This Week in Science

Science  27 Apr 2001:
Vol. 292, Issue 5517, pp. 597
  1. Anthrax Antidote

    Although there is a prophylactic vaccine available for anthrax, the natural rarity of the disease does not warrant widespread use except for military personnel. However, if there were a terrorist incident involving anthrax, a therapeutic vaccine would need to be available for immediate mass use. Sellman et al. (p. 695; see the Perspective by Olsnes and Wesche) have developed the raw materials for such a therapeutic by constructing mutants of a key molecule of the multicomponent anthrax toxin. These are dominant-negative mutants, meaning that just one mutant subunit assembling into the seven-subunit component called protective antigen will block the cellular activity of the entire toxin by inhibiting the translocation of the active components across cell membranes. Rats given unadulterated toxin components died within 2 hours, but those given the dominant-negative mutant toxin survived without symptoms until the end of the experiment.

  2. Glimpsing Our Galaxy's Halo

    The halo of the Milky Way galaxy is thought to be composed mostly of dark matter, which is detected by its gravitational effects on luminous galactic components, but otherwise the nature of this dark matter is unknown. Recent work has suggested that cool white dwarfs—compact, extremely dense stars that no longer produce energy by nuclear fusion and that are now cooling and fading from view—may be one component of the halo. Oppenheimer et al. (p. 698; see the 23 March news story by Sincell) surveyed a region near the South Galactic Cap and have now observed a population of extremely faint old white dwarfs with large space velocities that are consistent with these subluminous stars being part of the galaxy's halo. Furthermore, this population accounts for about 2% of the halo's “dark matter.”

  3. Mantle Flow in the South Pacific

    The volcanic islands of Fiji and Tonga in the southwestern Pacific Ocean are situated above a complex plate boundary. The Pacific plate is being subducted beneath the Indo-Australia plate along the Tonga trench and just west of the trench the Lau backarc basin is opening up. Smith et al. (p. 713) used seismic data from local stations on land and the ocean floor to infer the direction of flow in the mantle beneath these islands. The mantle flows north to south from Fiji toward the Lau basin and east to west from Tonga toward the Lau basin. Along the opening of the Lau basin, the mantle flow is coming up because the Samoan hot-spot plume infiltrates the basin.

  4. Selective Bond Breaking with Strong Laser Fields

    Bond-selective photochemistry has been a long-sought but elusive goal. One difficulty is that photoexcited states can rapidly redistribute energy in ways that can be difficult to predict. Levis et al. (p. 709; see the Perspective by Hurley and Castleman) now report that different reaction outcomes can be selected through a closed-loop feedback algorithm that tailors the phase and amplitude of “strong-field” pulses. For very intense laser pulses (1013 watts per square centimeter), the various eigenstates of the molecule can be resonance-ionized by multiphoton adsorption; the states come into resonance through the Stark shifting that occurs in the strong electric fields generated by the laser pulse. By “training” the pulse shape, they can control, for example, whether acetophenone dissociates to form C6H5CO and CH3 or rearrange to form toluene and CO.

  5. Early Cities of Preceramic Peru

    The first large settlements in the Americas seem to have developed about 5000 years ago on the coast of Peru. These settlements also marked the development of irrigation and more intensive agriculture, the beginning of the construction of monuments, and the beginning of urban planning. Shady Solis et al. (p. 723; see the news story by Pringle) provide new radiocarbon dates for one of the earliest and perhaps the largest known settlement, Caral, in the Supe Valley, Peru. The dates range back to about 4500 years ago. Caral was also inland, and along with more than a dozen other settlements in the Supe Valley, may mark the beginnings of complex society in the Americas.

  6. Sorting Out Carbon Nanotube Conductivity

    Carbon nanotubes, whether the multiwalled (MWNT) or the single-walled (SWNT) variety, are formed as mixtures of metallic and semiconducting tubes. Two reports examine how structure influences conductivity and how structures can be modified to select for metallic or semiconducting properties (see the Perspective by Dresselhaus). Both the “armchair” and “zigzag” configurations of SWNTs have been thought to be metallic, but recent theoretical work that has considered the distortion in the chemical bonds cast doubt on that perception. Ouyang et al. (p. 702) obtained low-temperature scanning tunneling spectra of individual and bundles of armchair and zigzag SWNTs, and clarify that the zigzag types are not true metals but display an energy gap at the Fermi level that depends on the diameter of the tube. Device applications would benefit from ways to control the conductivity of nanotubes that connect electrodes. Collins et al. (p. 706) show that an extreme approach—running high currents through the tubes in air—selectively removed the outermost layers of MWNTs or particular tubes from SWNT bundles to create either a metallic or semiconducting contact. Entire arrays of field-effect nanotube transistors were fashioned in this manner.

  7. Managing Mitochondrial Mortality Messages

    The proteins BAK and BAX are pro-apototic members of the “BH3-domain-only” family of proteins (so-called because they share with the BCL-2 family of proteins only the third of four BCL-2-homology domains). M. C. Wei et al. (p. 727) provide evidence that BAK and BAX are essential gatekeepers for apoptotic signals that act through the mitochondria. Signals from death receptors on the cell surface cause activation of tBID (another BH3-domain-only family member), which leads to release of cytochrome c from mitochondria and subsequent cell death. Cells that lacked BAK or BAX alone were still sensitive to tBID, but cells deficient in both proteins did not undergo tBID-induced apoptosis. A broad range of signals from the plasma membrane, nucleus, and endoplasmic reticulum all required the presence of BAK or BAX and thus appear to promote cell death through signals that converge at the mitochondria.

  8. Ripping Up RNA, One Molecule at a Time

    Analysis of the folding of single molecules provides insight into the biological forces and folding behavior that are not available through bulk measurements, which are often confounded by the presence of multiple species and multiple folding pathways in solution. In a study of RNA folding, Liphardt et al. (p. 733; see the Perspective by Fernandez et al.) separately tethered three species of RNA of increasing size and complexity between two polystyrene beads. One bead is caught in an optical trap; the other is linked to a piezoelectric actuator, which is used to rip the RNAs apart. The smallest RNA, a hairpin, unexpectedly comes apart all at once, rather than unzipping slowly, as predicted from bulk measurements, and the two smaller RNAs “hop” repeatedly between the folded and unfolded states when a constant force is applied. Unlike the small RNAs, the largest RNA has a “brittle” structure that is resistant to mechanical deformation but then “fractures” beyond a certain limit.

  9. Turning Somatic Cells into Stem Cells

    Stem cells offer great potential for tissue repair in degenerative diseases and in gene therapy, but one limitation has been the need to isolate stem cells from the individual patient. Wakayama et al. (p. 740) have taken cloning and stem cell research another step forward. They produced mouse embryonic stem (ES) cells from blastocytes, which were themselves made by somatic cell nuclear transfer. The ntES cells display full developmental potential by differentiating into various cell types, including specialized neurons and gametes. The ntES cell nuclei were then used for nuclear transfer to produce viable clones. This work should enable the generation of ES cell lines from complex genetic mutants and may have significant applications to human medicine.

  10. Creating a Catenated Viral Coat

    Many viruses undergo conformational changes during maturation. Conway et al. (p. 744) now provide a view of the transition from a precursor capsid (Prohead-II) to the mature capsid (Head-II) in bacteriophage HK97. The conformational change is caused mainly by domain rotations with concomitant refolding of two motifs. The expanded and thinner mature capsid is stabilized by an increase in the surface area buried at interfaces and by cross-links that result in a catenated topology. During packaging, repulsion between DNA and the negatively charged inner surface of Prohead II may trigger the transition.

  11. Submitting to DNA Testing

    The oral polio vaccine (OPV) developed by the Wistar Institute was administered to more than 1 million people in Central Africa. There have been claims that the vaccine was grown in chimpanzee kidneys contaminated with a simian immunodeficiency virus, thus giving rise to the AIDS pandemic. Poinar et al. (p. 743; see the news story by Cohen) used the polymerase chain reaction to test for the presence of chimpanzee DNA in samples provided by the Wistar Institute. Their results identify the primate source for the vaccine as macaque monkeys and provide no evidence to support the OPV hypothesis for the origin of AIDS.

  12. Weak Layers Beneath Tibet

    The Tibetan Plateau, a large area of very thick continental crust that includes the Himalayas at its southern end, was created by the collision of India with Asia. W. Wei et al. (p. 716) completed several north-to-south magnetotelluric surveys across the plateau to determine the rheology of the crust and understand its structural evolution. They found a shallow layer of highly conductive crust beneath the southern plateau that they attribute to the presence of fluids, partial melt, or both. A deeper layer of high conductivity is inferred in the mantle beneath the northern plateau and is attributed to partial melting. Thus, this major plateau is underlain by pervasive layers of weakness that account for the geothermal activity and volcanic deposits that manifest at the surface.

  13. Probing Polymerase Pausing

    RNA stem-loop structures, or hairpins, participate in transcription pausing and termination by RNA polymerase (RNAP). Various models have been advanced to explain how hairpins work. In some of these, RNAP is treated as a rigid body through which hairpin formation pulls the newly synthesized RNA away from the enzyme's active site. Other models have suggested that the hairpin may cause a conformational change in RNAP. Using cross-linking analyses and mutagenesis, Toulokhonov et al. (p. 730) conclude that a hairpin in a transcript that causes pausing touches part of the RNAP “flap” that extends over the RNA exit channel. Thus, this hairpin induces a conformational change in RNAP that blocks nucleotide addition and results in transcription pausing.

  14. Controlling Ozone

    Urban ozone (O3)-control strategies have successfully focused on reducing automobile sources, but large amounts of tropospheric O3 can also be made in rural areas where power plants produce NOx and vegetation produces volatile organic hydrocarbons, two precursors of O3. Ryerson et al. (p. 719) used field measurements from instrumented aircraft to quantify the strong dependence rates and yields of the plume O3 formation on two key variables, power-plant NOx emissions rate and geographic location. The control of tropospheric O3 formation could be significantly enhanced if both NOx emissions concentration and location were considered, rather than only the total amount of NOx emitted.

  15. Muscle Cells on the Move

    The ligand Slit and its corresponding receptor Robo are notorious for providing guidance signals to axons finding their initial pathways during early Drosophila development. Kramer et al. (p. 737) now show that the Slit/Robo combination is even more versatile than previously thought. Muscle cells also migrate during the early stages of muscle organization under the direction of the Slit/Robo team. The response switches from repulsive to attractive; initially repelled by Slit at the midline, muscle cells are later attracted by Slit at specific attachment sites.