This Week in Science

Science  07 Jan 2005:
Vol. 307, Issue 5706, pp. 13
  1. Through a Glass Slowly


    Electrons moving from donor to acceptor sites often must tunnel through the potential barrier set up by the intervening medium, such as the peptide chains in proteins. Most model studies of these processes have focused on systems in which the donor and acceptor sites are connected by a covalent bridge. Wenger et al.. (p. 99) have explored the effect of nonbonded contacts on tunneling by examining electron transfer rates for random arrays of donors and acceptors in frozen glasses of toluene and 2-methyltetrahydrofuran. The transfer rates are much slower than for covalently bonded alkane bridges at comparable distances.

  2. Fast Light Switch


    Certain organic salts form one-dimensional (1D) or 2D electronic bands that are partially filled and that can give rise to electronic and magnetic properties such as superconductivity or ferroelectricity. Chollet et al.. (p. 86) examined the organic salt (EDO-TTF)2PF6, where EDO-TTF is ethylenedioxytetrathiafulvalene, which forms a quasi-1D band that is one-quarter-filled with hole carriers. This material displays a metal-to-insulator (M-I) transition near room temperature that arises from structural changes that lead to charge ordering. The authors find that this M-I transition can be brought about very rapidly (in a few picoseconds) after photoexcitating only a very small fraction of the molecules within the crystal (about 1 in 500) at temperatures near ambient. This phase transition appears to be driven by a coherent phonon generation process caused by the interaction between the electrons and the lattice. Such properties may prove useful as an ultrafast molecular switch.

  3. Pentagonal Columnists

    Some shapes, like triangles and squares, can regularly pattern or tile a flat space, whereas regular pentagons are only able to tile a sphere. Chen et al. (p. 96) have synthesized molecules with three incompatible segments that form liquid-crystalline columnar phases. The columns then tile into either identical pentagonal cylinders or a structure composed of square shapes and triangular columns. This packing is possible because of the combination of order and mobility in the fluid state of this type of matter.

  4. Tracing Temple Timing

    Several of the Hawaiian islands contain relic temples that were built by their rulers and functioned as centers of control. Radiocarbon dates on wood and charcoal associated with the temples implied that they were built during a 250-year period as the Hawaiian societies evolved and grew. Coral was placed and enclosed in special compartments on these temples as part of their dedication, and Kirch and Sharp (p. 102; see the news story by Stokstad) dated preserved corals from temples in Maui and Molokai using the 230Th method, which provides more accurate dates for this time. The dates of the coral branches span about 30 years on Maui (just after A.D. 1600) and are slightly older on Molokai. The temples were all completed, and presumably rule was consolidated, much more rapidly than had been believed, perhaps within a single generation.

  5. Seek, Fortify, Then Destroy

    In clinical trials, “anti-angiogenic” drugs, which are designed to destroy the blood vessels that feed tumors, have limited efficacy when administered as single agents. However, when provided as a combination therapy, they enhance the efficacy of conventional cytotoxic drugs targeting tumor cells, even though the destruction of the tumor vasculature might be expected to impede drug delivery to the tumor. Jain (p. 58) reviews evidence supporting the counterintuitive notion that anti- angiogenic drugs initially fortify, rather than destroy, the tumor vasculature, thereby improving delivery of cytotoxic drugs to the tumor. If further substantiated, this hypothesis would have important implications for the optimal dose and scheduling of combination cancer therapies.

  6. Dissecting Malaria's Genetic Strategies

    Plasmodium parasites, the agents responsible for malaria, are of intense interest, but they have complex life cycles within their mosquito vectors and within their mammalian hosts that make molecular analysis difficult to untangle. A comparative genome analysis by Hall et al. (p. 82) shows that, apart from conserved central sections of chromosomes, there are genes evolving rapidly in response to life-cycle, stage-specific pressures. For example, transcriptional profiling and proteomic analysis of several species of parasite has helped tease apart aspects of the little understood sexual cycle of these parasites.

  7. Salt Survivors

    Immense salt deposits beneath the Mediterranean floor are the legacy of its having evaporated to dryness about 6 million years ago. Van der Wielen et al.. (p. 121) have explored the microbiology of deep hypersaline anoxic remnants. A picture emerges of whole microbial communities that are far from being biogeochemical dead-ends. Rather they are contributing to global cycles while thriving in some of the most saline environments known.

  8. Bioremediation Bug Genome Revealed

    Dehalococcoides ethenogenes is the only bacterium known to reductively dechlorinate groundwater pollutants, tetrachloroethene (PCE) and trichloroethene (TCE), to ethylene. Seshadri et al.. (p. 105) now present an analysis of the genome of D. ethenogenes. Multiple dehalogenases and reductases were identified which indicate that the organism is highly evolved to utilize halogenated organic compounds and H2. The analysis provides insight into the organism's complex nutrient requirements, and surprisingly suggests that an ancestor was a nitrogen-fixing autotroph. Because the organism is difficult to culture, the genome sequence contributes significantly to our understanding of the physiology of this organism and its bioremediation potential.

  9. Picky Eaters

    It is widely assumed in foraging theory that predators cannot balance their nutrient intake, but instead maximize their energy intake subject to prey size, abundance, and time constraints. Mayntz et al. (p. 111) show that this is not the case, using three species of invertebrates (ground beetles, wolf spiders, and web spiders) with widely different feeding biology. When the diet of the predators was manipulated to render them either protein- or lipid-deficient, the animals adjusted their feeding to make good the specific deficit. Compensatory nutrient selection occurred either by selecting among foods of different nutritional composition, by adjusting consumption of a single prey type, or by extracting nutrients selectively from within individual prey items.

  10. Calcium Channels in T Lymphocytes

    Calcium represents a critical signaling mediator in a number of biological systems, including excitable cells of such as neurons and in lymphocytes of the immune system. However, the identity of channels that mediate calcium entry in lymphocytes has been unclear. Badou et al. (p. 177; see Perspective by Winslow and Crabtree) find that T cells express two forms of voltage-gated calcium channel (Cav) that are required for mediating activation signals critical for normal T cell functions. Cav activity was increased directly by T cell receptor stimulation.

  11. A Spindle Here, a Spindle There


    During cell division, replicated chromosomes align on the mitotic spindle poised to segregate to opposite ends of the cell. To prevent errors during mitosis, a spindle checkpoint monitors proper attachment of chromosomes to the spindle microtubules as well as tension that presumably exists between the chromosomes and the spindle. Indjeian et al. (p. 130) now describe Sgo1, a protein found on kinetochores (the central region of chromosomes that become attached to the mitotic spindle) that also has a microtubule-binding domain. In mutant yeast lacking Sgo1, chromosomes no longer align correctly on the spindle, and cell cycle progression is blocked. Sgo1 is likely to represent part of the cell's tension sensing machinery when errors in chromosome-spindle interaction occur. Many tumor cells are characterized by increased genomic instability and chromosome segregation defects, and may possess extra microtubule-organizing centrosomes and multipolar mitotic spindles. Quintyne et al.. (p. 127) now find that cytoplasmic dynein-mediated centrosome clustering can help to prevent the formation of multipolar spindles in cells containing additional centrosomes. The authors suggest that the generation of spindle multipolarity in transformation may require two distinct steps—centrosomal amplification followed by centrosome separation.

  12. How Electrons Sink or Swim

    Hydrated electrons, which are of importance in radiolytic chemistry and biologically relevant electron transfer, have been studied by using gas-phase water clusters as proxies for bulk water. Do clusters of roughly 50 or more water molecules truly mimic the solvating cavity in the bulk, or do the excess electrons bind to the cluster surface? Verlet et al. (p. 93, published online 16 December 2004) used photoelectron imaging to garner evidence for two distinct water cluster types, which they assign to structures with either a surface-bound or internally solvated electron. The traditional method of cluster preparation yields the internally solvated structure and supports the applicability of prior studies to the bulk. In contrast, the surface-bound class, with significantly smaller electron binding energies, results from electron attachment to vibrationally colder neutral clusters.

  13. Massive Resistance

    The conductivity of single-walled carbon nanotubes (SWNTs) can be altered by chemically absorbed species, an effect that could be exploited in sensing applications. Romero et al.. (p. 89) present evidence showing that collision of gas atoms, even inert gases and weakly adsorbing molecules such as methane and nitrogen, change the resistance and thermo-power of metallic SWNTs. The effect scales as the third power of mass, and the authors present a model in which denting of the side of the nanotube by the collisions leads to scattering of the conduction electrons.

  14. The Nondegenerate Y

    Comparative genomics between Drosophila pseudoobscura and Drosophila melanogaster has revealed a surprisingly dynamic picture of the evolution of the Y chromosome. Carvalho and Clark (p. 108, published online 4 November 2004; see the Perspective by Graves) report that out of 15 genes and pseudogenes on the D. pseudoobscura Y chromosome, none were shared with D. melanogaster. All of the single-copy genes from the Y chromosome of D. melanogaster are located on an autosome in D. pseudoobscura and related species. The Y chromosome turnover probably occurred between 2 to 13 million years ago. Y chromosome genes underwent a major reorganization after they moved to the autosomes: From gigantic genes spread out across megabase spans of heterochromatin, they became normally sized, normally spaced, euchromatin-like genes.

  15. I'm a Phage, Let Me Out of Here

    The enterobacteriophage P1 secretes a protease, lysozyme, which degrades the host cell wall prior to lytic release of progeny phage. After the P1 lysozyme is secreted to the Escherichia coli periplasm, it is tethered to the bacterial membrane through an amino-terminal signal-arrest-release (SAR) domain in an inactive form. Upon membrane release, the P1 lysozyme is activated. Xu et al.. (p. 113) show that the P1 lysozyme is activated by an intramolecular thiol-disulfide isomerization that requires a cysteine in the SAR domain. The disulfide transitions occur without any change in the overall oxidation state of P1 lysozyme, but cause dramatic conformational changes in the amino-terminal domain that contains the catalytic residues.

  16. Synaptic Vesicle Recycling

    The mechanisms underlying exo- and endocytosis of neurotransmitter vesicles at synaptic terminals are still not fully understood. Although many endocytic pathways have been documented, there is little direct evidence on the molecular dependence of endocytosis. Yamashita et al. (p. 124) used capacitance measurements in combination with intraterminal molecular loadings at the calyx of Held to show that endocytosis and recycling of synaptic vesicles predominantly depend upon guanosine triphosphate hydrolysis by dynamin-1. The study also suggests that the rapid capacitance change, which has been proposed to represent kiss-and-run exo-endocytosis, is unrelated to transmitter release.