This Week in Science

Science  17 Jul 1998:
Vol. 281, Issue 5375, pp. 305
  1. Hot Electrons for Spin-Valve Transistors

    Layered structures of magnetic and nonmagnetic metals can produce a “spin-valve” effect useful in magnetic sensing—application of a magnetic field aligns the magnetizations of the magnetic layers and lowers the resistance for electrons aligned parallel to the field. For transistors based on the spin-valve effect, interfacial defects between metal and semiconductor layers have tended to limit the operation of these devices to cryogenic temperatures. Monsma et al. (p. 407; see the Perspective by De Boeck, p. 357) show that room-temperature devices can be prepared by vacuum bonding; a platinum layer bonds the silicon emitter to a metal-multilayer- silicon collector. The collector accepts only “hot” ballistic electrons transported through the layers rather than parallel to them, which makes the device highly sensitive to magnetic fields.

  2. Syphilis Genome

    The agent that causes syphilis, Treponema pallidum, cannot be cultured continuously in vitro, which has hampered studies of its pathogenesis and virulence. The complete genome sequence of T. pallidum has now been determined by Fraser et al. (p. 375; see the Policy Forum on syphilis elimination by St. Louis and Wasserheit, p. 353, and the news story by Pennisi, p. 324). Analysis of the metabolic pathways used by this spirochete should aid in efforts to culture the organism, and evaluation of the outer membrane proteins indicates possible vaccine candidates. Comparison with other bacteria, such as the Lyme disease agent Borrelia burgdorferi and another metabolically limited bacterium Mycoplasma genitalium, provide clues to the basis for microbial diversity.

  3. Pinning down Methyl Bromide

    Methyl bromide is the main atmospheric component containing bromine and plays a role in ozone destruction in the stratosphere. Many of the sources and sinks of methyl bromide are incompletely understood, which results in large uncertainties in estimating the relative importance of natural and anthropogenic sources and the residence lifetime of this compound (which is an important factor in determining how much will reach the stratosphere). Colman et al. (p. 392) now provide an independent estimate of the residence lifetime, independent from source and sink estimates, which is based on determining its spatial variability relative to other atmospheric compounds with known lifetimes. The result is consistent with previous estimates from source and sink strength.

  4. From Water to Ice

    Small amounts of water in silicate magmas, either dissolved as hydroxyl ions (OH) or present as molecular water, can change the bulk properties of the melt and alter the ascent rate of the magma, which affects not only eruptive processes observed on the surface but also the amount of chemical fractionation in magma reservoirs below. Richet and Polian (p. 396) determined the partial molar volume and bulk modulus of water dissolved in iron-free, andesitic glasses through density and Brillouin-scattering measurements. The volume and bulk modulus of the dissolved water was independent of the water concentration and similar to ice VII. Ice VII is a network of H4O tetrahedra formed by shortening of hydrogen bonds, which suggests that the main mechanism of compression of water in the glasses may also be hydrogen bond shortening through a clustering of OH ions.

  5. L.A.'s Seismic History

    The greatest seismic risk in Los Angeles may not be from the more distant San Andreas fault but from smaller nearby thrust faults, such as the ones that ruptured during the 1994 Northridge (magnitude M 6.7) and 1971 San Fernando (M 6.7) earthquakes. The size and frequency of potential earthquakes on these faults has been uncertain because the record of prehistoric ruptures is poorly resolved, and there are some suggestions that earthquakes greater than M 7 are unlikely. Rubin et al. (p. 398) obtained a paleoseismic record for the Sierra Madre fault, which extends along the base of the San Gabriel Mountains and ruptured in the 1971 earthquake. Offsets of markers in a trench through the fault and radiocarbon dates imply that the fault ruptured twice in large magnitude events (7.2 to 7.6) during the past 15,000 years.

  6. Digging into Old Diets

    Understanding the life-styles of extinct animals is a daunting task that has been made easier by findings of Poinar et al. (p. 402; see the news story by Stokstad, p. 319). They have determined that it is possible to amplify DNA sequences from a coprolite (ancient dung) of a ground sloth that lived in the Pleistocene era. This analysis could be performed only after the DNA was first freed from sugar-derived condensation products through an agent that cleaves chemical crosslinks. Analysis of the amplified DNA, which ranged in size from 153 to 273 base pairs, indicated that the sloth had ingested representatives of six families and two orders of plants.

  7. Self-Assembling Protein Gels

    Materials that can reversibly transform from liquids to gels with changes in temperature or pH have numerous uses, such as controlled release of drugs, yet the materials that show this behavior as hydrogels are complex and their behavior at the molecular level is often poorly understood. Petka et al. (p. 389) have used protein engineering to create reversible hydrogels that undergo transitions in gelation near neutral pH and ambient temperatures; leucine zipper domains were incorporated that can aggregate through coiled-coil interactions to control the formation of three-dimensional networks.

  8. Regulating Notch

    The specification of tissue boundaries in vertebrates and invertebrates has been shown to involve the Notch family of receptors. In Drosophila, Notch is broadly distributed in the wing; however, wing development requires tight regulation of Notch, limiting target gene activation to the dorsoventral boundary. One mechanism to limit Notch activity involves the protein Fringe, where Fringe modulates the sensitivity of Notch for its two ligands. Neumann and Cohen (p. 409) report another Notch regulatory mechanism. For this mechanism, the POU domain protein Nubbin competes with Notch-dependent activating signals to repress Notch target genes, further limiting Notch activation to cells near the wing margin.

  9. Getting the Guide into the Work

    The decoding of genetic information begins with RNA polymerase, the enzyme that uses double-stranded, helical DNA as a template to transcribe a single strand of RNA. The helix must be unwound and the hydrogen bonds between the deoxynucleotide pairs must be broken in order to sequentially order substrate ribonucleotides before polymerizing them into RNA. Nudler et al. (p. 424) have mapped the relative locations of portions of the enzyme complex and the DNA-RNA hybrid at various stages of transcription. The catalytic site where polymerization occurs is near the portion of the enzyme that guides the nascent strand of RNA. This juxtaposition may explain how double-stranded regions of RNA promote termination of transcription.

  10. Doubly Fast

    It has been widely accepted that a single neuron could only release one neurotransmitter, and even the discovery of peptides and slowly acting neuromodulators has not changed this general rule for fast neurotransmitters. Jonas et al. (p. 419; see the Perspective by Nicoll and Malenka, p. 360) show that inhibitory interneurons in the spinal chord corelease two fast neurotransmitters, glycine and γ-aminobutyric acid (GABA). They also provide evidence that a substantial amount of miniature postsynaptic currents are composed of a GABA and a glycine component.

  11. Adapter Protein Activities

    Some white blood cells express an “adapter protein,” SLP-76, which has been thought to bring together various types of signal transduction molecules in T cells after the T cell receptor binds an antigen. Yablonski et al. (p. 413) determined that SLP-76 is critical for transmission of activation signals through phospholipase C-γ1 and the Ras-signaling pathway. In an examination of SLP-76-deficient mice by Clements et al. (p. 416), SLP-76 was found to be required for the maturation of T cells, but not for B cells or other lineages. Together, these studies provide a framework for understanding the mechanism of action and the in vivo function of SLP-76.

  12. Beetle Chemistry

    The pupa of the squash beetle (Epilachna borealis) is the unusual source of a combinatorial library described by Schröder et al. (p. 428; see the cover and the news story by Campos, p. 321). The family, comprising a hundred polyamino macrocyclic structures, is derived from just three parent components. Termed polyazamacrolides, these compounds are involved in the defense of the insect against predation, although their precise mode of action and the possible evolutionary benefits conferred by the combinatorial phenomenon remain to be determined.

  13. Feasibility of a Colliding Beam Fusion Reactor

    N. Rostoker et al. proposed (21 Nov., p. 1419) a “colliding beam fusion reactor” with “an alternate confinement system, the field-reversed configuration [that would confine] beams of protons and boron-11.” The design might “solve the major problems of Tokamak reactors.” W. M. Nevins discusses “nonthermal ion distributions” and concludes that “the fusion gain … would be much lower than the value of 2.7 stated by Rostoker et al.” A. Carlson comments that “a large power input would … be required” and that maintaining equilibrium would be “a serious problem.” Rostoker et al. respond to Nevins by providing detailed calculations “of heating of electrons by boron scattering.” They address Carlson's concerns by presenting a “formula for the power density required to overcome the friction between proton and boron beams” that includes the force of the magnetic field. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/281/5375/307a

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