This Week in Science

Science  05 Jan 2007:
Vol. 315, Issue 5808, pp. 14
  1. Ancient Aerobics


    The emergence of animals in the Late Proterozoic (about 580 million years ago) may have been aided by the oxygenation of Earth's atmosphere and oceans, but little evidence for such an environmental change has been reported. Canfield et al. (p. 92, published online 7 December; see the 8 December news story by Kerr) examined the distribution of iron in rocks in Newfoundland that represent deep-ocean deposits from the Late Proterozoic. Their data imply that the deep ocean became oxygenated immediately after the last major Proterozoic glaciation. This change immediately preceded the appearance of the first animal fossils in these rocks.

  2. Sensitive Sidewalls

    The conductivity of the sidewalls of single-walled carbon nanotubes (SWNTs) changes when they are modified by defects or adsorbed molecules and, like graphite, the sidewalls of bulk samples can be modified by electrochemical oxidation. Goldsmith et al. (p. 77) have exploited this sensitivity by performing electrochemical oxidation and reduction on individual SWNTs mounted on electrodes so that their conductance G can be monitored. Oxidation in strong acids caused stepwise drops in G that the authors attribute to the formation of C-O groups that bond to the acid's conjugate base. Reduction recovers most but not all of the drop in conductivity, indicating that rather than reforming the pristine sp3 carbon framework, sp2 groups with minimal electron scattering, such as ether linkages, can form instead.

  3. Tracing a Stratospheric Journey

    Large volcanic eruptions inject material into the stratosphere and impact global climate, but a lack of observational data has made it difficult to determine if an ancient volcanic eruption, which might only be documented by deposited ash layers, affected the stratosphere. Baroni et al. (p. 84) now report that the isotopic composition of the sulfur in sulfate contained in Antarctic snow for the Agung (1963) and Pinatubo (1991) eruptions displays mass-independent fractionation in the sulfate concentration peaks. Because only photochemical reactions in the stratosphere can explain this pattern of isotope fractionation, the authors suggest that sulfate isotopic composition could be used to record whether volcanic ash entered the stratosphere.

  4. Massive Interference


    The Newtonian gravitational constant G is the fundamental constant that has been the hardest to determine accurately, in part because of the relative weakness of gravity. Traditional methods for measuring G tend to be mechanical, such as looking at the rotation of a torsion balance in response to a moving test mass. Fixler et al. (p. 74) show that the interference pattern of the de Broglie waves of cold cesium atoms shifts in response to the position of a 540-kilogram test mass made of lead. The authors claim that the technique is less prone to the systematic errors that plague the mechanical measurements and may ultimately allow for a more accurate determination of G.

  5. Greenhouse Gases in an Earlier Ice Age

    Numerous studies of Cenozoic climate have shown how climate and the carbon cycle are inked, but similar records much farther back in time are rare. Before the start of the current “icehouse,” around 35 million years ago (Ma) when large ice sheets began to form in Antarctica, the last period when Earth had sizable volumes of continental ice was during the late Paleozoic (between 265 and 305 Ma). Montañez et al. (p. 87) used a 40-million-year-long record of the stable isotopic compositions of minerals formed in soils, fossil plant matter, and shallow-water brachiopods to explore the relation between continental surface temperatures and the concentration of atmospheric CO2 during this interval when Earth drifted in and out of glaciated and fully deglaciated conditions. Changes in continental ice volume were strongly correlated with shifts in atmospheric partial pressure of CO2, and paleofloral data chronicle the repeated restructuring of paleotropical floral communities that accompanied the inferred climate shifts. These findings suggest that greenhouse gas forcing of climate occurred during remote times in a manner similar to the present era.

  6. Altered Aerobics

    A comparison of field observations and laboratory data on the eelpout (a North Sea and Baltic Sea fish species) by Pörtner and Knust (p. 95; see the Perspective by Wang and Overgaard) has revealed a mismatch between tissue oxygen supply and temperature-dependent oxygen demand that is causing a loss of species abundance during hot summers. Temperature-dependent constraints in oxygen supply are likely to affect many functions, including behavior, growth, reproduction, and interaction with other species, and thereby influence the long-term fate of populations and species in various climates.

  7. Targeting MicroRNAs to the Nucleus

    MicroRNAs (miRNAs) are small ∼22-nucleotide (nt) noncoding RNAs found in most eukaryotes that regulate the translation and/or stability of target RNAs. miRNAs are grouped into families that are related by their highly conserved 5′ “seed” sequences that are important in defining the complementary target RNAs. The 3′ sequences are generally less conserved within families, which has raised questions about their functional significance. Even so, 3′ sequences can be very highly conserved (even identical) across species for individual miRNAs, which suggests the presence of powerful selective constrains. Hwang et al. (p. 97) now show that human miR-29b is localized to the nucleus and that this localization is driven by a 6-nt sequence in the 3′ half of the molecule. The authors raise the intriguing possibility that miR-29b might regulate the transcription or splicing of target transcripts.

  8. Biased Inheritance

    Although chromosome segregation is generally considered to be random relative to daughter cell inheritance, nonrandom segregation of mouse chromosome 7 has been reported for certain cell types. Armakolas and Klar (p. 100; see the Perspective by Sapienza) examined molecular components that participate in nonrandom chromatid segregation. Mutation of a gene encoding the microtubule motor left-right dynein (LRD), shown previously to affect left-right body-axis determination, differentially affected chromatid segregation in specific cell types.

  9. Giving Lotus the Nodule


    The nodulation of roots in legumes is a key factor in nitrogen fixation. Working in the leguminous plant Lotus japonica, Trichine et al. (p. 104, published online 16 November) and Murray et al. (p. 101, published online 16 November) have identified how the hormone cytokinin fits into the signaling cascade by which leguminous plants establish nitrogen-fixation nodules filled with symbiotic bacteria (see the Perspective by Oldroyd). Gain-of-function mutation in a cytokinin receptor results in spontaneous formation of bacteria-free nodules, whereas loss of function results in too few nodules, despite aggressive formation of bacterial infection threads.

  10. Choosing the Right Path

    Dendritic cells of the immune system present antigen to T cells in the context of either class I or class II molecules of the major histocompatibility complex (MHC) as a means of generating two distinct arms of T cell immunity: Class I-restricted CD8+ T cell responses and CD4+ T cell help. Dudziak et al. (p. 107) present evidence that each pathway dominates in distinct subsets of dendritic cells. Using chimeric antibodies specific for cell surface markers present on each specific subtype of dendritic cell, it was possible to target antigens to the class I or class II MHC pathways and so elicit CD8 or CD4 responses, respectively.

  11. Enzyme Kinetics in Living Color

    Innovative methods are required to study the spatial regulation of enzymatic activity inside living cells. Yudushkin et al. (p. 115) describe such a method based on the detection of enzyme-substrate complexes using fluorescence lifetime imaging microscopy. The detection of the enzyme interacting with the substrate ensures utmost specificity and enables evaluation of the localized activity of a particular enzymatic species. The technique was used to investigate the spatial regulation of growth factor signaling by the tyrosine phosphatase PTP1B and revealed that PTP1B exists inside cells as kinetically distinct, spatially separated subpopulations.

  12. Cellular Accounting

    Fluorescent labeling has allowed quantification of proteins in single cells, but potential problems arise from interferences between reporter molecules themselves and interactions with other molecules within the cell. Huang et al. (p. 81) have devised a microfluidic chip for counting fluorescent proteins within a cell. Single cells are captured and lysed, and their contents separated by electrophoresis and quantified by single-molecule fluorescence detection. This method was applied to natively fluorescent compounds (the phycobiliprotein subcomplexes in individual cyanobacterial cells grown under nitrogen-limited conditions) and proteins labeled with fluorescent antibodies β2 adrenergic receptor) present in low copy numbers.

  13. Cellular Mechanics Up Close and Personal

    As they move, vertebrate cells can transmit mechanical information from their insides to their outsides and vice versa. A major pathway for this is through integrin-mediated adhesive interactions between the actin cytoskeleton and the extracellular matrix. Hu et al. (p. 111) focus on defining the protein interface mediating the interaction between the cytoskeleton and the extracellular matrix that are part of integrin-based focal adhesions by using sophisticated live cell microscopic analysis. The motion of the actin cytoskeleton is differentially transmitted through focal adhesions, with the efficiency of transmission gradually decreasing from actin-binding proteins to integrins, defining the focal adhesion as a hierarchical molecular clutch. The internal molecular dynamics of focal adhesions thus represents a key element in the mechanics of cell morphogenesis during migration.