This Week in Science

Science  27 Feb 2004:
Vol. 303, Issue 5662, pp. 1253
  1. Increasing Epitope Densities


    Proteins such as laminin present specific peptide epitopes that can enhance cellular proliferation and differentiation. Silva et al. (p. 1352) have synthesized a molecule that self-assembles in dilute aqueous solution to form nanofibers that present the neurite-promoting epitope of laminin at much higher densities (about a 1000-fold increase) than does the native protein. This high density promotes in vitro differentiation of murine neural progenitor cells into neurites in 1 day while suppressing the formation of astrocytes. This nanofiber architecture also prevents the passive diffusion of the epitope away from the site of injection in tissues.

  2. Clouds and Smoke in Amazonia

    Biomass burning, which mainly occurs in the tropics and subtropics, creates aerosol particles that can affect cloud properties and climate (see the Perspective by Graf). Andreae et al. (p. 1337) present in situ measurements that reveal how aerosols produced by biomass burning in the Amazon affect the water cycle there. High levels of aerosols suppress low-altitude precipitation and washout so that the smoke particles ascend through the clouds and into the free troposphere. This process gives the clouds a “smoking” appearance. These smoke aerosols and their effect on cloudiness can alter the radiation budget, facilitate long-range transport of the smoke, and cause stronger updrafts and intensified thunderstorms. The implied redistribution of heat in clouds due to these effects could alter regional atmospheric circulation. Further insight into the complexity of the radiative effects of smoke from biomass burning is provided by Koren et al. (p. 1342). They analyze satellite data on changes in the abundance of trade-cumulus clouds in the Amazon, the most common variety in that region, during burning season. They find that trade-cumulus coverage is reduced from an average of 38% in clean-air conditions to zero in heavy smoke, and that the resulting reduction in cloud cover can overwhelm the scattering of incoming solar radiation by smoke from biomass burning and actually reverse the sign of the local radiative forcing. This may help explain why aerosol forcing did not counteract the effect of accumulating atmospheric greenhouse gases, and why there was an increase in global average surface temperature, over the last century.

  3. Organizing Scattered Starlight


    Young stellar systems, such as T Tauri stars (about 1 solar mass) and their bigger brothers, Herbig Ae/Be stars (about 10 solar masses) are enshrouded in gas and dust that make it difficult to see how they formed and evolved. Perrin et al. (p. 1345) used laser guide star adaptive optics to image the structure of the dust and gas around Hebig Ae/Be stars LkHα198, LkHα198-IR, and LkHα 233. They found evidence for a biconical nebula, jets, and a thick circumstellar disk consistent with disk models, in which the infalling gas envelopes are rotationally flattened by the conservation of angular momentum. Thus, the more massive Herbig Ae/Be stars probably form like the lightweight T Tauri (and our Sun).

  4. Life or Death Transcription Factors

    Tight control of the levels of the transcription factor and proto-oncogene c-Jun determine in part whether the outcome for the cells is survival or programmed cell death or apoptosis. Two independent groups have identified c-Jun-specific ubiquitin ligases that control the turnover of c-Jun in mammalian cells. Nateri et al. (p. 1374) characterized c-Jun degradation stimulated by the ubiquitin ligase complex SCFFbw7. The activity of the SCFFbw7 pathway in promoting the degradation of c-Jun allows neurons to survive conditions that would otherwise lead to cell death. Wertz et al. (p. 1371) report that the DCXhDET1-hCOP1 ubiquitin ligase complex can also promote c-Jun degradation and thereby protect mammalian cells from apoptosis. Because c-Jun controls a large number of cellular processes, it is not surprising that more than one ubiquitin ligase controls c-Jun levels.

  5. Minimal Transcription Initiation Machinery

    In the initiation of transcription, the promoter is “melted” so that the RNA polymerase holoenzyme (RNAP) can access the template strand. The mechanism of promoter melting is unknown; however, models invoking DNA torquing, DNA bending, and/or base flipping have been proposed. Young et al. (p. 1382; see the Perspective by deHaseth and Nilsen) now show that only a small portion—less than one-fifth the size of the core Esherichia coli RNA polymerase molecule—along with the bacterial initiation factor σ, is capable of promoter melting. DNA binding, as well as capture of the melted state, was essential for the melting process, and base flipping appeared to be largely responsible for initial promoter melting. Similar processes occur at both prokaryotic and eukaryotic promoters, and during DNA replication and repair.

  6. Nanobelts Come Full Circle

    Oxide semiconductors such as zinc oxide (ZnO) can be synthesized as thin belts (10 to 30 nanometers thick and about 100 nanometers in width). Recent work has shown that the long-range electrostatic interactions set up by the polar faces of these crystals can induce coiling into a structure resembling a spring. Kong et al. (p. 1348; see the Perspective by Korgel) have now identified synthesis conditions in which the ZnO nanosprings coil tightly enough so that as belts grow, the edge of the newly formed belt fuses with the existing coil to create an extended nanoring. Transmission electron microscopy was used to identify this growth mechanism, which can occur along two different crystallographic directions.

  7. How Nitrogen Fixation Depends on Calcium

    Leguminous plants form symbiotic relationships with rhizobial nitrogen-fixing bacteria and mycorrhizal fungi, which set up residence in nodules in the roots of host plants. Nodulation results from a signal transduction pathway induced in the roots of legumes by rhizobial signals called Nod factors. Two reports by Ané et al. (p. 1364) and Lévy et al. (p. 1361) define genes from the leguminous plant Medicago truncatula involved in the earliest steps of nodulation. One of the genes likely encodes a calcium- and calmodulin-dependent protein kinase, and the other a ligand-gated cation channel. Thus, calcium signaling appears to be critical for the establishment of a successful plant-microbe symbiosis.

  8. Distant Organic Dust

    Organic material may form in cold molecular clouds in interstellar space and develop distinctive hydrogen, carbon, and nitrogen isotopic fractionations. Isotopic anomalies in H and N have been seen in interplanetary dust particles, but corresponding C isotopic anomalies have remained elusive. Floss et al. (p. 1355) have now measured 15N enrichment correlated with 13C depletion in an anhydrous interplanetary dust particle. These results suggest that these organic materials originated from a cloud in interstellar space.

  9. Meiosis Mutant Library

    To explore the mechanisms that distinguish meiosis (and the generation of haploid gametes) from the process of normal mitotic cell division, Marston et al. (p. 1367) conducted a large-scale screen for mutants of budding yeast that showed defects in chromosome segregation. Three of the genes identified in the screen were important in holding together sister chromatids at the centromere, thereby preventing inappropriate chromatid separation during meiosis I. Analysis of the other mutants from the screen will provide further insight into regulation of the meiotic cell division cycle.

  10. Let Me Out of Here


    Legionnaires' disease is a threatening respiratory infection associated with poorly maintained water and air conditioning systems. It can persist in such inhospitable environments because its natural hosts are aquatic amoebae, and humans are infected when they come into contact with contaminated aerosols. To disperse and infect human cells, the bacterium Legionella pneumophila has to escape the host amoeba. Chen et al. (p. 1358) searched for so-called effector genes in the L. pneumophila genome whose products might be involved in the infection and dissemination pathway. Two putative effectors, lepA and lepB, in the bacterium resembled mammalian SNARE proteins important in membrane trafficking. A specialized secretory system transfers the Lep proteins out of the intracellular vacuole in which the bacteria reside and into the host cell cytoplasm. These effectors then appear to commandeer a protozoan exocytic membrane trafficking pathway to release the bacteria.

  11. Specifying Pancreas of Liver

    To begin to understand how tissue specificity is established and maintained, Odom et al. (p. 1378; see the Perspective by Kulkarni and Kahn) have looked at the genes regulated by the transcription factors HNF1α, HNF4α, and HNF6 in human hepatocytes and pancreatic islet cells. Microarray and immunoprecipitation analyses were used to determine that HNF4α binds to almost half of the actively transcribed genes. HNF6 was involved in regulatory motifs that have been associated with sensitivity to sustained inputs. Lack of these HNF transcriptional regulators can cause a type of diabetes, and thus an understanding of these circuits may help future treatment of this disease.

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