This Week in Science

Science  26 Apr 2002:
Vol. 296, Issue 5568, pp. 613
  1. In Brevia ...

    Ajayan et al. (p. 705) report that carbon single-walled nanotubes ignite and combust when hit with the light from a photographic flash.

    CREDIT: AJAYAN ET AL.
  2. Electrons Racing with Nuclei

    The Born-Oppenheimer approximation allows theoreticians to treat the motion of the much faster and lighter electrons in molecules separately from nuclear motion. In the reaction of H2 with the open-shell halogen atoms, F and Cl, three potential energy surfaces (PESs) describe the reaction. If the Born-Oppenheimer approximation holds, only the lowest energy PES should contribute to product formation. Indeed, for F, this is the case to within 10%, and for the heavier Cl species, one would expect an even smaller contribution. Nonetheless, recent experimental results have suggested that for the Cl reaction, the higher energy spin-orbit PESs are the dominant reaction channel. A theoretical analysis by Alexander et al. (p. 715; see the Perspective by Manolopoulos) now shows that the spin-orbit PES does not appear to contribute to the reactivity, except at very low energies, and that ineleastic reaction channels open up from the ground state. The origin of the disagreement between theory and experiment is unclear and will warrant further study.

  3. The Role Not Taken

    Although water vapor is the most important greenhouse gas, it is not even agreed whether an increase in its concentration due to global warming will cause more warming or cooling. Warming (a positive feedback) will occur if the biggest effect is to absorb more outgoing long-wave radiation. Cooling (a negative feedback) will occur if the major result is to form more clouds, which would reflect more sunlight. Soden et al. (p. 727; see the Perspective by Del Genio) test these two possibilities by using observations of the atmospheric cooling and drying that occurred after the eruption of Mount Pinatubo in 1991 as targets for an atmospheric general circulation model. The cooling that followed the blast is consistent with the measured decrease in water vapor only if the vapor provides positive feedback. These results suggest that current climate models have correctly chosen the role of water vapor.

  4. Maintaining Continental Slopes

    Continental slopes maintain a consistent angle of 2° to 4°, which indicates that some form of steady state is achieved between continental erosion and ocean dynamics. Cacchione et al. (p. 724) have determined that semidiurnal internal tides with angles less than the continental slope are reflected toward the sea floor and increase the velocity and shear stresses along the slope and sea floor. This excess energy keeps fine-grain sediment from collecting along the slope and thus maintains the angle. Their model, which is consistent with observations along the northern California and New Jersey coasts, may be used to understand the coupling between continental and oceanic processes.

    CREDIT: CACCHIONE ET AL.
  5. Seeing Double with Quantum Cloning

    The uncertainty inherent in quantum mechanics places constraints on the ability to make perfect copies or “clones” of quantum states. Much theoretical work has focused on how good of copies can be made. Lamas-Linares et al. (p. 712) now present an experimental demonstration of the cloning of the polarization state of a photon. The coincidence of a photon with a frequency-doubled photon in a nonlinear crystal results in the production of two photons, one a clone of the input photon. The measurement of fidelity, 0.81, is close to the theoretical limit of 0.83.

  6. When Less Is More

    Experimental studies of grassland ecosystems have supported the paradigm of a positive relation between diversity and ecosystem productivity. Paine (p. 736) provides experimental evidence, replicated over several years, from a rocky intertidal community that challenges this paradigm's generality. The intact biotic assemblage containing the natural complement of kelp, herbivores, and predators results in a high-diversity plant assemblage that is maintained in large measure by selective grazing effects of the system's dominant herbivore. However, when this single herbivore species was experimentally removed, a single plant species dominated and overall productivity increased, thus creating a negative relation between plant species diversity and production.

  7. Fickle Finches

    Continuous long-term studies of evolution in natural populations of vertebrates are very rare, and we have few insights into the consistency of selection, the frequency of evolutionary responses or hybridization, or the extent to which change is gradual or episodic. Grant and Grant (p. 707; see the news story by Zimmer) have studied Darwin's finches on the Galapagos for more than 30 years, and their data set offers a long-term look at the interplay of predictable and unpredictable components in the evolutionary process.

  8. Still Searching for Schizophrenia Genes

    The search for genes involved in schizophrenia has been a long and confusing one. Levinson et al. (p. 739) present results of an ongoing effort by a multicenter collaboration to use a sample of about 800 families with schizophrenia to evaluate evidence of genetic linkage in chromosomal regions that have been “nominated” by more than one study. Although previous reports (including one published in Science) had strongly implicated region 1q as being involved, this study did not find any evidence of linkage. Nonreplication suggests that the affects of 1q on schizophrenia are, if present, likely to be very small at the population level.

  9. Murine Toxin Misnomer

    Plague is caused by the bacterium Yersinia pestis and is transmitted via flea bites. Closely related bacteria do not take this tortuous route between human hosts. Hinnebusch et al. (p. 733) reveal that the so-called Yersinia murine toxin is in fact key to flea transmission. The toxin is in fact an enzyme, phospholipase D, whose activity is important in establishing an infection in the flea gut.

  10. Not All Foraging Is the Same

    A comprehensive understanding of how genes and the environment interact to influence behavior is becoming a realistic goal. Ben-Shahar et al. (p. 741; see the news story by Pennisi) report that the same gene known to influence foraging behavior in Drosophila via genotypic variation also influences foraging in honey bees via regulation of expression during adult behavioral maturation. The gene, foraging, encodes a guanosine 3',5'-monophosphate-dependent protein kinase, a key player in many signal transduction pathways. This gene thus influences behavior across strikingly different time scales, both evolutionary and developmental.

    CREDIT: BEN-SHAHAR ET AL.
  11. Rousing the Resistance in Plants

    The “gene-for-gene” hypothesis, long useful in describing how particular plants resist infection by particular pathogens, may now be in for complicating elaboration. An additional plant gene, Rcr3, is involved in the interaction between a particular fungal elicitor and its corresponding plant-encoded protein, localized in the plant cell membrane. Krüger et al. (p. 744) show that Rcr3 is a protease secreted by the plant cell in response to infection by this fungus. Rcr3 function is required for a robust response to pathogen invasion.

  12. Healing Touch of T cells

    Dendritic epidermal γδ T cells (DETCs) are thought to contribute to the integrity of the skin. Jameson et al. (p. 747) have extended previous studies which showed that DETCs express growth factors in response to damaged keratinocytes by demonstrating that these cells are involved directly in wound repair. Mice deficient in γδ DETC displayed a twofold lower rate of wound healing relative to mice with normal numbers of DETCs, as well as a marked reduction in keratinocyte proliferation. In a skin organ culture system, reduced wound repair and keratinocyte proliferation in the absence of γδ DETC could be corrected by addition of keratinocyte growth factor. Skin γδ T cells therefore appear to play a prominent role in allowing rapid wound healing.

  13. Slowly Changing Partners

    A wealth of genomic and proteomic information now makes it possible to analyze fundamental properties and evolution of protein interaction networks. To estimate evolutionary rates, Fraser et al. (p. 750) compared putative orthologous proteins in Saccharomyces cerevisiae and Caenorhabditis elegans. As expected, given that highly interactive proteins are more likely than their less connected counterparts to be required for viability, highly connected proteins also appear to evolve more slowly. However, the authors' analysis indicates that what limits evolutionary rates of interacting proteins is that a greater proportion of the molecules themselves takes part in physical interactions. Changes in interacting partners are expected to only be maintained if a reciprocal change in the other partner also occurs. This requirement appears to require similar evolutionary rates for the interacting partners.

  14. Variations on Some Genes

    To identify specific genes that control transcription levels, Brem et al. (p. 752) compared gene expression patterns from a laboratory strain of yeast and a natural isolate from the wild that were grown under identical conditions, and found more than 1500 genes that were differentially expressed. The authors analyzed progeny from crosses between the two strains and found that 308 of the differences segregated in a Mendelian fashion and could be mapped. Variation in gene expression typically was controlled by many genes; eight appeared to be trans-acting, with widespread transcriptional effects.

  15. Anion Determines Source or Sink

    Hydrotalcites, minor mineral phases found in crustal rocks and soils, are important sinks or sources for divalent metals that may contaminate water supplies, depending on the composition of the hydrotalcite. Allada et al. (p. 721) used calorimetry to measure the heats of formation of cobalt-aluminum hydrotalcites and show that the properties of the anion are more important than the cation in determining the solubility of hydrotalcites in water. For example, if the anion is CO32−, then the hydrotalcite does not dissolve readily in water and it is a potential sink for metal contaminants, whereas if the anion is NO32−, then the hydrotalcite does dissolve more readily in water and becomes a potential source for metal contaminants.

  16. Only the Right Partners

    An achiral object can become chiral if confined within a place. Grzybowski and Whitesides (p. 718) studied magnetic millimeter-sized plates with tails that make them right-handed (R) and left-handed (S) when confined to a fluid-air interface. Under the influence of a rotating magnetic field, these plates rotate but because of their handedness, create different local flow fields. The coupling of the R plates to the fluid flow leads to attractive interactions between the plates, as evidenced by tracer dye and tracer particle analysis, whereas the S plates show repulsive interactions. A consequence is that R plates will form dipoles, through a complex interaction catalyzed by free R plates, while the S plates are repulsed by both S and R objects. Altering the plates by giving them either two tails, or by narrowing their shape, changes the types of aggregates that form.

  17. Judging the Blooms

    Phytoplankton blooms, such as the one that occurs in the North Atlantic every spring, fix huge amounts of atmospheric CO2. In the early 1950s, Sverdrup developed a simple model describing the conditions needed for a bloom to occur, in which he assumed that photosynthesis is limited only by light and is a linear function of local irradiance. However, estimates of the level of light where total amount of organic carbon production by photosynthesis equals total organic carbon loss by respiration and other processes—called the compensation irradiance—have varied by more than an order of magnitude. Siegel et al. (p. 730) use a combination of satellite data from SeaWiFS and hydrographic measurements to characterize the North Atlantic spring bloom during the period from 1998 to 2000 to provide an estimate of heterotrophic production over wide spatial scales. Their estimates suggest that phytoplankton respiration accounts for roughly one-half of community respiration.

  18. Working with the Mediator

    The mechanisms involved in transcriptional activation of genes transcribed by the eukaryotic RNA polymerase II are not yet fully understood. Transcriptional activators recruit coactivators to a promoter, which in turn associate with and regulate the polymerase molecule. In yeast, the Mediator coactivator complex is recruited to promoters through interactions between transcriptional activator proteins and specific subunits of the Mediator. Stevens et al. (p. 755) extend this model to higher eukaryotics by demonstrating that specific activators can mediate transcriptional responses to the Ras-mitogen-activated protein kinase signal transduction pathway through specific interactions with the Sur2 subunit of the mammalian Mediator complex.

  19. Electron Microscopic Characterization of Silver Nanowire Arrays

    Hong et al. (Reports, 12 October 2001, p. 348) reported that they had synthesized arrays of single-crystal silver nanowires approximately 0.4 nanometers in diameter and up to a micrometer in length, supporting that assertion with high-resolution electron microscope (HREM) images accompanied by selected-area electron diffraction (ED) patterns. In a comment, Ijima and Qin argue that the samples shown “actually represented not arrays of nanowires but highly defected needlelike silver crystals,” with the observed array-like geometry due to stacking faults and twinning planes aligned parallel to the needle axis. Hong et al. respond that the details of the ED patterns, electron energy loss spectra, and infrared spectra do not support Ijima and Qin's suggestion that the images represented bulk silver, and instead are more consistent with the nanowire interpretation. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/296/5568/611a

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