This Week in Science

Science  16 Jul 2004:
Vol. 305, Issue 5682, pp. 305
  1. Pulsar Hot Spot

    CREDIT: NASA/COMPTON GAMMA RAY OBSERVATORY

    X-ray Multimirror Mission-Newton observations of the thermal emission gathered from the surface of the Geminga pulsar as it rotates shows evidence of a hot spot at one pole. Caraveo et al. (p. 376) suggest this hot spot is sustained by the heating of accelerated magnetospheric particles. The hot spot may help explain how the x-ray emissions are linked to the gamma-ray emissions from pulsars.

  2. Where CO2 Goes

    Based on the amount of fossil fuel that has been burned in the industrial era, the increase in atmospheric CO2 concentration is only half of the expected value. The fate of the rest of the CO2—in the ocean and the terrestrial biosphere—is the focus of two Research Articles (see the Perspective by Takahashi). Feely et al. (p. 362; see the cover) use a new data set of total alkalinity and chlorofluorocarbon measurements to estimate CaCO3 dissolution rates and to examine the long-term impacts on the carbon speciation in the global ocean. They also discuss the effects of these changes on calcium-carbonate-secreting organisms, including corals, foraminifera, pteropods, and coccolithophorids. Sabine et al. (p. 367) use marine inorganic carbon measurements from two global ocean surveys conducted in the 1990s to estimate the size of the oceanic sink for the period from 1800 to 1994. They find that the oceans have absorbed 48% of the CO2 emissions from fossil fuel burning and cement manufacturing during that period. Because the atmospheric CO2 levels are equivalent to about two- thirds of the anthropogenic CO2 produced, the terrestrial biosphere must have been a net source during that interval.

  3. Gleevec: The Sequel

    The tyrosine kinase inhibitor imatinib (Gleevec) is now a frontline treatment for chronic myelogenous leukemia (CML). As with most cancer therapies, however, a subset of patients develop resistance to imatinib, a problem that has been traced to the acquisition of specific mutations in the oncogenic BCR-ABL kinase targeted by the drug. Guided by these mutational data as well as crystallographic data detailing how imatinib binds to the ABL kinase domain, Shah et al. (p. 399) have isolated a new tyrosine kinase inhibitor that appears to have all of the favorable characteristics of imatinib—and more. Preclinical studies of this second-generation drug (an orally available thiazolecarboxamide) in a mouse model and in cultured bone marrow cells from CML patients indicate that it is more potent than imatinib and, importantly, retains activity against the majority of imatinib-resistant BCR-ABL mutants without significant toxicity.

  4. Keeping a Stiff Lower Tip

    Unlike the scanning tunneling microscope, which probes states near the Fermi level of surfaces, the atomic force microscope (AFM) can in principle be sensitive to deeper lying states that can have strong directional character. However, achieving the needed resolution with an AFM depends critically on the nature of the tip. Hembacher et al. (p. 380, published online 10 June 2004; see the Perspective by de Lozanne) show that the needed resolution can be achieved by turning the experiment “upside down”—a light atom (carbon in a graphite sheet) is used as the “tip,” and the tungsten tip is used as the sample. By measuring higher-order force derivatives under cryogenic conditions, they can image the covalent bonds in the tungsten metal.

  5. Size Matters

    Developmental size control is very poorly understood, especially in vertebrates. Why do different body parts attain particular relative and absolute sizes, and then stop growing? In the case of limbs, continued growth and patterning of the embryonic bud depends upon the maintenance of a positive feedback loop between Sonic hedgehog and fibroblast growth factors. Scherz et al. (p. 396) now describe the timing mechanism that terminates this feedback loop and hence determines the length of the digits and the number of phalanges they contain. Over time, an expansion occurs in the number of cells in the posterior limb bud that cannot support the intermediate steps of the feedback loop downstream of Sonic hedgehog. When the size of this nonresponsive zone grows broader than the range of Sonic hedgehog diffusion, the loop is broken.

  6. Mantle Convection Inhibitors

    Magnesium- and iron-rich silicate perovskite is the dominant phase in the lower mantle. Determining the properties of this mineral at the high pressure and temperature conditions of the lower mantle will provide information about mantle rheology. Badro et al. (p. 383) conducted high-pressure experiments on the spin state of iron in perovskite and found a transition from the high- to low-spin state at the base of the lower mantle where there is seismic evidence of a distinct layer (called D″). The presence of the low-spin state would alter thermal conductivity, such that convection would be more sluggish and the D″ layer more stable.

  7. Molecular Evolution of Influenza

    CREDIT: SMITH ET AL.

    The inclusion of antigenic data in the analysis of virus evolution is crucial because virus evolution is generally thought to be driven by antigenic properties. Smith et al. (p. 371, published online 24 June 2004) examined 35 years of antigenic evolution of influenza A virus, subtype H3N2, from the 1968 pandemic until 2003. By comparing and contrasting the antigenic evolution of influenza virus with its genetic evolution, they show that antigenic evolution is clustered, whereas genetic evolution is continuous. The authors are also able to calculate the antigenic effect of amino acid substitutions in the hemagglutinin gene of influenza. This method can be applied to the development of diagnostics, surveillance policies, and vaccines for a variety of antigenically variable pathogens.

  8. Green Ocean Turns to Red

    The eukaryotic phytoplankton are critically important to the global carbon cycle and marine ecosystem structure. Falkowski et al. (p. 354) review the past billion or so years of their evolutionary history, with a particular focus on the remarkable radiation of dinoflagellates, diatoms, and coccolithophores during the Mesozoic. The authors argue that this radiation was triggered by the increasing oxygenation of the planet. Thus, through geological time, an increasingly oxidized ocean would eventually shift from “green” phytoplankton, which prefer reducing conditions, to “red” ones that thrive in an oxidizing environment.

  9. Staying Cool in the Hive

    Honey bees don't have a manager who allocates work, yet each bee senses what tasks it needs to perform. In such a situation, what is the good of genetic diversity? Jones et al. (p. 402, published online 24 June 2004) found that the nest of a genetically homogeneous set of bees showed erratic temperature shifts, whereas the genetically diverse set keep the nest at a more constant temperature. Genetically heterogeneous workers start fanning at different temperatures, which allows the colony to better modulate its response to changes in temperature.

  10. Selective Hearing in One's Mate

    CREDIT: SISNEROS ET AL.

    Like many other vertebrates, the mating season for the Pacific midshipman fish brings changes in behaviors. Males migrate to their nesting territory and begin to hum, producing a sound that their female counterparts find irresistible (although human ears have likened the sound to that of a motorboat). Sisneros et al. (p. 404; see the Perspective by Zakon) now show that the acoustic sensitivity of the inner ear in females is actually adjusted to better match the frequencies put out by the mating hum in response to seasonal changes in circulating steroid hormones. Thus, the effectiveness of the male's siren song is enhanced by seasonally tuned hearing in females.

  11. Watch One, Do One, Teach One

    Imitation is a simple form of social learning, and behavioral and neurophysiological studies have documented the capability of reproducing observed movements in animals. Can a monkey learn not only how to make a sequence of movements but also the underlying rule for doing so? Subiaul et al. (p. 407) argue that, after watching a trained counterpart perform the same task on a neighboring computer screen, two monkeys can learn to specify the uniquely rewarded order of four arbitrary items by pointing to photographs on a touch-sensitive screen.

  12. Switching from Nonspecific to Specific DNA Binding

    Most DNA binding proteins first bind nonspecifically and then translocate along the DNA to their specific DNA binding site. How do the proteins switch between nonspecific and specific binding? To address this question, Kalodimos et al. (p. 386; see the Perspective by von Hippel) determined the solution structure and dynamics of a complex of a dimeric lac repressor DNA binding domain (DBD) with nonspecific DNA and compared it with the structure of the dimeric DBD bound to its operator. The same set of residues switch from nonspecific, electrostatic interactions with the DNA backbone to specific binding with base pairs of the target DNA sequence. In the nonspecific complex, the protein-DNA interface remains flexible, a feature that may allow it to probe potential binding sites.

  13. Eat Less, Live Longer

    Health problems in the elderly are thought to result from the cumulative effects of cell loss over time. Caloric restriction and genetic manipulations that extend life-span typically reduce stress-induced apoptotic cell death. In yeast, caloric restriction increases life-span by activating the Sir2 deacetylase. Cohen et al. (p. 390, published online 17 June 2004) now find that mammalian Sir2 (SIRT1), a regulator of cell defenses, is induced in calorically restricted rats and in human cells treated with serum from these animals. One way in which caloric restriction extends mammalian life-span may be by increasing SIRT1 expression and thus the survival of key cells.

  14. Easing the Break Up

    Cytokinesis, the act of separating two daughter cells after mitosis, is a complicated cascade of molecular events, including actomyosin ring assembly and contraction, vesicle fusion, and plasma membrane fission. Dobbelaere and Barral (p. 393) show that, in budding yeast, proteins known as septins form diffusion barriers on each side of the contractile ring. The septins maintain the localization of a variety of diffusible factors involved in vesicle fusion, actomyosin ring contraction, and membrane fission to the vicinity of the contractile apparatus. This activity of the septins is required for proper actomyosin ring function and completion of cell cleavage.

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