This Week in Science

Science  06 Dec 2002:
Vol. 298, Issue 5600, pp. 1843
  1. In Brevia

    An upper bound of 20% for the contribution of the dissolved organic carbon flux to support respiration in the dark ocean has been estimated by Aristegui et al. (p. 1967) based on a data set they assembled.

  2. How Our Planet Shapes Up

    Recently, Cox and Chao (2 August, p. 831) determined that Earth's dynamic oblateness had increased sharply in 1998. Now Dickey et al. (p. 1975) show that the increase is related to oceanic mass redistribution caused by the Pacific Decadal Oscillation and the El Niño-Southern Oscillation and an increase in subpolar glacier melting. Once these short-term climatic effects are excluded, the longer-term decrease in oblateness caused by postglacial rebound can be refined to estimate the viscosity of the lower mantle.

  3. Sonochemistry in Green Solvents

    Sonochemistry takes advantage of the high temperatures that can be generated in solution by the collapse of cavitation bubbles. Kuijpers et al. (p. 1969) show that radicals formed by cavitation can be used to induced polymerization reactions of methyl methacrylate in high-pressure liquid CO2. High conversions result because the polymer is poorly solvated under these conditions and precipitates, thus maintaining a low-viscosity solution.

  4. In the Original Olmec

    A written language emerged in Mesoamerica sometime in the first millenium B.C. It was used in southeastern Mexico by the Olmec, who were responsible for many of the first monuments in the New World and who developed large cities, and the language formed the basis for later Mayan writing. However, the time, location, and developers of Mesoamerican writing have been widely debated. Pohl et al. (p. 1984; see the news story by Stokstad) have uncovered a seal and plaque holding glyphic inscriptions from near La Venta, Mexico, an ancient Olmec center. Radiocarbon dates and associated pottery imply that these artifacts date 650 B.C. These finds imply that this region was the origin of Mesoamerican writing and the calendar system, rather than cultures in southwestern Mexico.

  5. Heavy Bombardment Formed Martian Valleys

    The surface of Mars shows evidence for at least 25 large impact events about 3.5 billion years ago and for the formation of many of the extensive river valley networks at that same time. Segura et al. (p. 1977; see the news story by Kerr) modeled the effects of impacts on valley network formation and found that one 500-kilometer- diameter bolide or a dozen 200-kilometer-diameter bolides could have created the 50-meter-thick global layer of water needed to form the valley networks. Impact events may have created warm and wet conditions on Mars, but only for a very short time after the heavy bombardment. Thus, habitability for life would have been limited to a brief period of comfortable quiescence.

  6. More Is Less

    It is commonly assumed that plants will produce more biomass if the atmospheric CO2 concentrations continue to rise, but CO2 is not the only agent that effects plant growth. Shaw et al. (p. 1987; see the Perspective by Morgan) describe the responses of a grassland ecosystem to a suite of realistic changes in CO2, temperature, precipitation, and nitrogen deposition, alone and in combination. The response of net primary production (NPP) to elevated CO2 varies strikingly, depending on the status of other global-change factors. As a single factor, elevated CO2 increases aboveground growth, but when elevated CO2 is combined with other global changes, elevated CO2 tends to decrease NPP.

  7. Refuge Networks

    Networks of marine reserves have been suggested as a potential way to conserve biodiversity at scales larger than single isolated reserves. However, often there is not enough information on biodiversity at medium and large scales (beyond data on species richness) to design appropriate networks. To address this problem, Sala et al. (p. 1991) have collected information on patterns of biodiversity, ecological processes, and connectivity in reef habitats in the Gulf of California that cover a distance of about 1000 kilometers. Using this information, as well as data on fishing pressure and socioeconomic factors, they developed an algorithm for the selection of reserves to produce a network of reserves that fulfills all conservation goals and permits coexistence with local fisheries.

  8. Easily Lost Electrons

    The extremely low ionization energy of cesium has long been exploited in photoelectric cells. Cotton et al. (p. 1971) now report on a molecular solid whose ionization energy beats out cesium by a few tenths of an electron volt. This compound contains a specially ligated tungsten-tungsten quadruple bond, and it is the electron in the weakest δ bond that is emitted at such low energies. Quite unlike cesium, the molecule is relatively stable at ambient conditions and may find use as a reducing agent.

    CREDIT: COTTON ET AL.
  9. Poly(A) Tails and Spermatogenesis

    A gene-regulation mechanism that operates during spermatogenesis has been identified by Kashiwabara et al. (p. 1999). Male mice that carry a targeted disruption of the gene for testis-specific, cytoplasmic poly(A) polymerase (TPAP) are infertile because of an arrest in spermiogenesis at the round spermatid stage. The reduced expression of a subset of haploid-specific genes demonstrates that TPAP is essential for spermiogenesis. In addition, TPAP is involved in poly(A) tail extension of particular transcription factor messenger RNAs (mRNAs) in round spermatids, and the elimination of TPAP affects nuclear transport of the TAF10 transcription factor. Hence, TPAP is critical for mRNA control during mammalian gametogenesis through posttranscriptional and posttranslational events.

  10. Stem Cell Tails

    Many recent experiments on stem cells have focused on whether their differentiative capabilities allow switching of their developmental lineages. Echeverri and Tanaka (p. 1993; see the Perspective by Stocum) now follow the fate of stem cells in the relatively normal context of a regenerating salamander tail. Using real-time, live observation of individually labeled cells, the authors find that cells labeled in the spinal cord contribute to muscle and cartilage of the regenerating tail, as well as to more neuronal cells. Although amphibians show a greater capacity to regenerate limb and tail than do mammals, these observations of amphibian cell plasticity may lead to insights into the factors that control mammalian cell plasticity.

    CREDIT: ECHEVERRI AND TANAKA
  11. Blocking Malarial Entry

    The widespread resistance of human malaria parasite Plasmodium falciparum to the few effective antimalarial drugs has led to a search for new therapeutic targets. Greenbaum et al. (p. 2002) used a chemical proteomics screen to identify a cysteine protease, falcipain-1, that is essential for the parasite merozoites to invade human red blood cells. By screening chemical libraries, the authors identified inhibitors that specifically blocked only falcipain-1 activity and parasite invasion of erythrocytes, but not other cysteine proteases or other stages in the parasite life cycle.

  12. Wnt Signaling Through Transducins

    Wnts are signaling molecules with critical roles in development that act though a receptor called Frizzled. Although Frizzled proteins resemble heterotrimeric guanine nucleotide binding protein (G protein)-coupled receptors, Wnt signaling is best understood in cases where Wnts control transcription through a β-catenin-dependent signaling mechanism. Ahumada et al. (p. 2006) describe a different mechanism for Wnt signaling in which the rat Frizzled-2 protein appears to be coupled to the G protein transducin, better known for its central role in signaling in visual tissues that respond to light. In cultured cells, activation of transducin was required for Wnt-dependent activation of guanosine 3',5'-monophosphate (cGMP)-dependent phosphodiesterase, as well as subsequent decreases in the intracellular concentration of cGMP and increased release of calcium from intracellular stores. Wnt signaling during zebrafish gastrulation could be inhibited when phosphodiesterase activity was reduced after application of pharmacological inhibitors.

  13. Wakeful Smells in the Morning

    It is hard to sleep late when your household is up and about at the crack of dawn. Such social influences are seen in other animals, but how are these signals communicated? Levine et al. (p. 2010) have examined mixed Drosophila populations of wild-type (rhythmic) and short-period (arrhythmic) fly populations and found that arrhythmic flies destabilize the rhythms of wild-type flies. The information seems to be communicated by chemosensory cues—flies that lack the ability to sense chemical signals ignored their neighbors' activity.

  14. Where Baby Talk Ends Up

    How do we learn to understand speech? Dehaene-Lambertz et al. (p. 2013) have begun to explore the cerebral origins of language acquisition in humans by adapting the technique of functional magnetic resonance imaging for use on 3-month-old infants. They find evidence for activity in brain regions known to subserve language processing in adults, predominantly on the left side, in response to speech.

  15. Diatom Decay

    Marine diatoms protect their silica (opaline) skeletons from dissolution by surrounding them with a protective organic matrix. When diatoms die, they sink, during which time their opaline and organic carbon parts may dissolve at different rates. Bidle et al. (p. 1980) report that the relative preservation efficiencies of Si and C in diatoms is a function of temperature, which controls the bacterial activity responsible for the breakdown of the organic matrix around the opal. They propose a mechanism that may help explain the regional and depth-dependent differences in Si/C ratios that occur between many oceanic systems, as well as the geographic pattern of opal accumulation on the sea floor. These results could help produce better reconstructions of oceanic paleoproductivity.

  16. From General to Specific

    In vitro studies and work done in yeast have suggested that the transcription DRAP1 functions as a “general” transcriptional regulator that represses transcription by preventing the interaction of TFIIB with TBP (the TATA box-binding protein of TFIIB). Iratni et al. (p. 1996) examined the function of DRAP1 during early mouse development and found that the mutant embryo exhibited gastrulation defects consistent with increased activity of Nodal, a secreted morphogen of the transforming growth factor-β family that is the primary inducer of mesoderm during gastrulation. Nodal signaling is inhibited in the early embryo by DRAP1, most likely through its interaction with FoxH1. Thus, a factor that was previously thought to be a general transcriptional regulator displays a specific role in embryonic patterning through the regulation of Nodal's positive feedback loop, providing a mechanism for regulation of morphogen signaling.

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