This Week in Science

Science  09 May 1997:
Vol. 276, Issue 5314, pp. 873
  1. Signaling to friendly troops

    In an unusual interaction of three trophic levels, plants respond to attack by certain insects by calling in predators of these insects. Alborn et al. (p. 945; see the Perspective by Farmer, p. 912) have identified the chemical signal from a caterpillar that induces the wounded corn seedling to emit volatile signals attractive to parasitic wasps, the natural enemy of the caterpillar. This initial signal, volicitin, resembles precursors of eicosanoids and prostaglandins and may be part of the octadecanoid signaling pathway.

  2. Mesoporous materials

    Two reports focus on mesoporous materials, which have cavity diameters of a few nanometers and which have potential applications in separations, and catalysis. Feng et al. (p. 923) were able to coat the inside of a mesoporous silicate with a functionalized organic layer. The coating contained thiol groups and made the material an efficient scavenger of mercury and other heavy metals from both aqueous and organic solutions. Most mesoporous materials are insulators that have amorphous wall structures. Tian et al. (p. 926) have synthesized a crystalline mesoporous manganese oxide that is semiconducting and highly thermally stable.

  3. Hydrocarbons in the Kuiper belt

    In the last 5 years, our view of the Kuiper belt, a band of objects just beyond the orbit of Neptune, has become clearer with the direct observation of about 40 objects. Detection of these distant objects is hampered by their small size and unknown composition. Brown et al. (p. 937) used the Keck I telescope to obtain a near-infrared spectrum of Kuiper belt object 1993SC and infer that the surface of 1993SC is composed of a hydrocarbon ice. The similarity of the spectrum of 1993SC to that of Pluto and Triton suggests a Kuiper belt origin for these bodies.

  4. Comet x-rays and the solar wind

    Comets were not considered to be strong x-ray emitters until they were observed recently from comet Hyakutake. Häberli et al. (p. 939) modeled the interaction of the solar wind with the cometary plasma. A three-dimensional, single-fluid magnetohydrodynamic model reproduced the energy and shape of the x-ray emissions caused by charge exchange excitations of solar wind ions with neutral molecules or atoms in the comet's plasma.

  5. Started with squash

    Recent radiocarbon dates have implied that agriculture in North America originated about 6000 years ago with the use of maize, beans, and other crops, although there have been earlier suggestions and some dates implying that the origin was perhaps 10,000 years ago. Smith (p. 932; see the cover and the news story by Roush, p. 894) analyzed and dated squash seeds, peduncles, and fruit rind fragments recovered from Archaic period deposits in Guilá Naquitz cave in Oaxaca, Mexico. The seeds are larger than wild types and, along with other features, imply that these seeds and fragments were from an early domesticated plant. The calibrated radiocarbon ages on the seeds and peduncle specimens vary up to about 10,000 years ago.

  6. Uncommonly fast

    The enzyme that catalyzes the conversion of orotidine 5′-monophosphate (OMP) to the corresponding uridine nucleotide, OMP decarboxylase, accelerates the rate of this reaction rate by several orders of magnitude. The origin of this unsurpassed acceleration, which occurs in the absence of metals or cofactors, has been unclear. Lee and Houk (p. 942), on the basis of quantum mechanical computations, propose that protonation of the substrate occurs in concert with decarboxylation to yield an unconventional stabilized carbene as an intermediate. This protonation can occur only within a nonpolar environment, which would account for the great advantage of the enzyme active site.

  7. Inhibiting tyrosine kinases

    Activity of protein tyrosine kinases contributes to stimulation of cell growth and can contribute to uncontrolled growth in cancer cells, and inhibitors of such enzymes may have therapeutic value. Mohammadi et al. (p. 955) present the crystal structure of the tyrosine kinase domain of the fibroblast growth factor receptor 1 with relatively specific inhibitors based on an oxindole group. Although the inhibitors bind to the adenosine triphosphate binding site that is similar in the various kinases, they preferentially inhibit certain kinases. The results reveal the structural basis of the specificity and may permit development of more effective inhibitors.

  8. Contrasting result

    The overall level of contrast in a visual scene can vary greatly, yet our sight adapts rapidly. How do we adjust our cortical neurons, which have limited contrast sensitivity, to operate within the appropriate range? Helmholtz studied the psychophysics of this phenomenon in the 19th century (see the Perspective by Barlow, p. 913), and physiological studies, carried out with extracellular electrodes, have led to theories of contrast gain control. Carandini and Ferster (p. 949) present data obtained from intracellular recordings that support an unexpected interpretation—chronic excitatory input is used to adjust neurons to the most recent average level of contrast. This mechanism functions independently from the well-known fluctuations in membrane potential that encode the changing visual scenery.

  9. Biasing Brownian motion

    Small particles suspended in a liquid can undergo Brownian motion, which on average leads to no net displacement of the particle. In an article, Astumian discusses how nonequilibrium fluctuations, such as those introduced by asymmetric electrical or chemical potentials, can take advantage of Brownian motion to produce motion in a single direction. Such effects might occur in microscopic biological motors and pumps and could find applications in particle separations.

  10. Protein NMR on ice

    Determination of protein structure by nuclear magnetic resonance (NMR) spectroscopy is usually performed with solution methods, but for very large proteins that tumble slowly in solution, or for proteins bound in membranes, solid-state NMR methods could provide an important alternative approach. Hall et al. describe a method for enhancing signals using dynamic polarization transfer in the solid state from unpaired electrons of nitroxide-free radicals to nuclear spins with microwave irradiation. Model studies were performed for arginine and T4 lysozyme in frozen water-glycerol solutions.

  11. Discharging ice

    It has been difficult to evaluate whether the Greenland ice sheet is in balance or is shrinking or growing. Mass has been thought to be lost primarily by ablation and by the discharge of icebergs. Rignot et al. used satellite radar interferometry to identify the grounding line—beyond which the ice sheet is floating—in northernmost Greenland and used this information to evaluate the mass balance of the ice sheet. The data imply that most of the ice there is being lost by melting at the base of the ice sheet where it is floating.

  12. Slowing cell division

    Stimulation of cell division and other biological processes occur through the phosphorylation and activation of protein kinases in the mitogen-activated protein (MAP) kinase cascade. Such signals are balanced by dephosphorylation and inactivation of the MAP kinase kinase MEK, probably by protein phosphatase 2A (PP2A). Hériché et al. report a new mechanism by which activity of PP2A may be regulated. In resting cells, but not in cells stimulated with a growth factor, PP2A associated with a protein previously described as the catalytic subunit of the protein kinase casein kinase 2 (CK2). Overexpression of the CK2 subunit inhibited mitogen-stimulated activity of MEK and MAP kinase and inhibited transformation of cells by the oncogenic form of the guanosine triphosphatase Ras. Such regulation of PP2A by the CK2 subunit appears to be the opposite of that mediated by small tumor antigen of the SV40 virus, which leads to uncontrolled cell growth.

  13. HIV drug performance

    Measurements of virus levels in the blood have been important in determining the existence of powerful drug combinations that can act against human immunodeficiency virus-type 1 (HIV-1). However, it is also necessary to evaluate how well those drugs are working in the lymphoid tissue, where most of the virus is produced. Cavert et al. measured virus load in tissue biopsies of the tonsils of HIV-1-seropositive individuals during 24 weeks of treatment with a triple combination of viral protease and reverse transcriptase inhibitors. Although some infected mononuclear cells still remained, treatment eliminated more than 99 percent of the virus from the lymphoid tissue. [See the news story by Cohen.]

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