This Week in Science

Science  20 Jun 1997:
Vol. 276, Issue 5320, pp. 1765
  1. Old low-latitude ice

    The most detailed continental paleoclimate records are available from ice cores. These are concentrated in Greenland and Antarctica; those that have been obtained from low-latitude ice caps have extended perhaps back to the Last Glacial Maximum, about 18,000 years ago, and generally just through the Holocene. Thompson et al. (p. 1821; see cover) present an ice core record from the Guliya ice cap (Qinghai-Tibetan Plateau) that extends in detail back past 100,000 years ago and may contain ice deposited 500,000 or more years ago. Oxygen isotope data and comparison with methane data from the polar ice cores imply that the hydrologic and global methane cycles are linked.

  2. Metal-free phosphors

    Most white phosphors, such as those used in fluorescent lighting, contain heavy metals and are excited by mercury emission, all of which pose potential environmental problems. In a search for new materials, Green et al. (p. 1826) report that highly emissive broadband phosphors can be synthesized by incorporating small amounts of carboxylic acids into silicates through sol-gel methods. The luminescent species may arise from a defect substitution of carbon into the silicate lattice.

  3. Disk drives

    It takes about a million years for a Jupiter-mass planet to form by accretion in a disk. This time scale presents a problem for explaining the formation of Jupiter and Saturn and extrasolar giant gas planets (one to several Jupiter masses) because the gas in the disk is predicted to dissipate on the same time scale. Boss (p. 1836) used a three-dimensional hydrodynamic code to show that giant gas planets can form by gravitational instability. An isothermal or adiabatic instability produces spiral arms of material spinning toward the edge of the disk, which eventually break off into clumps of giant gas protoplanets. These structures can then form planets more rapidly.

  4. Ganymede's oxygen origin

    Optical reflectance spectra indicate that oxygen is present on Ganymede, Jupiter's largest satellite, but the source and nature of this oxygen has not been well understood. Vidal et al. (p. 1839) attempted to match the Ganymede spectra in the lab using solid oxygen, condensed water-oxygen mixtures, and irradiated ice. The spectra of solid oxygen and water-oxygen mixtures at 26 kelvin matched best and suggests that oxygen exists as ice on Ganymede. It is either protected somehow from the daily surface warming by the sun or it may float in a thin haze above the surface.

  5. Don't shoot

    Inhaled therapies can be valuable for treatment of respiratory disorders and for the benefit of patients who can have an alternative to more invasive forms of therapy, such as injections. Edwards et al. (p. 1868) have developed an inhalation aerosol that is characterized by particles of small mass density and large size. These particles were able to avoid the lungs' normal clearance mechanisms. High levels of insulin or testosterone were delivered over relatively long periods of time in rats.

  6. Macrophage factories

    At a time during the course of human immunodeficiency virus (HIV) disease when the prime viral targets, CD4+ T cells, are decreasing in number, what is responsible for the increased levels of virus? Orenstein et al. (p. 1857) studied biopsies of infected human lymph nodes and concluded that macrophages can be very productive sources of HIV. Their ability to act as factories for HIV production was dramatically increased if the tissues showed signs of infection by opportunistic pathogens such as Mycobacterium or Pneumocystis. Thus, therapies to control opportunistic infections may also serve to control HIV production.

  7. Shocking structure

    An antimicrobial protein, the bactericidal-permeability-increasing protein (BPI), helps protect us against certain kinds of septic shock induced by the lipopolysaccharide (LPS) from the outer membrane of bacteria by binding this lipid, which kills the bacteria. Beamer et al. (p. 1861) found that the x-ray crystal structure of the recombinant human protein has an unexpected structure; BPI is a boomerang-shaped molecule with two lipid binding sites. The structure suggests how the LPS is bound and how related human proteins may bind and transport lipids.

  8. Carotenoid cleavage

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    Vitamin A and other apocarotenoids have a variety of important functions, including regulating development in animals and serving as photosensory pigments. However, the pathways by which biomolecules are synthesized from carotenoids in vivo have remained obscure. Schwartz et al. (p. 1872) have determined that the VP14 protein of maize catalyzes the cleavage of carotenoids to synthesize abscisic acid. Plants carrying a mutation in VP14 are deficient in abscisic acid hormone and defective in seed maturation.

  9. CMV assisting HIV infection

    A chemokine receptor encoded by the human cytomegalovirus (CMV) allows entry of human immunodeficiency virus-type 1 (HIV-1) into CD4+ cells. In order to infect human CD4+ T cells, different strains of HIV must use coreceptors. Pleskoff et al. (p. 1874; see the news story by Balter, p. 1794) found that human CMV encodes a chemokine receptor, US28, that can allow infection by macrophage-tropic HIV strains and fusion of CD4+ cells with cells expressing HIV-1 envelope proteins. Many HIV-infected individuals are infected by CMV, and it has been suspected that coinfection affects the progress of HIV disease.

  10. Enhanced ocean mixing

    How mixing occurs and energy is redistributed in the deep ocean has been unclear, in large part because observations have been limited. Lueck and Mudge measured currents and the dissipation of energy around Cobb Seamount in the northern Pacific. The data show that ocean mixing is enhanced greatly near the seamount and implies that mixing is concentrated near topographic boundaries.

  11. Silicates, weathered or not

    As the Himalayas uplifted during the past 50 to 60 million years, large volumes of material eroded. The ratio of strontium isotopes (87Sr/86Sr) in samples that reflect ocean composition 40 million years ago have suggested significant weathering of minerals high in Sr content; if these were silicates, this process would have consumed large amounts of atmospheric CO2. Quade et al. examined fossil shells and paleosol carbonate from Himalayan river deposits, and their reconstructed 87Sr/86Sr record suggests that the weathering was mainly of carbonate minerals (metalimestones) in the central and western Himalayas. These results suggest that the Sr data may not be an accurate record of silicate weathering.

  12. Ending a glacial age

    Understanding the abruptness and severity of climate changes during the Holocene, the climate period we are now in, is important for assessing potential future climate changes. Stager and Mayewski studied a major climate excursion recognized in many Northern Hemisphere records of the Holocene about 8000 years ago, near the final transition from glacial to full postglacial conditions. They show that the climate event is also seen in a new ice core record from Taylor Dome, Antarctica, and can also be seen, within resolution, in records from Lake Victoria (near the equator) and in Greenland ice cores.

  13. Nuclear transport

    Two reports focus on the regulation of protein transport across the nuclear membrane. The small guanosine triphosphatase (GTPase) Ran is required for nuclear import; Richards et al. report that export of proteins that carry a nuclear export signal (NES) also requires Ran. GTPases like Ran exist in an active form bound to guanosine triphosphate (GTP) and are inactivated by GTP hydrolysis. Although protein import requires GTP hydrolysis by Ran, export seems to require the GTP-bound form of Ran, but not hydrolysis of GTP. The results indicate that nuclear import and export share a requirement for Ran, but the role of Ran is different in each case and the two processes may not be tightly coupled. Nuclear transport is essential for gene expression and a target for viral disruption; Her et al. report that the matrix (M) protein of vesicular stomatitis virus blocks both import and export in Xenopus oocytes. The M protein apparently interferes with transport dependent on Ran-TC4 and its associated factors. [See the Perspective by Goldfarb.]

  14. Oncogenic lipid kinase

    Phosphoinositide 3-kinase (PI 3-kinase) has long been recognized as a critical nodal point in cellular growth signaling. PI 3-kinase is recruited to the cell membrane when cells are stimulated by growth factors, and it transmits the growth signal by phosphorylating specific lipid substrates. Chang et al. isolated a retrovirus from a chicken hemangiosarcoma and found that it encodes a homolog of the host gene for the catalytic subunit of PI 3-kinase. Both the viral gene and its cellular counterpart have potent transforming activity in chicken fibroblasts, suggesting that PI 3-kinase may have a role in tumorigenesis.

  15. Virus association with multiple myeloma

    Multiple myeloma (MM) is a cancer of the bone marrow; its precursor, monoclonal gammopathy of undetermined significance (MGUS), affects nearly 1 million people in the United States alone. Rettig et al. have found an association of MM and MGUS with the herpesvirus thought to cause Kaposi's sarcoma. The virus was not found in malignant cells but in dendritic cells of the bone marrow. The virus could be involved in the transformation of MGUS to MM and virally encoded growth factors (specifically, viral interleukin-6) could be involved in promoting growth of malignant cells. [See the news story by Gura.]

  16. Divergence despite gene flow

    How much population divergence can occur in the presence of high rates of gene flow? Not much, according to traditional thinking, which argues that small, isolated populations are the main sources of morphological diversity. However, Smith et al. present evidence that gene flow combined with selection does result in evolutionary change. They correlated rates of genetic exchange with morphological diversity in rainforest and ecotone (the transition zone between rainforest and savanna) populations of little greenbuls; large morphological differences were found between the two types of habitat (but not within them) despite substantial gene flow, which indicates that natural selection occurred. A further implication of the work is that ecotones may make an important, previously overlooked, contribution to rainforest biodiversity. [See the news story by Enserink.]

  17. Early auxin response

    Auxin, a plant hormone, regulates plant growth and development in a variety of ways. The genes that form part of the early response pathway for auxin effects contain identifiable sequence motifs in their promotors. Ulmasov et al. have found a protein that binds to these elements, thereby activating transcription of these genes in response to auxin. The protein contains both DNA- and protein-interaction domains, indicating that the search for auxin-responsive transcription factors has only just begun.

  18. Turning on G proteins

    Phosphorylation of proteins on tyrosine residues is an important regulatory mechanism in eukaryotic cells. Another major mechanism for cellular signaling is stimulation of cell surface receptors coupled to activation of heterotrimeric guanine nucleotide binding proteins (G proteins). Activation of G protein-coupled receptors increases tyrosine phosphorylation of certain proteins. Umemori et al. now present evidence that tyrosine phosphorylation of the a subunit of a G protein is actually required for receptor-mediated activation. Inhibitors of protein tyrosine kinases blocked activation of G proteins by their associated receptors, and tyrosine phosphorylation influenced the interaction of the G proteins with receptors.