This Week in Science

Science  05 Jan 2001:
Vol. 291, Issue 5501, pp. 9
  1. Rapid Planet Formation

    To understand star and planet formation, the locations, spatial distribution, and properties of low-mass objects that could be proto-stars or young stars need to be determined. Briceño et al. (p. 93; see Perspective by Kastner) have begun a large-scale photometric and spectroscopic survey of the Orion star-forming region. Their survey indicates that there are 168 new pre-main sequence stars, smaller than one solar mass, in the Orion OB1b and OB1a molecular clouds with an age range of 1 to 10 million years. Many stars are found in regions without disks or gas, which suggests that any planet formation around these stars is complete and thus that these planets formed within a few million years.

  2. A Silver Setting for Fluorescence

    Silver clusters with unusual fluorescence dynamics have been made on demand by a photochemical route. Peyser et al. (p. 103) photoreduced silver oxide nanoparticles with light to create small fluorescent silver clusters stabilized within an oxide matrix. Excitation of these small silver particles with green light led to strong red fluorescence. Excitation with blue light led to unusual blinking dynamics with large spectral shifts—the same particle could switch between emitting green, red, or yellow light over time, with silver oxide likely transferring energy to the silver clusters. These results offer possibilities for writing data with blue light (creating clusters) and reading it with green light (as red fluorescence).

  3. Electrochemical Separation of Olefins

    Olefins such as ethylene and propylene are basic feedstocks for the chemical industry, and most of their cost comes not from their synthesis but from expensive cryogenic separation. Electrochemical approaches for separation, in which an olefin could be bound selectively to a carrier and later released, have been explored but have suffered from poisoning by feedstream impurities or from irreversible reactions. Wang and Stiefel (p. 106; see the Perspective by Crabtree) show that certain nickel dithiolene complexes form olefin complexes. Binding appears to occur at sulfur instead of at the metal center, and thus aliphatic olefins could bind even in the presence of typical contaminants such as H2S, H2O, H2, and CO.

  4. Do Nature Reserves Work?

    Globally, nature reserves are set aside for the protection of biodiversity, but despite much discussion, there are few data to indicate the effectiveness of these reserves in slowing the pace of habitat destruction. Bruner et al. (p. 125) paint a guardedly optimistic picture of the position in tropical regions. Their survey of 93 parks in 23 countries shows that a degree of protection is achieved even if funding is scarce, but that there is also a direct correlation between the resources devoted to a park and its effectiveness.

  5. All Together Now

    Most examples of quantum tunneling through barriers in molecules involve protons. Tunneling events can be synchronized in highly symmetric molecules, but environmental effects often break the symmetry and make such coherent tunneling difficult to observe. Horsewill et al. (p. 100) make a case for coherent proton tunneling in a network of four hydrogen bonds between the OH groups in calix[4]arene. They observed a peak at 35 megahertz in the proton spin-lattice relaxation rate in solid-state samples below 80 K, which they attribute to tunneling of excited state levels.

  6. Topological Junctions in Nanotubes

    Theoretical studies of carbon nanotubes have indicated that the chirality, and hence the electronic properties, can change along the length of the nanotube. Far from being unwanted defects, modeling data suggest that these intramolecular junctions may function as molecular-sized metal-metal and metal-semiconductor building blocks for applications in molecular electronics. Ouyang et al. (p. 97) use scanning tunneling microscopy to verify the existence of such intramolecular junctions.

  7. Southern Warmth

    Ice core records of surface air temperatures in Greenland and Antarctica show that both regions experienced many of the same oscillations between warmer and colder climates during the last glacial period. It has, however, become apparent that these events were not synchronous. Blunier and Brook (p. 109; see the Perspective by Shackleton) present a relative chronology for these regions, extending back to 90,000 years before the present, using the atmospheric concentration of methane as a correlative tool. During this period, seven millennial-scale interhemispheric warming events occurred. In each case, Antarctic warming preceded warming in Greenland.

  8. Location Does Matter

    Action potentials and excitatory post-synaptic potentials (EPSPs) interact during normal nervous system function. It has been assumed that action potentials always reset the membrane potential during an EPSP. However, the original data came from spinal cord motor neurons and may not apply to neurons with extensive dendritic trees. Häusser et al. (p. 138) have analyzed EPSP shunting produced by antidromic action potentials and found that synaptic events with a brief underlying conductance were shunted more effectively than longer-lasting responses. Synaptic potentials initiated in dendrites were less affected than somatic ones. Thus, the interaction between action potentials and EPSPs depends on the EPSP type and location.

  9. Grammar Acquisition

    In evolutionary history, the emergence of language ranks as one of the most important breakthroughs. Linguistic research has suggested that humans possess a “universal grammar”—an innate set of mechanisms that is required for language learning. Nowak et al. (p. 114) formulate a mathematical theory for the population dynamics of grammar acquisition, showing how natural selection can determine the length of the learning period and limit the size of the searching space within which learning procedures evaluate sample sentences.

  10. An RNA Snowplow

    Processing of messenger RNA (mRNA) requires intervening sequences to be excised in a process mediated by the mRNA splicing machinery. An early step in assembling the spliceosome is forming a helical, double-stranded RNA-protein complex, which provides for sequence recognition, and then rearranging this structure. RNA helicases have been shown by genetic means to be involved in this rearrangement. Jankowsky et al. (p. 121) describe the biochemical role of NPH-II, an RNA helicase, in dislodging the protein (U1A) from the double-stranded RNA. The helicase appears to act as a snowplow either before or in concert with unwinding of the helix.

  11. Breaking and Entering

    Malaria parasites have complex life cycles. After inoculation of the parasite when a mosquito feeds on humans, the sporozoite stages motor around the body seeking out the liver. Mota et al. (p. 141; see the news story by Enserink) show that during this stage they are unfussy about which cells they invade and do so simply by breaking through the plasma membrane and then exiting the same way. However, the parasite will stop to replicate if it has entered by a pathway in which it becomes enfolded in host membranes, forming a parasitophorous vacuole. Perhaps the migratory phase allows the parasite to accumulate host factors on its surface, which in turn trigger signalling for a subtle form of host-cell entry that allows the parasite to reside under cover.

  12. Fighting Mildew

    Flor's gene-for-gene hypothesis has been seminal in describing plant responses: if the plant does have a resistance (R) gene for a specific pathogen, then that pathogen is arrested by induced local necrosis of host cells. But what farmers really want is broad-spectrum resistance to a range of plant pests. Xiao et al. (p. 118) have identified a new class of resistance genes in the model plant Arabidopsis, called the RPW8 locus, which confers broad-spectrum resistance to a range of important agronomic fungal pathogens called the powdery mildews.

  13. A Migration Signal

    During normal animal development, cells often migrate from one area to another. Duchek and Rørth (p. 131) have examined a two-step cell migration event that occurs during oogenesis in Drosophila: a cluster of somatic follicle cells migrates posteriorly between the nurse cells toward the oocyte and then proceeds dorsally toward the germinal vesicle. The epidermal growth factor signaling pathway is involved, with the ligand Gurken specifically directing the dorsal migration.

  14. Hair and Hair-after

    Chemotherapy-induced alopecia (hair loss) is a frequent and emotionally distressing side effect of cancer treatment. The epithelial cells in the hair follicle are especially susceptible to cytotoxic drugs because they divide so rapidly. Davis et al. (p. 134; see the news story by Marx) used structure-based methods to design small-molecule inhibitors of cyclin-dependent kinase 2, a protein that promotes cell cycle progression. Topical application of these compounds to neonatal rats prior to administration of chemotherapy significantly reduced hair loss in the animals.

  15. Dome Concordia

    The exchange of carbon between the ocean, the atmosphere, and the terrestrial biosphere is highly dynamic. Thus, the concentration of atmospheric CO2 during transitions from glacial to warm periods must be detailed before the role of the carbon cycle in climate change can be understood. Monnin et al. (p. 112) have constructed a record for the last deglaciation by analyzing the composition of air bubbles trapped in ice at the Dome Concordia, Antarctica. Changes in CO2 levels occurred in tandem with changes in atmospheric methane and were closely related to variations of local temperature.

  16. Quick Fixes

    One topic of study in population genetics is the genetic events that have occurred to produce the divergent genomes of the Drosophila siblings D. melanogaster and D. simulans. Previous work indicated that a new D. melanogaster gene, Sdic, arose as a fusion of two adjacent genes on the X chromosome. Nurminsky et al. (p. 128) demonstrate that the reduction of polymorphism in Sdic results from the process of selective sweep in which favorable mutations are incorporated so rapidly that linked alleles can ‘hitchhike’ and also become fixed.