and thus provide very long coherence times (the time that determines how long a calculation can be done).

  3. Nanoscale Organic Spheres and Tubes

    Interactions between molecules can lead to their assembly into larger aggregates, but controlling such aggregation can require several competing effects. Amphiphilic, polyhedron-shaped p-sulfonatocalix[4]arene building blocks, which have been shown previously to assemble into bilayers in an antiparallel fashion, have now been assembled in a parallel alignment by Orr et al. (p. 1049; see the cover). Spherical and helical tubular structures resulted through the addition of pyridine N-oxide and lanthanide ions. The relative amount of pyridine N-oxide added changed the curvature of the assembling surface and controlled whether spheres or extended tubules formed.

  4. Iron Batteries Forge Ahead

    The alkaline batteries that power portable electronics and flashlights use a zinc anode and a manganese oxide (MnO2) cathode. Their energy capacity is cathode limited. Licht et al. (p. 1039; see the news story by Hellemans) show that an unusual iron oxidation state couple, from Fe(IV) to Fe(VI), can provide a 50% increase in energy capacity, mainly by using a three-electron couple instead of just two. The ferrate (FeO42−) species used for the cathode is a known species that has often been considered unstable; the authors show that this instability can be avoided by eliminating metal impurities, such as nickel or cobalt, that catalyze ferrate decomposition. The cells show significant rechargeability, and batteries in the “AAA” configuration have been made.

  5. Different Ways of Taking the Heat

    Even for simple surface reactions, such as the catalytic oxidation of carbon monoxide (CO) to CO2 on transition metal surfaces, the role of the surface in activating molecules is unclear. If CO and O atoms are coadsorbed on a flat ruthenium metal surface and then heated in a vacuum, CO desorbs and no reaction occurs. Nonetheless, Bonn et al. (p. 1042) found that if they rapidly heated the surface with femtosecond infrared laser pulses, CO2 was formed. Although desorption of this final product was caused by thermal energy (phonons, the vibrations of the surface), the hot surface also generated electrons that initiated the oxidation reaction before the phonons could cause the CO could desorb.

  6. Still Waters Running Deep

    Manganese crusts in the ocean are providing valuable geochemical records of past ocean chemistry that can be used to infer past rates and locations of continental weathering, ocean circulation, and climate change. Chemical tracers with short residence times are poorly mixed and reflect local processes and fluxes. Lee et al. (p. 1052) now present an ocean hafnium isotope record from two widely separated manganese crusts in the Pacific Ocean. Hafnium has a longer residence time than that of more commonly used lead and neodynium tracers. The data imply that, particularly after 20 million years ago, the Pacific Ocean bottom waters became more isolated from the Atlantic Ocean, probably because of increasing separation of Australia from Antarctica.

  7. Separations Through Fluctuations

    Thermal fluctuations, or Brownian motion, normally randomize particles and molecules, but the application of time-dependent asymmetric forces can bias diffusion processes and lead to the transport of molecules and even separations. Van Oudenaarden and Boxer (p. 1046) have experimentally realized a geometrical Brownian ratchet in which charged lipids are driven over an asymmetrical barrier by an applied electric field. Different diffusion rates over this barrier leads to a separation in the direction orthogonal to the applied field of two phospholipids bearing either a single or a double negatively charged head group; no separation was seen in the absence of the barrier. Such an approach may allow membrane-bound molecules to be separated without extraction from their native membrane.

  8. Earlier Emergence of Eukaryotes

    Fossils need not be bones and teeth or even preserved soft tissues—organisms can leave their signature in the fossil record through characteristic biomolecules. Brocks et al. (p. 1033; see the Perspective by Knoll) report the discovery of the oldest molecular fossils. Hydrocarbons (2α-methylhopanes), preserved in northwestern Australian sediments 2700 million years old, are biomarkers that indicate the existence of eukaryotes (cyanobacteria) half a billion years earlier than hitherto recognized. They also find evidence for cyanobacterial oxygenic photosynthesis 600 million years before the atmosphere became oxidizing. These findings will necessitate a radical overhaul of current ideas about the evolution of early life and a recalibration of the eukaryotic molecular clock.

  9. The Missing Piece for the Pilus

    An important structure involved in the pathogenesis of enteric and urogenic Escherichia coli are their pili. Two reports present detailed structural evidence that gives insight into pili assembly (see the Perspective by Eisenberg). Choudhury et al. (p. 1061) have solved the crystal structure of the FimH-FimC complex, where FimH is the adhesin at the tip of the type I pilus and FimC is the chaperone that regulates where and when it is added. Sauer et al. (p. 1058) solved the structure of the PapK-PapD complex, in which the chaperone PapD is bound to the “joint” protein at the top of the rod of the P pilus, PapK. Both of these complexes have a similar structure in that the chaperone “donates” a strand to form an atypical immunoglobulin-like fold for the structural protein, a process termed donor strand complementation. The model proposes that these folded structural proteins, and those that comprise the rods of the pili, all fit together during pilus formation through donor strand exchange, in which each subunit completes a typical immunoglobulin-like fold of its neighbor.

  10. Cocaine and Clocks

    The circadian clock comprises a protein-based feedback loop that marks the passage of approximately 24-hour periods. Drosophila engineered to lack each of these genes—period, clock, cycle, timeless, and doubletime—were tested by Andretic et al. (p. 1066) for cocaine sensitization, in which a second exposure to the drug produces an exaggerated response. Cocaine sensitization was absent in all but the mutant fly strains missing the timeless gene. This implication of a subset of the circadian clock genes in drug responsiveness echoes recent suggestions that at least some of these genes may act in more places and in more functions than just the brain's clock.

  11. Catalyzing Fusion

    Intracellular transport of vesicles is regulated at several distinct steps. A transport vesicle must first recognize its target membrane. Once docked in the appropriate place, the two membranes fuse and the contents are mixed. Although several proteins have been identified that ensure the specificity of the early steps in this process, regulation of the final membrane fusion event has not been so clear. Peters et al. (p. 1084) report that an enzyme, protein phosphatase 1 (PP1), catalyzes intracellular membrane fusion. PP1 was also found in a complex with the calcium binding protein calmodulin. Hence, it appears that a distinct cellular machinery is at work to drive bilayer mixing.

  12. Betwixt and Between

    Integrins are transmembrane proteins located at the cell surface that integrate and mediate reciprocal communication between the extracellular matrix and the cell's interior cytoskeleton. As reviewed by Giancotti and Ruoslahti (p. 1028), integrins not only anchor cells to substrates but also generate intracellular signals that affect cell morphology, movement, growth, survival, and differentiation. Molecular details of the integrin signaling machinery indicate that potential cross talk with other signaling pathways may regulate complex cell behaviors.

  13. Tracking Abiogenic Methane

    Natural gas is mostly formed by the digestion of organic compounds by microorganisms or by the thermal decomposition of organic matter. Recently, it has been suggested that methane may also form via inorganic (or abiogenic) mechanisms in Earth's crust, but few criteria for distinguishing abiogenic methane exist. Horita and Berndt (p. 1055) show that methane forms rapidly in the presence of even small amounts of hydrothermally formed nickel-iron alloy under conditions resembling those in oceanic crusts. Isotopic compositions are similar to those of microbial methane, as are the ratios of methane to ethane and methane to propane. Abiogenic methane formation may thus be more widespread than commonly assumed.

  14. Correcting Errors in Translation

    In protein synthesis, the enzymes that “charge” tRNA molecules with their corresponding amino acids are called aminoacyl-tRNA synthetases. To avoid errors in protein composition, it is critical that the tRNAs bring in the correct amino acid to the protein synthesis machinery. Although amino acids can vary by side groups as small as a methyl group, the error rate is exceedingly small, which suggests that an efficient editing mechanism is involved. Silvian et al. (p. 1074) have examined the crystal structure of isoleucyl-tRNA synthetase complexed with tRNAIle and murpirocin (an active site-directed inhibitor). The synthetic and editing active sites are separated by 34 angstroms and are located at the bottoms of two deep clefts. Structural features suggest that the editing function may result from shuttling an incorrect product between the two active sites. This mechanism is similar in some ways to that used by DNA polymerase.

  15. Viewing the Insulin Receptor

    Structural information on large protein complexes is rare because they are difficult to crystallize as intact molecules and may be too large to analyze by spectroscopy. Luo et al. (p. 1077) take an alternative approach to determine the quaternary structure of the full-length heterotetrameric insulin receptor bound to insulin. Seven hundred electron micrographs were obtained to generate a three-dimensional image that was then fitted to available high-resolution structures of several receptor subdomains. Their 20 angstrom resolution reconstruction revealed that the receptor need only bind to one molecule of insulin to achieve a conformation in which the catalytic regions are poised to autophosphorylate. The spatial relation of the receptor's domains also confirms previous biochemical studies on receptor activation. This reconstruction approach may be useful for determining the organization of other large protein complexes that are refractory to crystallization.

  16. Unraveling the Regulation of RAG Genes

    Immunoglobulins and the T cell antigen receptor gene fragments recombine during development. The recombinase activating genes (RAG1 and RAG2) may have entered the vertebrate genome as a mobile genetic element and are closely linked on the chromosome. Yu et al. (p. 1080) developed many strains of transgenic mice that carried large pieces of chromosomes (in the form of BACs, bacterial artificial chromosomes) containing the RAG genes. The RAG gene of interest was replaced by the gene for yellow or green fluorescent protein. Expression of the RAG1 or RAG2 genes was regulated by distinct genetic elements in T and B cells, and both genes were regulated by genetic elements 5' of the RAG2 gene.

  17. (Mg,Fe)SiO3-Perovskite Stability and Lower Mantle Conditions

    G. Serghiou et al. (Reports, 26 June 1998, p. 2093) studied (Mg, Fe)SiO3-perovskite, under high temperature and pressure in “three different experiments.” They observed that perovskite remained as a single stable phase (to the pressure and temperature limits of their experiments) and did not decompose. This result supports the idea that perovskite is “the major component of the lower mantle” of the Earth.

    L. S. Dubrovinsky et al. comment that their own earlier study and one other “show that perovskite may not be stable in the lower mantle and dissociates into its component oxides.” They discuss details of the methods used in the report, and state that the results therein “appear to be mostly the result of different ways of characterizing the conditions of experiments and specimens.”

    In response, Serghiou et al. state that “the previously observed decomposition is most likely the result of large temperature (and possibly pressure) gradients present in those experiments, resulting from the use of a [Nd-yttrium-aluminum-garnet] Nd-YAG laser and, more important, the lack of a thermally insulating, soft pressure medium.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/285/5430/983a

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