Editors' Choice

Science  01 Jul 2005:
Vol. 309, Issue 5731, pp. 21

    Plains on Fire

    Before elaborate programs of fire suppression were instituted in North America in the 20th century, wildfires occurred frequently and were critical contributors to the health and maintenance of many different ecosystems. Abundant evidence for a link between fires and climate exists for many forested regions, but less attention has been paid to nonboreal environments. The grasslands of the Northern Great Plains, which have replaced the extensive spruce forests that stood there at the start of the Holocene, are one such system.

    In order to establish how drought and fire might be related in this region, Brown et al. constructed a 4500-year-long record of charcoal, grass pollen, and soil carbonate at Kettle Lake in North Dakota. They find that charcoal production was highest during moist intervals, when grass cover (fuel) was plentiful, and that fires did not happen at regular intervals. Spectral analysis of the data showed that for much of the late Holocene, fires recurred in cycles with a period of around 160 years, but secular trends, including any evidence of the effects of anthropogenic warming, are more difficult to detect. — HJS

    Proc. Natl. Acad. Sci. U.S.A. 102, 8865 (2005).


    Nippy Inoculation

    For a vaccine to generate protective immunity at a level comparable to that produced by an infection, one or more secondary booster shots over an extended period of time may be required. Thus, finding ways by which the initially primed memory T cells might be more efficiently bolstered could help to increase vaccine efficacy.

    Badovinac et al.show that mice vaccinated with dendritic cells that have been coated with peptides derived from the bacterium Listeria monocytogenes could mobilize a memory CD8+ T cell response to a booster challenge considerably faster than mice given an attenuated bacteria vaccine. Furthermore, the dendritic cell-vaccinated mice also showed significantly greater resistance to infectious bacteria, consistent with an increased level of protective immunity. Vigorous memory responses were also generated to a range of other booster immunizations, including those from a noninfectious source, and were apparent even toward weak antigens. Vaccination with coated dendritic cells in this setting was at its most efficacious when inflammatory signals were minimal, which appeared to accelerate the rate at which CD8+ T cells acquired a memory phenotype during the priming phase. — SJS

    Nat. Med. 10.1038/nm1257 (2005).


    Pores for Strength

    It is now possible to make bulk metallic glasses (BMGs) from a wide range of alloy compositions and to fabricate parts where the minimum dimension is at least a few millimeters. In comparison to the corresponding crystalline alloys, BMGs are almost twice as strong. Their downfall is a lack of plastic strain, which leads to softening and abrupt failure associated with shear bands, even under compression. Although they can be rendered more ductile by including a dispersed crystalline phase, this reduces the yield strength.

    Wada et al.used four distinct hydrogenation treatments to create BMG rods with porosities between 0 and 4%. Compressive tests showed only a small decrease in the Young's modulus (about 10%), but with a strain at rupture as high as 18% and a significant increase in the rupture energy, which is the total energy under the stress-strain curve before failure. Structural analysis of the fractured samples showed that the pores acted as stress concentrators for the shear bands, causing an increase in the shear banding as the material deformed, thus increasing its toughness. — MSL

    Appl. Phys. Lett. 86, 251907 (2005).


    Safer Sodium

    Metallic sodium and potassium, as well as their alloys, are useful for their potency in chemical reduction reactions. However, their instability when exposed to oxygen is inconvenient, and their highly exothermic reaction with water is a severe fire hazard in the laboratory.

    Dye et al.have addressed this problem by mixing the metals with silica gel. Liquid Na-K alloy combines with silica at room temperature, producing a black powder that remains air-sensitive but is easily handled under nitrogen. Heating the powder to 150°C (or heating a pure Na/silica mixture to 165°C) yields a new product, stable for months, that retains much of its reducing capacity even on exposure to dry oxygen. This so-called stage I powder can be packed in columns and used for reductions or dehalogenative couplings of eluted aromatics. Further heating of the Na-stage I powder to 400°C gives a slightly less powerful stage II reducing agent that can be handled in humid air and used for drying organic solvents or for controlled reaction with water to generate small quantities of hydrogen. Preliminary experiments suggest that stage II formation involves chemical decomposition of the silica to produce Na4Si4 particles. — JSY

    J. Am. Chem. Soc. 10.1021/ja051786+ (2005).


    A Scuba Gel

    In the phylogenetically ancient eumetazoan jellyfish, between the endoderm and ectoderm lies the mesoglea, a noncellular gelatinous mix of macromolecules that provides hydrostatic support. Thuesen et al.have asked whether this material might contribute in some fashion to the ability of jellyfish to thrive in eutrophic environments. Using a fiber optic oxygen probe, they detected an oxygen gradient decreasing from the convex to the concave side of the mesoglea, consistent with oxygen consumption by the metabolically active subumbrellar musculature. Furthermore, the gel appeared to be able to store substantial quantities of oxygen, enough to allow the jellyfish to survive hypoxic conditions (30% air-saturated water) and to move about vertically and vigorously in a stratified tank—100% air-saturated at the surface and only 5% saturated at 60 cm depth. — GJC

    J. Exp. Biol. 208, 2475 (2005).


    Enumerating Choices

    Alternative splicing increases the complexity of the proteome of multicellular organisms by allowing for the generation of a large number of mRNA and protein isoforms from a relatively small number of genes. To identify alternative splicing events in the fruit fly Drosophila melanogaster, Blanchette et al.developed a microarray assay in which they examined the target pre-mRNAs of four splicing regulators. The largest number (319) of splicing events was affected by the regulator dASF/SF2, whereas the smallest number (43) was affected by PSI, suggesting that the former is a general splicing factor and the latter a more specialized one. Intermediate numbers of splicing events were affected by B52/SRp55 (107 events) and by hrp48 (90 events), and this fits well with the estimated range of 10,000 to 40,000 alternative splice junctions and roughly 200 splicing factors in Drosophila. In addition, cooperation was observed such that hrp48 partnered with PSI in alternative splicing events, and antagonistic regulation was also present, albeit rarely, between SR proteins (such as dASF/SF2) on the one hand and hrp48 and PSI on the other. — BAP

    Genes Dev. 19, 1306 (2005).

  7. STKE

    Suppressor Screens

    Two groups have used RNA interference (RNAi)-based screening of human cell lines to identify tumor suppressor genes. Westbrook et al.infected immortalized mammary epithelial cells with a retroviral RNAi library in which each RNA was tagged with a DNA barcode and looked for clones that showed anchorage-independent growth (indicative of malignant transformation). Array-based comparative genomic hybridization indicated that two genes associated with the formation of anchorage-independent colonies, TGFBR2 (which encodes the known tumor suppressor transforming growth factor-β receptor II) and REST (RE1-silencing transcription factor), were frequently deleted in colorectal cancers. Knockdown of REST promoted signaling through the PI3K (phosphoinositide 3-kinase) pathway, and expression of a dominant negative form of the PI3K regulatory subunit inhibited transformation, consistent with REST acting by suppressing PI3K signaling.

    Using a similar approach on immortalized fibroblasts (which can be transformed by activated RAS), Kolfschoten et al.identified the homeodomain pituitary transcription factor PITX1. Knockdown of PITX1 enhanced RAS signaling and produced a phenotype similar to that seen with overexpression of activated RAS. PITX1 expression was reduced in colon cancers that expressed wild-type RAS. The promoter of the GTPase-activating protein RASAL1 contained a PITX1 binding site; transfection with PITX1 enhanced RASAL1 mRNA abundance, whereas PITX1 knockdown reduced RASAL1 mRNA. Thus, PITX1 appears to function as a tumor suppressor that acts through RASAL1 to repress RAS signaling. — EMA

    Cell 121, 837; 849 (2005).

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