This Week in Science

Science  09 Feb 2007:
Vol. 315, Issue 5813, pp. 733
  1. Going Nonlinearly with the Flow


    Nonlinear processes can be highly disruptive or turbulent, and thus would be unlikely candidates for being time reversible (see the Perspective by Epstein). Fuerstman et al. (p. 828) exploit the linear and smooth flow associated with microfluidics to create a system that shows nonlinear behavior in the motions of entrained droplets that can be reversed with time. As the fluid flows toward a T-junction, the entrained droplets must choose one branch or the other, and this decision has a nonlinear dependence on the fluid flow and the rate of droplet production. After the two streams recombine, the droplets take on a particular repeat pattern or encoding that can be decoded through a reversal of the fluid flow. Prakash and Gershenfeld (p. 832) exploit nonlinear behavior of bubbles in a microfluidic system to create bubble logic, in which the bubbles represent bits of information similar to the ones and zeros used in digital computation. They create a series of simple devices including AND, OR, and NOT gates, as well as more complex signal amplification and processing.

  2. All the Same

    The predictable brightnesses of type Ia supernovae make them some of the most useful probes of cosmological distances. Observations of these supernovae suggested the existence of dark energy, but uncertainties linger, and better knowledge of the physics of these stellar explosions is needed to improve their use as distance indicators. By mapping the expansion of the supernova explosion in different spectral lines, Mazzali et al. (p. 825) show that all type Ia supernovae form from the explosion of a star of the same mass, and their ejecta reach out to similar distances. Simulations of the explosions and analysis of the recorded changes in brightness over time support this finding. These results point to a single explosion mechanism for supernova Type Ia progenitors, supporting their cosmological use.

  3. Small Sinking Surprise

    The concentration of CO2 in the atmosphere is kept in check partly by how much CO2 is consumed by marine photosynthetic organisms, although the only ones that sequester CO2 “permanently” are those that sink to the bottom of the ocean and wind up in sediments. It is generally thought that the most important class of organisms responsible for exporting carbon from the surface to deeper waters are the large phytoplankton such as diatoms that can sink rapidly enough to avoid being consumed by microbes on the way down. Richardson and Jackson (p. 838; see the Perspective by Barber) challenge this assumption by describing how the very small autotrophs, called “picoplankton,” that dominate ocean primary production over large regions, can also contribute to export through the processes of aggregation and incorporation. This finding implies that the amount of picoplankton that contribute to the flux of carbon to the deep ocean has been significantly underestimated, and that the importance of these tiny photo-synthesizers as “carbon pumps” has been overlooked.

  4. Viral Prefusion Conformation Revealed

    The rhabdoviruses, which include human pathogens like rabies virus, enter the cell through the endocytic pathway. Viral membrane fusion with cellular endosomal membranes is triggered by a pH-dependent structural change of a transmembrane viral glycoprotein (G). Now Roche et al. (p. 843) have determined the prefusion form of the fusion G protein from vesicular stomatitis virus. Comparison with the postfusion structure reveals the structural reorganization between these forms and suggests a pathway for the dramatic, but reversible change.

  5. Directly Observed


    Atmospheric deep convection, which transports air from the surface to the upper troposphere is difficult to measure, and our understanding of this process has mainly been based on modeling. Bertram et al. (p. 816; published online 5 January; see the Perspective by Jaegle) provide direct observational constraints to the process, with measurements of a suite of trace gases and aerosols made in the summertime continental upper troposphere over the eastern United States and Canada. Using the distribution of chemical species whose kinetics are well understood to determine the amount of time that air spends in the upper troposphere, they calculated important dynamical parameters such as the extent to which convection perturbs the continental upper troposphere during summer, the fraction of boundary layer air present in convective outflow, and the convective overturn rate of the upper troposphere. These results present a challenge to current ideas about processes that control upper tropospheric ozone and its impact on climate.

  6. Documenting Diversity

    Human genetic diversity includes polymorphisms at individual points within genes, as well as variations in copy number. Stranger et al. (p. 848) present a whole-genome survey of the effect of such differences on gene expression of nearly 15,000 transcripts. In order to understand complex patterns of inheritance (such as quantitative traits), it will be necessary to look at both types of variation.

  7. Make Your Move

    Motility in bacteria is important for pathogenesis and for bacterial chemotaxis. So-called gliding motility in the bacterium Myxococcus xanthus is powered by two distinct engines: S motility (powered by type IV pili) and A motility, of unknown mechanism. By following the localization pattern of an A motility-specific protein, Mignot et al. (p. 853; see the Perspective by Kearns) have now discovered that the most popular hypothesis for A motility, directed slime secretion, is likely to be incorrect and that intracellular motor-coupled adhesion complexes power movement. Thus, bacteria and eukaryotic cells may use a similar transient adhesion-based mechanism for motility.

  8. A Receptor for Vitamin A Uptake


    Vitamin A has a large number of biological functions, including roles in vision, reproduction, immunity, tissue regeneration, and neuronal signaling. The existence of a cell surface receptor for the vitamin A carrier retinol binding protein (RBP) was proposed more than 30 years ago. Kawaguchi et al. (p. 820, published online 25 January) now report the identification and characterization of the long-sought RBP receptor. The RBP receptor is a multipass-transmembrane domain protein with robust RBP binding and vitamin A uptake activity and is localized to the expected cellular locations for vitamin A uptake.

  9. The Grim Reaper on Autopilot?

    The proteins Bax and Bak are key mediators of cell death signals that function at the mitochondria to promote apoptosis. There is evidence for multiple modes of regulation of Bax and Bak in cells. Some studies have proposed that other members of the Bcl-2 family of proteins interact with and directly activate Bax and Bak—a scenario in which cell survival seems to be the default state of the cell. However, activity of Bax and Bak is also held in check by interaction with pro-survival proteins, and it may be that relief of this inhibition determines the cell's fate. Willis et al. (p. 856; see the Perspective by Youle) present evidence that the default state of Bax and Bak would lead to cell death. Cells lacking the direct activators of Bax and Bak still undergo apoptosis in response to overexpression of upstream components of the cell death pathway.

  10. Two Wings Good, Four Wings Better

    Some flying insects have two wings, whereas others have four. The common housefly, which possesses two wings, makes use of the vestigial hindwing (the pendulum-shaped haltere) as a source of mechanosensory input to the neural centers that support stable flight. Sane et al. (p. 863; see the Perspective by Alexander) have asked whether moths, which have four wings, possess a similar kind of flight control mechanism. Instead, the antennae appear to serve a haltere-like function by providing mechanosensory input through hairs or bristles located at their base, whose deflections are translated into afferent neural signals.

  11. Mind the Gap

    Cyclobutane pyrimidine dimer (CPD) lesions in DNA are induced by exposure to UV light and are a major cause of skin cancer. The cell deals with CPDs by nucleotide excision repair, one pathway for which is transcription-coupled DNA repair, triggered by RNA polymerase II stalling at a CPD. Brueckner et al. (p. 859) now show that the yeast RNA polymerase II does not stall because of distortions in the DNA caused by bulky CPDs, but because movement of the second T of the CPD into the polymerase active site allows the slow misincorporation of U, bringing the transcription machinery to a grinding halt. During DNA repair, the CPD, the mRNA, and RNA polymerase II might thus be removed as a unit.

  12. Integrating Cell Signaling Pathways in Development

    During development signaling by the fibroblast growth factor (FGF) receptor tyrosine kinase enhances mesoderm induction by transforming growth factor (TGF)-β and Activin-like growth factors. Cordenonsi et al. (p. 840, published online 18 January) combine Xenopus embryology with biochemical analyses to show that FGF signaling promotes phosphorylation of the N-terminal region of the tumor-suppressor protein p53. This modification enables the formation of a complex between p53 and TGF-β-activated Smad, which regulates gene expression and promotes Xenopus mesoderm induction.

  13. Capturing an Iron(V) Oxo

    The iron(V) oxo motif has long been postulated as a highly reactive intermediate in a variety of enzymatic oxidations, as well as certain Fe/H2O2 systems used for oxidative waste remediation. Tiago de Oliveira et al. (p. 835, published online 21 December) find that this elusive species can be stabilized at low-temperature upon treatment of a tetraamido-coordinated Fe center with m-chloroperbenzoic acid. The complex persists for hours at −60°C, and was characterized spectroscopically and analyzed by density functional theory calculations. Both theory and experiment support a low-spin electronic configuration. The compound cleanly transfers oxygen to triphenylphosphine, as evidenced by an isotopic labeling study.

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