This Week in Science

Science  09 Mar 2001:
Vol. 291, Issue 5510, pp. 1855
  1. Toward Attosecond Optics

    The development of femtosecond-scale optical pulses, comprising only a couple of cycles of the optical field, has opened the window on looking directly at atom and molecular dynamics. However, there are some other processes, electronic relaxation and ionization, that proceed on still shorter time scales. Observing the dynamics of these processes will require the development of yet shorter pulses and the ability to measure them. Drescher et al. (p. 1923; see the Perspective by Reid) now describe a technique for the generation and measurement of x-ray pulses 2.5 femtoseconds in duration that are shorter than the carrier field used for their generation.

  2. The Quantum Mechanics of Micromechanics

    In classical mechanics, a vacuum is a pretty dull place, but in quantum mechanics, it's quite busy. Quantum fluctuations of the electromagnetic field give rise to fleeting moments in which virtual photons are created and annihilated. If these events take place in the space between two uncharged plates, they cause a pressure differential that pushes the plates together—the Casimir effect. For large-scale separations, these events have little consequence, but as length scales are reduced, as in the case of microelectromechanical systems now being used as actuators, the effect could be significant. Chan et al. (p. 1941) demonstrate that as separations are reduced to the nanometer scale, quantum fluctuations do indeed play a significant role and must be considered in describing the operation of such microfabricated devices.

  3. Losing Oxygen

    The polar flux of O+ out of Earth's atmosphere suggests that the atmosphere has lost an amount oxygen equivalent to 18% of its current oxygen content during the last 3 billion years. Seki et al. (p. 1939; see the Perspective by Lundin) estimated the amount of O+ that is actually lost through four escape mechanisms in the magnetosphere. Their results suggest that only 2% of the oxygen has been lost during this period through these four escape routes. Either the oxygen is being recycled in the magnetosphere back into the ionosphere or a yet unknown escape mechanism lurks.

  4. Extinction Clues in Space Dust

    Interplanetary dust particles (IDPs) carry a higher abundance of 3He than do terrestrial rock samples because this species is implanted by the solar wind. If small IDPs are not unduly frictionally heated by Earth's atmosphere, they will enrich the surface of minerals with 3He when they are deposited. Thus, the measurement of 3He normalized to 4He in terrestrial sedimentary rocks can provide an estimate of IDP flux. Mukhopadhyay et al. (p. 1952) examined clays emplaced near the Cretaceous-Tertiary (K-T) boundary and found that the IDP flux was constant during this time period. No additional helium was added from the bolide impact, which is consistent with the interpretation that the bolide mostly vaporized. They also estimate a sedimentation rate of the K-T boundary clay layer of 11,000 years, which indicated that the K-T extinction event was rapid.

  5. Oxide Nanobelts

    CREDIT: Z. W. PAN ET AL.

    Nanotube and nanowire morphologies are known for many materials. Pan et al. (p. 1947) have now found that several metal oxides (including zinc, tin, indium, and gallium) can form a “nanobelt” morphology after thermal evaporation the bulk oxide at high temperature and deposition of the vapor at lower temperatures on an aluminum oxide surface. The nanobelts are between 30 and 300 nanometer wide, have width-to-thickness ratios of 5 to 10, and can extend for several millimeters. These oxide semiconductors have several applications in sensing and electronics that could be enhanced by this high-area morphology.

  6. Switching Sides to Get Ahead

    As the nervous system of the developing vertebrate embryo is laid down, the guidance molecule netrin attracts the growth cones of advancing axons to the embryo's midline. However, as soon as the growth cones cross the midline, they switch allegiance, ignoring netrin and instead responding to Slit, which repels the growth cones away from the midline. Two papers tackle how growth cones make their way through competing signals (see the Perspective by Dickson). Stein and Tessier-Lavigne (p. 1928–; see the cover) show that this switch is the result of the cytoplasmic domain of the Slit receptor, Robo, binding to the cytoplasmic domain of the netrin receptor DCC. Thus, the dwindling effects of netrin on growth cones is directly linked to their increasing responsiveness to the repellent effects of Slit. This interlocked silencing mechanism prevents the growth cone from being trapped in a tug-of-war between the attractive and repellent activities of two opposing guidance molecules. Stein et al. (p. 1976) address the mechanism by which netrin exerts its effects. In contrast to a recent suggestion that netrin acts through the adenosine A2B receptor, the authors show that netrin binds directly to the DCC protein and that A2B activation is not required for netrin's effects on axon outgrowth or neuron attraction. Thus, DCC is the primary receptor responsible for netrin's actions.

  7. Bypassing Checks and Balances

    The hepatitis virus type C tricks the host's ribosomes into recognizing and initiating translation of its RNA even though its RNA lacks a 5' cap, which is added to host messenger RNAs and is the usual prerequisite for initiation. It does so by utilizing an internal ribosome entry sequence (IRES) that lies directly upstream of the protein-coding region of its RNA. Spahn et al. (p. 1959) used electron microscopy to find that the IRES binds to the decoding subunit of the mammalian ribosome. Binding triggers a conformational change that brings the adjacent protein-coding region into the decoding site and locks it into place in preparation for initiating translation and protein synthesis.

  8. Mysterious Resistance Revealed

    Resistance against the penicillin family of antibiotics has almost become universal among clinically important strains of staphylococci bacteria. When penicillin binds to a sensor on the surface of bacteria, a signal is transmitted across the cell membrane to cause removal of a DNA repressor protein and to allow the transcription of the various regulatory genes for expression of β-lactamase or of low-affinity penicillin-binding protein substitutes. Zhang et al. (p. 1962; see the Perspective by Archer and Bosilevac) now show that penicillin binding to the sensor triggers self-proteolysis. The cleavage product then binds, either directly or via intermediates, to the DNA binding repressor protein. The repression is lifted in a second proteolytic step when the DNA binding protein is released and transcription of the antibiotic-resistance genes starts.

  9. Turn Signals

    CREDIT: T. M. GOMEZ ET AL.

    Growth cones at the tips of extending neurites dictate axonal guidance by translating extracellular cues into intracellular signals that then determine the appropriate direction to turn. Gomez et al. (p. 1983) report that localized calcium transients occur in filopodia that extend from the growth cones. The frequency and amplitude of these calcium transients depend on the extracellular substrate, implicating the involvement of integrins in regulating the turning process. These calcium transients also appear to propagate back to the growth cone and affect global calcium dynamics.

  10. Recovering Ebola Virus

    Ebola virus, and its ally Marburg virus, are nonsegmented negative-strand RNA viruses and members of the family Filoviridae. They cause fatal hemorrhagic diseases, but so far their occurrence has been in fairly restricted localities, mostly in western and central Africa. The “wild hosts” for these viruses are currently unknown. Molecular tools are needed for therapeutic development and for responding appropriately to outbreaks of these and any new filoviruses. Volchkov et al. (p. 1965) have established a reverse-genetics system for rescuing Ebola virus from cloned DNA and generated mutants incapable of transcriptional editing of a key nonstructural glycoprotein. This mutant form of the virus expressed greater amounts of the envelope glycoprotein and was more cytotoxic. Further investigations should lead to insights into replication and pathogenicity of these viruses.

  11. Do You Hear What I Hear?

    The recognition of pitch is a two-part process in which the ear collects musical sounds, which are then extensively processed by the brain to produce pitch perception. To elucidate the contribution of genetics and environment to pitch perception ability, Drayna et al. (p. 1969; see the news story by Holden) applied the Distorted Tunes Test (DTT) to 284 monozygotic and dizygotic twin pairs. With the DTT, subjects have to recognize notes with incorrect pitch in simple popular melodies. There was a significantly higher correlation on DTT scores between monozygotic compared with dizygotic twin pairs, which indicates that there is a large inherited component to pitch perception.

  12. Boosting Transmitter Release

    In vertebrates, presynaptic neurotransmitter receptors are generally thought to inhibit transmitter release. However, Schmitz et al. (p. 1972) show that activation of high-affinity presynaptic kainate receptors at hippocampal mossy fibers facilitates glutamate release from the synaptic terminals. This mechanism contributes in part to the unusually large frequency facilitation during trains of stimuli seen at mossy fiber synapses.

  13. Sheets of Melt Beneath a Segmented Ridge

    Mid-ocean ridges mark the boundaries between plates and are also the regions where new oceanic crust is generated by magma reservoirs below the ridge axes. Dunn et al. (p. 1955) have imaged the P-wave velocity structure of the upper mantle below an overlapping spreading center (OSC) along the East Pacific Rise. They found evidence for a continuous magma reservoir below the OSC, which indicates that the ridge segmentation is related to changes in the velocity of the spreading of the plates rather than to a discontinuous magma supply. These results are consistent with sheetlike upper-mantle upwellings and provide clues to the kinematics of plate interactions.

  14. Fullerenes Get Organized

    The potential applications for fullerene molecules could be expanded if their limited solubility in water authors could be overcome. Recently it has been shown that a chemically modified form of C60 fullerene, the Ph5C60 anion (where Ph is a phenyl group), dissolves in water as the potassium salt after an iterative dissolution process. Zhou et al. (p. 1944) have performed light-scattering studies to show that this soluble species exists as a spherical, 17-nanometer-wide vesicle formed from a bilayer of ions.

  15. Keeping Up in the Cold

    Low temperatures slow down most life processes, but if they became very slow in cold polar waters then that could result in significant developmental and eventually ecological problems for invertebrates. Marsh et al. (p. 1950) have found that an Antarctic sea urchin overcomes the handicap of living in subzero temperatures by compensatory mechanisms that allow it to have equivalent rates of protein and RNA synthesis to its temperate relations, but at low metabolic rates. It does this in part by elevating rates of RNA synthesis to maintain high cellular concentrations of messenger RNA and by minimizing the energetic costs of protein metabolism to a level about 25-fold below that so far found in any organism. The biochemical adaptations that permit these compensations have yet to be discovered, but would be expected to have some exciting biotechnological potential.

  16. Making Connections

    T cells respond to antigens through multiple signals, many of which are driven by proteins that coassociate at the cell membrane. SLP-76 is a primary signaling molecule that is recruited by the integral membrane protein LAT, and together they initiate several signaling pathways. Yoder et al. (p. 1987) show that GADS, a small adaptor protein, is critical in mediating this association and thus has a significant impact on T cell development. Thymocytes from mice deficient in the expression of GADS displayed defects in cellular proliferation as well as in positive and negative selection. By connecting SLP-76 and LAT, GADS facilitates important SLP-76-dependent signals at different stages of T cell development.

  17. Examining Evidence for Reproductive Isolation in Sockeye Salmon

    Hendry et al. (Reports, 20 Oct. 2000, p. 516) offered evidence of significant reproductive isolation in two salmon populations after only 13 generations of divergent selection. Howard et al., in a comment, argue that the observed measures of genetic differentiation do not establish that the populations are indeed genetically divergent, that the migration estimates of the study are poorly supported, that Hendry et al. “have provided no evidence that observed phenotypic differences have a genetic basis,” and that explanations for genetic differentiation other than reproductive isolation were not explored. Hendry et al. respond that “large genetic differences were neither expected nor crucial to our conclusions” given the brief time since the populations diverged. They also respond to the objection regarding migration estimates, provide indirect evidence that observed phenotypic differences “were at least partly genetic,” and critically examine some alternative hypotheses. “We certainly do agree with Howard et al.” they conclude, “…that much work remains to be done.”The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/291/5510/1853a