This Week in Science

Science  06 May 2005:
Vol. 308, Issue 5723, pp. 753
  1. Splicing Dicer


    Small noncoding microRNAs (miRNAs) are potential regulators of gene function and have been shown to affect specific developmental processes in invertebrates. Null alleles of Dicer, a key enzyme in the production of miRNAs, are embryonic lethal in fish and mice. Giraldez et al. (p. 833, published online 17 March 2005) eliminated mature miRNAs in zebrafish by removing maternal and zygotic Dicer. These embryos show no overt abnormalities in patterning and cell fate specification but display severe defects in morphogenesis, particularly of the brain. Injection of a family of developmentally regulated miRNAs rescued brain morphogenesis.

  2. Excitons Prevail

    When a material is confined to one dimension, its electronic band structure can exhibit features termed van Hove singularities, which have been invoked to explain the sharp absorption spectra in materials such as single-walled carbon nanotubes (SWNTs). This model predicts a sealike photoexcited state of free electrons and holes. Recently, however, support has emerged for an exciton picture, in which light absorption creates excited electrons that remain strongly correlated with the positive holes left behind. Wang et al. (p. 838) present firm evidence for the exciton model in isolated SWNTs. Their experiment takes advantage of the selection rules that exciton creation imposes on one- versus two-photon absorption. The two-photon spectra are consistent with exciton-binding strengths near 0.5 electron volt, which are much higher than in bulk semiconductors.

  3. Implications of Sunny Days


    Many studies have reported direct or indirect evidence of a significant decrease in insolation (S), the amount of solar radiation reaching Earth's surface, during most of the past 40 years. How much S has varied, and why it may have changed, is poorly understood. Pinker et al. (p. 850; see the Perspective by Charlson et al. and the related Brevia by Wielicki et al.) analyzed satellite records of S for the period from 1983 to 2001 and concluded that while there was a decrease in the earlier part of the record, the negative trend reversed around 1990 and was followed by an even larger increase. The recent upward trend is corroborated by Wild et al. (p. 847), who examined a large set of surface-based measurements of S starting in 1990. This dimming and subsequent brightening could have resulted from changes in cloud coverage, the abundance of atmospheric aerosols, or atmospheric transparency after explosive volcanic eruptions. Changes in insolation appear in numerous paleorecords from both high and low latitudes, but not all parts of the world responded concurrently. The differences in the nature and timing of their responses are thought to be important clues to the mechanisms that cause that asynchrony. Wang et al. (p. 854; see the news story by Kerr) present a precisely dated record of oxygen isotope variations in a stalagmite from Dongge Cave, China, which they interpret as a proxy for Asian Monsoon intensity. Their data, which extend back 9000 years to near the beginning of the Holocene, reveal important correlations between the strength of the monsoon and changes in solar output. They also discuss how the Dongge Cave record is related to climate records from Greenland, and implications for the mechanisms that have controlled the Asian Monsoon.

  4. A Reductionist Approach in Gene Screening

    Cellular signaling pathways, such as Wnt in vertebrates or Wingless in flies, have traditionally been pieced together one step at a time. Technical advances now allow a more thorough probing of the genes whose products contribute to such a regulatory system. DasGupta et al. (p. 826, published online 7 April 2005; see the Perspective by Fearon and Cadigan and connection maps of the signaling pathways at Science's STKE linked to the online paper) designed a high-throughput screen in Drosophila cells that evaluated effects on Wingless signaling when expression of nearly every gene (about 22,000 of them) was decreased, one by one, by RNA interference. The 238 genes identified included about 15 known components of the signaling pathway. The remaining group comprised approximately equal numbers of genes with known functions not previously associated with Wingless signaling. Half of the implicated genes appear to have orthologs in humans, and a substantial proportion of these human genes show mutations linked to disease.

  5. Not Going with the Flow

    Tiny zooplankton reside in the ocean at constant depth, despite the movement of currents. Genin et al. (p. 860) show that these organisms maintain their position by swimming against upwelling or downwelling currents at speeds of up to 10 body lengths per second. High-frequency, multibeam sonar was used to track more than 300,000 individual zooplankters. Combining these field measurements with a simulation model, the authors show that this behavior creates the dense zooplankton accumulations that become feeding grounds.

  6. Maintaining Magma Temperatures

    Earth's oldest rocks date to only about 4 billion years ago, but a few of these contain recycled zircons. These minerals formed in even earlier magmas, dating back to 4.4 billion years ago, or nearly the age of the Earth, and provide clues about Earth's earliest environment. Watson and Harrison (p. 841) have developed a means to probe the temperature of magmas from the titanium content of zircons and calibrated this thermometer using both laboratory data and study of magmas with known or independently calibrated temperatures. They find that magmas that are more water-rich tend to be cooler. Application of this finding to these ancient zircons implies that they were formed from magmas that were similar in temperature and water content to those today. Thus, the nascent Earth may have been generating granitic magmas that were no hotter than those today.

  7. Some Like It Hot

    The efficiency of enzymes makes them attractive catalysts in industrial reactions. However, in many industrial applications the enzymes must operate at elevated temperatures, and designing active thermostable enzymes that maintain dynamic motions important for function is a challenge. Korkegian et al. (p. 857) have used a computational approach to identify three mutations that significantly stabilize the enzyme cytosine deaminase (CD) without reducing its catalytic efficiency. CD is a demanding model system because it forms an active dimer and displays complex folding behavior. Bacteria expressing the redesigned enzyme showed increased, temperature-dependent growth under conditions where an active enzyme would be required.

  8. Reforming Fuel Cells

    The hydrogen for fuel cells, at least in the near term, will come ultimately from the hydrogen available in hydrocarbon sources through a process called reforming. This process requires heat, so if this step can be completed “on board” a vehicle, it can take advantage of the heat provided by the fuel cell reaction to increase efficiency. However, the solid-oxide membrane fuel cells that can process hydrocarbons in this way have nickel anodes that tend to be deactivated by “coking,” the depositing of unreacted carbon. Zhan and Barnett (p. 844, published online 31 March 2005) describe the preparation and operation of solid oxide fuel cells with a reformer layer (CeO2/RuO2) placed over the anode to produce CO and H2 before the iso-octane fuel can reach the anode. They achieve power densities of 0.3 to 0.6 watt per square centimeter.

  9. Swapping Partners for Perfect Pairing

    Meiosis is the special “double” cell division in eukaryotes that results in the formation of haploid (germ) cells from diploid parent cells. Homologous chromosomes must pair during the first division so that they can be segregated equally between the two daughter cells. Tsubouchi and Roeder (p. 870) now show that, against expectations, initially nonhomologous pairs of chromosomes form during meiosis. Nonhomologous pairs are then resolved into homologous pairs as meiosis progresses, ensuring the correct segregation of chromosomes.

  10. Spatial Memory Maps


    Attractor networks have been the major hypothesis for the neural mechanism of memory. When rats explore two similar environments, neurons called place cells learn to distinguish between them (a process known as “remapping”). Wills et al. (p. 873; see the Perspective by Poucet and Save) provide evidence for coherent and complete transitions from one (attractor) state to another under conditions when sensory inputs change in a steady, incremental manner. Animals first explored two environments that differed in color, texture, and odor, as well as shape and, after the cells had remapped, were transferred to environments which varied along a single dimension (shape). The place cell representations of intermediate-shaped environments evolved into the (attractor) representations of either one or other initial shape: All simultaneously recorded cells coherently changed their firing pattern as a function of the intermediate shape. This direct evidence for the existence of attractor dynamics helps to provide a model for the representation of distinct contexts in context-dependent memory.

  11. Environmental Sensing

    Candida species are important human pathogens that bind to host epithelial cells via specific adhesive molecules, encoded by the yeast EPA genes. Most of the EPA genes are encoded in subtelomeric regions and suppressed by Sir3-mediated chromatin silencing, a mechanism that relies on the availability of the coenzyme nicotinamide adenine dinucleotide (NAD+). Domergue et al. (p. 866, published online 17 March 2005) discovered that EPA6 silencing was specifically lifted in urinary tract infections but not in blood stream infections. Strikingly, unlike other yeasts, Candida glabrata cannot manufacture NAD+ from tryptophan, and the pathogen is reliant on the host to supply the precursor, nicotinic acid. The pathogen exploits this limitation to sense its environment: In the urinary tract little nicotinic acid is available, so the adhesion gene suppression is lifted, and the cells can specifically attach to uroepithelial cells.

  12. The Target, the Transmitter, and the Synapse

    Despite its profound importance, neurotransmitter release probability has never been directly determined at central synapses, and it is not known what factors are involved in its regulation. Koester and Johnston (p. 863, published online 17 March 2005; see the Perspective by Thompson) show that release probability deduced from postsynaptic calcium transients is similar between synapses onto the same cell, but differs between those onto different cells. This normalization of release probabilities in specific synaptic connection indicates that additional release sites in neocortical synaptic connections not only reduce synaptic variability but also provide every targeted neuronal compartment with information at the same rate.