This Week in Science

Science  05 Feb 1999:
Vol. 283, Issue 5403, pp. 757
  1. Fast Winds from Coronal Holes

    Coronal holes are regions of low density and temperature within the solar corona. Observations in the 1970s and 1980s suggested that high-speed solar wind is ejected from areas of the coronal holes where the magnetic field lines are open. Hassler et al. (p. 810; see the cover) present Doppler velocity measurements of emitted Ne7+ from a polar coronal hole and the equatorial region of the sun obtained from the SOHO spacecraft. Fast outflows of Ne7+ ions from the coronal hole (at speeds as high as 20 kilometers per second) are correlated with sheets or intersections of sheets of intense silicon II radiation in the chromosphere below the corona. Most of the Ne7+ is flowing downward in the equatorial region between the intense Si II regions. Thus, energy is being brought up from along chromospheric boundaries or intersections and accelerating material along the open magnetic field lines in coronal holes.

  2. Solar Nebula Chemistry

    Highly refractory calcium-aluminum-rich inclusions (CAIs) in meteorites are considered to be the most pristine remnants that accreted from the gas and dust in the solar nebula. Hiyagon and Hashimoto (p. 828) analyzed the oxygen isotopic abundances of another, less refractory inclusion type that accreted later, olivine inclusions (OIs), in the Yamato-86009 and Murchison chondrites. The OIs have oxygen-16 enrichments similar to those in the CAIs, which indicates that the two different types of inclusions formed in the same oxygen environment. This unexpected similarity favors a common chemical process for 16O enrichment in the solar nebula rather than an exotic 16O-rich phase residing exclusively within CAIs.

  3. Nanodisc Formation

    Mixing of anionic and cationic surfactants leads to the formation of molecular bilayers at low concentration that may form vesicles or flexible cylinders. Usually, excess salt in the solution screens their electrostatic interactions and tends to destabilize the aggregates. Zemb et al. (p. 816) show that in a system without excess salt, achieved by using only H+ and OH counterions, stiff nanodiscs of 3-nanometer thickness form spontaneously and have high charge density at the strongly curved edges; their diameter can be turned from a few micrometers to 30 nanometers. The structures may be used as templates for orienting molecules or for inorganic polymerization.

  4. On the Double

    Excessive duplication of centrosomes is found in cancerous cells, but normal cells maintain strict control of duplication of centrosomes during cell division because formation of more than two spindle poles would lead to improper chromosome segregation. Hinchcliffe et al. (p. 851; see the news story by Pennisi) report that replication of centrosomes is regulated through activity of the cyclin-dependent kinase 2-cyclin E (Cdk2-E) complex. In Xenopus egg extracts arrested in the S phase of the cell cycle, centrosomes underwent multiple rounds of replication that was strictly dependent on the activity of the Cdk2-E complex. The results help elucidate the molecular basis by which a single round of centrosome replication is coupled to the machinery that controls progression through the cell division cycle.

  5. Organic Transistors with Oxide Gates

    Although all-organic transistors offer the possibility of inexpensive components with low fabrication costs, they typically require high switching voltages (tens of volts). Dimitrakopoulos et al. (p. 822; see the news story by Hellemans) now show that field effect transistors based upon pentacene with insulating gates made from amorphous barium zirconate titanate can be fabricated on plastic substrates in an room-temperature process. High carrier mobilities could be achieved at operating voltages typical of silicon devices (5 volts).

  6. Long-Toothed Browsers

    High-crowned fossil horse teeth suggest a diet of mostly abrasive grasses (C4 plants) typical of grazers in a tropical or temperate open grassland, whereas short-crowned teeth suggest a diet of soft and leafy plants (C3 plants) typical of browsers in a wooded or aquatic environment. MacFadden et al. (p. 824; see the news story by Morrell) have measured carbon isotopes in the tooth enamel from six species of 5-million-year-old horses that lived in Florida during a time of changing ecosystems. These extinct species had high-crowned teeth, but the isotopic data indicate that five of these species either browsed and grazed or were entirely browsers. The authors suggest that once these species evolved high-crowned teeth, they did not revert to short-crowned teeth despite the change in their diet, possibly because of their rapidly changing environments.

  7. Hydrothermal Brew

    One possible location for the synthesis of chemicals needed for the origin of life is submarine hydrothermal systems, where sea water circulates through and reacts with the oceanic crust. Imai et al. (p. 831) designed an experiment that simulates aspects of this process. Sea water containing glycine was repeatedly circulated through and heated in a flow reactor, which led to the formation of oligopeptides. Addition of copper ions helped further elongate the peptides.

  8. Hooked Together

    The sterile alpha motif (SAM) domain is present in a variety of proteins ranging from protein and lipid kinases to transcription factors. This diversity extends to the homotypic and heterotypic protein interactions mediated by this module. Thanos et al. (p. 833) report that a SAM domain monomer can interact in two distinct ways with another monomer. Repeated combinations of these interfaces could form higher order oligomers and possible platforms for recruiting other proteins. This mechanism of interaction may explain how proteins utilize SAM domains to regulate different cellular processes.

  9. Diagnosis: Iron Limitation

    Understanding the factors that limit photosynthesis in the oceans is problematic because of the vast areas involved and the lack of ready diagnostic techniques. Behrenfeld and Kolber (p. 840; see the Perspective by Mullineaux) have identifed a physiological response in phytoplankton that can be used as an indicator of iron limitation. In the open ocean and in laboratory cultures of marine Synechococcus, iron-limiting conditions are associated with characteristic diel patterns in variable fluorescence; these patterns are lost upon enrichment with iron. Application of the method suggests that, unlike the Atlantic, much of the South Pacific Ocean is iron-limited.

  10. Coral Bleaching Basics

    Are episodes of coral bleaching evidence of human interference with the natural environment or natural variation? Bleaching is characterized by the expulsion of zooxanthellae (symbiotic algae). Fagoonee et al. (p. 843) present a 6-year field study of zooxanthellae dynamics in the coral Acropora formosa in a shallow lagoon in Mauritius. Bleaching has a complex causation; considerable (and unexpected) natural temporal variation in zooxanthella contributed such factors as a strong seasonal cycling and density-dependence effects, in addition to significant environmental influences, such as nitrate loading.

  11. Homing Signals

    Stem cells that replenish the hematopoietic system reside in the bone marrow, but transplanted stem cells often fail to establish themselves and repopulate mature blood cells. The factor SDF-1 is a potent chemokine that attracts cells expressing its receptor, CXCR4. In an in vivo model of human CD34+ stem cell engraftment, immunodeficient NOD/SCID mice are transplanted with human bone marrow cells. Peled et al. (p. 845) report that blocking the SDF-1-CXCR4 system inhibits the repopulation of the NOD/SCID mouse's hematopoietic cells with human cells generated from the transplant. The induction of higher CXCR4 expression ex vivo, before transplanting the stem cells, increased the numbers of engrafting stem cells. This approach could potentially increase the efficacy of human bone marrow transplants.

  12. Mycobacterial Toxin

    Bacterial virulence is often mediated by secreted toxins. For the mycobacteria, which are responsible for such diseases as tuberculosis and leprosy, the only species for which there has been evidence for a toxin has been Mycobacterium ulcerans, which causes Buruli ulcer, a severe tropical skin disease. George et al. (p. 854) have now purified this toxin, mycolactone, and identify it as a polyketide containing a 12-membered ring. This cytopathic toxin induces similar types of skin lesions in guinea pigs. This finding may aid in the identification of other mycobacterial toxins.

  13. CD8 Control of HIV-1

    Although there has been circumstantial evidence that cell-mediated immune responses are important for controlling human immunodeficiency virus-type 1 infections, definitive evidence would require a “knock-out” type of experiment. Schmitz et al. (p. 857) provide such data by using an antibody to deplete CD8+ T cells from the blood and lymph nodes of rhesus monkeys. In cases of primary infection, removal of this T cell population was associated with increased viral replication and more rapid progression of disease. Viremia was also increased in cases of chronic infection but was suppressed again when the CD8+ cells reappeared. These results bolster strategies of vaccine design that rely on induction of cell-mediated immunity.

  14. Callisto's Atmosphere

    Callisto, the outermost of the four Galilean satellites that orbit Jupiter, is presumed to be geologically inactive. Carlson 820 used an off-limb scan of Callisto by the near-infrared mapping spectrometer on Galileo to search for a carbon dioxide (CO2) atmosphere from airglow emissions. He found a possible tenuous CO2atmosphere that may extend to 100 kilometers above the surface and that has a temperature about equal to that of the surface. This observation leads to several possibilities for the source of CO2 on Callisto that may relate to the source of CO2on other planets and satellites in the solar system. Perhaps the CO2 is produced by outgassing of Callisto's interior, or perhaps it comes solely from the surface through the release of accreted cometary CO2 by radiation and charged particle bombardment.

  15. Scattering Polarized Electrons

    Chiral molecules can rotate the plane of polarization of polarized light. Similar asymmetric scattering effects have been predicted and observed for interactions of molecules with polarized electrons in the gas phase, but the effects have typically been small. Larger asymmetry effects have been predicted, however, for oriented molecules. Ray et al. 814 show that for organized thin films of chiral organic molecules, the asymmetry in the resulting scattering is three to four orders of magnitude greater than that seen in the previous gas phase studies, which points to a supramolecular effect that enhances the asymmetry.

  16. Pathogens and Epidemics

    Recent years have seen a synergistic interaction between the disciplines studying infectious disease and the molecular basis of evolution. Levin et al. 806 review the insights that have been generated by this convergence. Knowledge of the genetics of a pathogen population can lead to better understanding of the course of an epidemic and allow us to predict more effective mechanisms to fight disease.

  17. Changing the Channels

    Activation of T cells requires signals mediated by increases in the intracellular concentration of Ca2+. Initially, Ca2+ is released from intracellular stores. Depletion of such stores leads to opening of calcium channels in the plasma membrane known as CRAC (calcium release-activated Ca2+) channels. Similar channels function in other cells as well. When these channels conduct Ca2+, the current is too small to allow single-channel recording. However, Kerschbaum and Cahalan 836 were able to make single-channel recordings when Na+ was used as the current-carrying ion. Their measurements show that conductance of the channels is much larger than previously estimated. Thus, the estimated number of these channels per cell (100 to 400) is much lower than previously thought.

  18. Propelling Proliferation

    When T cells are activated, not only do signaling pathways that participate in proliferation get initiated, but also pathways that inhibit growth, such as the activation of cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA). How then does a T cell that has received the appropriate signals for activation “stay-the-path”? Li et al. 848 report that phosphodiesterases (PDEs), and in particular the T cell-specific isoform PDE-7, also get activated and keep the level of cAMP down. Interfering with PDE-7 inhibits T cell proliferation and may provide a different approach for therapeutic regulation of T cell activity.

  19. Temperature Changes During the Younger Dryas in New Zealand

    C. Singer et al. (Reports, 7 Aug. p. 812) studied “pollen records of deglacial sequences from northwest Nelson, New Zealand,” and found “that there was no significant temperature decline” associated with the Younger Dryas (about 10,000 years ago) there.

    R. Newnham comments that the report does not account for “well-established limitations” of late glacial pollen records such as “poor chronological resolution” and “uncertainties surrounding pollen representation and provenance.” He states that “the existence of Younger Dryas cooling cannot unequivocally be confirmed or refuted from these data.”

    In response, J. Shulmeister et al. discuss details of the “site hydrology effect” and their use of the term “warm indicator” for Halocarpus pollen. They state that the “whole series of deglacial pollen records” are “unambiguously contrary to a Younger Dryas with a large thermal event in New Zealand.”

    The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/283/5403/759a

Log in to view full text

Via your Institution

Log in through your institution

Log in through your institution