This Week in Science

Science  09 Feb 1996:
Vol. 271, Issue 5250, pp. 737
  1. Icy breakup

    Recently, several large ice shelves in Antarctica have broken up or diminished in size. These ice shelves border large ice sheets. Rott et al. (p. 788) analyze radar images from the ERS-1 satellite and show that the Larsen Ice Shelf, which covered an area of 4200 square kilometers, fractured and disintegrated within a few days. In a Perspective, Fahnestock (p. 775) amplifies some of the possible implications for understanding the origin and dynamics of the larger ice shelves.

  2. Pulling hard on DNA

    Laser tweezer methods make it possible to manipulate large molecules in solution. Attachment of latex beads to the ends of DNA strands allows tension to be applied to the strand so that its mechanical properties (extension, given applied force) can be determined. Two reports by Cluzel et al. (p. 792) and Smith et al. (p. 795) show that at relatively high forces (70 piconewtons), there is a transition where the DNA suddenly stretches by 70 percent. Smith et al. show that the force plateau matches that for single-stranded DNA, suggesting that nicks along the double-strand lead to unraveling of the base pairs. These results may be relevant to recombination—such a transition may reduce the energy needed for proteins such as RecA to stretch DNA.

  3. Water on acid catalysts

    Solid acid catalysts, such as aluminosilicates and silicoaluminophosphates, are workhorses of industrial and petrochemistry, yet the nature of acidic species generated at their surfaces is difficult to determine experimentally. Water is often used as a probe for acidity—does it form the H30+ cation, or is only hydrogen-bonded water present? Smith et al. (p. 799) used powder neutron scattering and infrared spectroscopy to study water bound to a microporous synthetic catalyst, HSAPO-34, that converts methanol to light alkenes. The ordered structure of the catalyst allowed both H30+ and hydrogen-bonded water to be identified at two different sites. In a Perspective, Sauer (p. 774) discusses these results in relation to recent quantum chemical calculations.

  4. Under the reaper

    In Drosophila, the reaper gene (rpr) appears to play a major role in mediating apoptosis, and sequence data suggest similarities with mammalian apoptotic pathways. Two reports focus on mechanistic aspects of reaper expression. White et al. (p. 805) generated transgenic flies that allowed control of expression of the rpr gene. Overexpression of rpr in the fly retina resulted in compound eye ablation in a dosage-dependent manner. Pronk et al. (p. 808) induced expression of the RPR protein in Drosophila Schneider cells and showed that this increased ceramide production. In both reports, the apoptotic effects could be blocked by protease inhibitors. This result suggests that an interleukin-1β converting enzyme (ICE)-like protein plays a role in activating apoptosis.

  5. Making mast cells

    Understanding the origins and development of mast cells, which play a key role in allergic and inflammatory reactions, is of relevance to both basic and clinical science. Rodewald et al. (p. 818) report the identification of a committed mast cell precursor. It can be distinguished both morphologically, by the presence of cytoplasmic granules, and by its cell surface phenotype. It appears early in ontogeny, being found at day 15.5 of gestation in mice in the fetal blood; thus, commitment to the mast cell lineage can precede tissue emigration.

  6. Insulin secretion

    Diabetics are frequently treated with a group of drugs known as sulfonylureas. These compounds stimulate secretion of insulin from pancreaticβ cells by inhibiting certain potassium channels. Eliasson et al. (p. 813) now show that in addition to the known indirect effects on plasma membrane channels, these drugs directly interfere with the cell's secretory machinery.

  7. Cold comfort

    To adapt to the cold, poikilothermic animals restore fluidity to cold-rigidified membranes by increasing the unsaturation of membrane phospholipids. Tiku et al. (p. 815) have cloned the enzyme, δ9-desaturase from carp that incorporates the first degree of unsaturation into saturated fatty acids. They also examined the enzyme's regulation in carp during the exposure to cold. Cold induced an increase in δ9-desaturase activity that was the result of increased transcription and the activation of latent desaturase.

  8. B cell activation

    Human X-linked agammaglobulinemia is caused by a defective signaling pathway involving Bruton's tyrosine kinase (BTK) in B cells that results in a drastic loss ofγ -globulin and antibody production. Several studies have suggested that BTK interacts with SRC family kinases. Rawlings et al. (p. 822) now show that SRC kinases transphosphorylate BTK at one site and that autophosphorylation of BTK at a second site results in activation. These same sites are also phosphorylated when B cells are stimulated by immunoglobulin M. Certain mutations in BTK identified in patients map to the activation loop of BTK.

  9. Following the flow

    The hydrologic cycle of evaporation and precipitation on land provides nearly all of the fresh water used to sustain humanity and other life. Postel et al. (p. 785) provide an accounting of the use of this renewable fresh water by humans, including for agriculture, navigation, drinking, and other necessary activities. They conclude that humans appropriate 24 percent of evapotranspiration globally, and 54 percent of the runoff that is generally accessible (for example, flood waters are excluded).

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