This Week in Science

Science  28 Nov 2003:
Vol. 302, Issue 5650, pp. 1475
  1. Distinctive DNA Wraps

    In order to exploit fully the electronic properties of carbon nanotubes, methods will be needed to fully disperse the bundles in which they are formed and to then separate the various tubes by size and type. Several materials, including DNA segments, have been used to solubilize the nanotubes. Zheng et al. (p. 1545) now show that for certain sequences, the DNA wraps itself around the nanotube differently depending on tube diameter. These wrappings led to different electronic properties for each nanotube. Partial separations based on diameter and type (semiconductors versus metallic) were achieved with anion exchange chromatography.

  2. Inverse Doppler Effect

    In the conventional Doppler effect, the pitch of a sound coming from an approaching object, such as a siren, rises as it comes toward you and decreases as it goes away. Theory suggests that under particular circumstances and in materials with novel dispersion characteristics, an inverse Doppler effect can occur in which the pitch drops during the approach. Seddon and Bearpark (p. 1537) present experimental verification of such counterintuitive behavior in which reflection of electromagnetic radiation from a receding shock front propagating down a specially designed transmission line shifted to higher frequencies. The frequency shift is determined by the propagation speed of the shock front and thus could be tuned.

  3. Tales of Warming

    A large and rapid warming event 55 million years ago, the Paleocene-Eocene Thermal Maximum (PETM), raised the temperature of the high latitudes by 10°C and of the deep ocean by 5°C. Its effect on the low-latitude surface ocean has remained unresolved due to the absence of reliable sea surface temperature (SST) records from the tropics. Zachos et al. (p. 1551) now present an SST reconstruction based on a sediment core from the tropical Pacific Ocean. After measuring both oxygen isotopes and Mg/Ca in the skeletons of long-dead surface-dwelling foraminifera, they produced a record of temperature and salinity and found that SSTs rose by approximately 5°C. This result is consistent with model studies which assume that the warming was caused by a two- to threefold increase in the concentration of CO2 in the atmosphere.

  4. Optical A Cappella with Attosecond Pulses?

    High harmonics of an optical laser's wavelength that extend into the soft x-ray regime can be generated when an intense laser pulse interacts with a gas. If the phase of these harmonics can be synchronized, then very short pulses can be generated. However, for the generation of attosecond (10−18 second) x-ray pulses, Mairesse et al. (p. 1540) show that the high harmonics are inherently not synchronized. However, they could optimize the electrodynamical processes to create the equivalent of “a cappella” harmony and improve the synchronization to produce 130-attosecond pulses of soft x-rays.

  5. False Color

    Measurements of the concentration of chlorophyll in the surface ocean from satellites can be used as a proxy for biological productivity. However, recent studies have found enhanced production in regions of the subtropical oceans where an absence of nutrients usually leads to low productivity. Dandonneau et al. (p. 1548; see the Perspective by Claustre and Maritorena) combine satellite observations, an ocean-mixing model, analyses of surface winds, and in situ chlorophyll data to conclude that these high concentrations of chlorophyll are not derived from thriving photosynthetic communities fed by upwelling driven by Rossby waves, as has been previously suggested. The authors argue that the signals are caused by the accumulation of floating particles in areas where the winds associated with Rossby waves converge.

  6. Molecular Electronics Beats the Heat

    One near-term goal in molecular electronics is to create small memory devices that serve as dynamic random access memories (DRAMs). However, the engineers who run fabrication lines will subject such a memory, along with the rest of the chip, to temperatures of ∼400°C. Liu et al. (p. 1543) show that a class of porphyrin molecules bonded to the Si(100) surface survive such extreme temperatures in an inert atmosphere and still display voltage-induced switching behavior. This switching behavior is maintained after 1012 cycles, which corresponds to operating a DRAM over a 10-year lifetime.

  7. Molecular Dinner Invitations

    Apoptotic cells undergoing programmed cell death are potentially harmful, and normally they are removed by phagocytes that engulf the dying cells. The phagocytes are thought to recognize the apoptotic cells by the presence of phosphatidylserine in the outer leaflet of the plasma membrane. Two reports show that animals lacking phosphatidylserine receptors have defects in the engulfment of cell corpses (see the Perspective by Savill et al.). Wang et al. (p. 1563) show that in the nematode worm, Caenorhabditis elegans, the phosphatidylserine receptor appears to interact with one of two known signaling pathways that lead to the removal of cell corpses during development. Li et al. (p. 1560) show that mice lacking the receptor have defects in development of the lung and brain caused by accumulated dead cells.

  8. A Tortoise and Hare Race in Mutation Rates

    Predictions of the speed of evolution depend on mutation rates, yet little is known about these rates for the stationary or arrested growth stages that make up the bulk of the lifetime of many microorganisms. Loewe et al. (p. 1558) discovered that genetic changes are far more prevalent in stationary phase than reported previously for exponentially growing cultures. Earlier models of evolution rates for bacteria based on log-phase cultures may thus be considerable underestimates.

  9. Fish Go with the Flow

    In the presence of artificially generated vortices in flowing water, fish swim in a manner that allows them to save energy. Trout wired for electromyography were monitored by Liao et al. (p. 1566; see the Perspective by Müller) in a setup that allowed simultaneous recording of vortices generated in flowing water. The strategy of the swimming fish appeared to be one that minimized power input as they dodged, rather than intercepted, the vortices.

  10. Vancomycin-Resistant Staph

    Staphylococcus aureus is a cause of multiple-antibiotic resistant hospital-acquired infections. In June 2002, a case of resistance to the “last-ditch” antibiotic, vancomycin, was reported in a dialysis patient in the United States. Resistance was conferred by the multidrug resistance-conjugative plasmid pLW1043 into which the mobile element, Tn1546, was integrated. This transposon carries alternative cell-wall biosynthesis genes. Weigel et al. (p. 1569; see the news story by Ferber) present the sequence of the plasmid, which also contains additional resistance elements for other antibiotics and antiseptics. A similar plasmid isolated from Enterococcus faecalis in the same patient was probably the source of the vancomycin resistance.

  11. Molecular Chopsticks in Nuclear Import

    When cells are deprived of cholesterol, the transcription factor SREBP-2 (sterol regulatory element-binding protein 2) is imported into the nucleus, bound directly to the nuclear import adaptor, importin-β. Lee et al. (p. 1571; see the Perspective by Stewart) have determined the crystal structure of the importin-β:SREBP-2 complex at 3.0 angstrom resolution. The importin-β molecule changes its conformation to accommodate the symmetric SREBP-2 dimer and probably binds other dimeric substrates in a similar manner. The SREBP-2 dimer is gripped by long helices in importin-β that act like chopsticks. Binding of Ran guanosine triphospate by importin-β opens the chopsticks and releases SREBP-2 into the nucleus.

  12. Kinase Secrets Revealed

    Protein kinases function not only as catalysts but also as scaffolds and docking targets. Papa et al. (p. 1533) now show how an unanticipated conformation-dependent activation of the kinase Ire1 activates the “unfolded protein response” from within the endoplasmic reticulum. This response stimulates the production of protein chaperones and other factors to enhance the cell's ability to cope with the buildup of misfolded proteins within the secretory pathway. The conformational change can be mimicked by a bulky adenosine 5'-triphosphate (ATP) analog when the ATP binding site is modified. The activation of the RNA splicing activity promoted by Ire1 is surprisingly independent of phosphorylation.

  13. Genetics of a Coronary Vascular Disorder

    The genetic factors influencing coronary artery disease (CAD) remain poorly defined. Wang et al. (p. 1578) studied a large family with high incidence of a vascular condition resembling CAD, which in most of the cases led to myocardial infarction. A genome-wide linkage analysis identified an autosomal dominant locus that included MEF2A, an endothelial transcription factor that is thought to participate in vascular development. A deletion-mutation within MEF2A that conferred altered cellular distribution of the protein specifically segregated with CAD family members. It remains to be seen if the MEF2A pathway plays a role in the common form of CAD resulting from atherosclerosis.

  14. Do Not Disturb

    The terrestrial biosphere absorbs approximately one-quarter of annual global anthropogenic CO2 emissions. Eddy covariance studies, in which the concentration of CO2 measured at various heights in and above the forest canopy is used to infer how much the forest is absorbing or emitting at that location, suggest that old-growth tropical rainforests in general, and the Amazon in particular, are substantial carbon sinks. However, if uptake by Amazonian forests is so large, there also must be locations from which CO2 is emitted to the atmosphere, because atmospheric inversion studies of regional CO2 exchange show that Amazônia is a much weaker sink than that projected from eddy covariance estimates. Saleska et al. (p. 1554) report that some old-growth Amazonian rainforest can be a net source of carbon, and suggest that earlier studies may have overestimated the strength of the sink there by neglecting to consider the effect of recent natural disturbance events. The seasonality of forest carbon exchange in Eastern Amazônia was unexpectedly strong and opposite in phase from that normally expected. These findings underscore the difficulty of making regional projections of carbon sequestration from local results, and indicate that Amazonian forests might respond to climate cycles like El Niño differently than has been hypothesized.

  15. A Muscular Fountain of Youth

    When muscle fibers are injured, skeletal muscle precursor cells proliferate and differentiate to regenerate and repair the injured muscle. The ability to regenerate skeletal muscle decreases with age, and Notch signaling is necessary for normal adult muscle repair. Conboy et al. (p. 1575) now show that Notch also functions in the age-related decline in mouse skeletal muscle regeneration. Old and young muscles possessed equivalent numbers of muscle precursor cells, but cells from older mice were less able to induce the Notch ligand, Delta, and to activate Notch signaling in response to muscle injury. Forced activation of Notch-1 restored the regenerative potential of old muscle, whereas inhibition of Notch-1 impaired regeneration of young muscle.

  16. Controlling Immunological Signaling

    Activation and nuclear translocation of the immune-response transcription factor NF-κB depends on signals that emanate from a variety of cell-surface receptors. Two membrane-proximal caspase-recruitment proteins, CARMA1 and Bcl10, form part of the chain of signaling events, but other critical remaining steps in the pathway have yet to be established. Ruefli-Brasse et al. (p. 1581; see the Perspective by Yu and Lenardo) show that the caspase-like protein, paracaspase, is essential for lymphocyte function and acts downstream of CARMA1 and Bcl10. Impaired paracaspase-dependent activation of the kinase complex that normally facilitates NF-κB activation caused T and B cells from paracaspase-deficient mice to respond poorly to receptor signals.