This Week in Science

Science  17 Dec 1999:
Vol. 286, Issue 5448, pp. 2229
  1. Quantum Ratchets

    Particles can be made to move in one direction by applying a time-varying asymmetric potential. For proteins and small molecules, these “ratchets” act classically, but quantum-mechanical effects have been predicted for particles that can tunnel through their potential barriers. One manifestation is that the ratchet would reverse the net direction of particle flow as a function of temperature. Linke et al. (p. 2314) have now verified this effect experimentally for electron flow in a semiconductor heterostructure and present a simple model that explains why current reversals occur.

  2. Coherent Increases in Condensates

    Recent work has shown that Bose-Einstein condensates exhibit superradiance—the interaction between coherent light and the condensate can set up a grating of atoms with well-defined momentum. Kozuma et al. (p. 2309) have controlled this emission process by using a small, well-defined seed condensate and by applying well-timed laser pulses that add more atoms to the grating. They demonstrated amplification of the initial matter wave by a factor of 10. Moreover, they show that this amplification is coherent—it maintained the phase of the initial seed condensate. This matter wave amplifier is analogous to the optical wave amplifier that revolutionized the optical laser.

  3. Viewing Impurities

    Dislocations and other defects have long been known to affect the distribution of impurity atoms in the material, which are enriched in the vicinity of the defects. Such “Cottrell atmospheres” lead, for example, to strain-aging in steels. However, direct imaging of this effect, which could shed light on enrichment rates, has been hampered by instrument limitations. Blavette et al. (p. 2317; see the Perspective by Miller) used a three-dimensional atom probe technique to image such enrichments on an atomic scale in iron-aluminum alloys. The increased solute concentrations are accompanied by an unexpected depletion in aluminum, which is likely to affect the plasticity of the material.

  4. Dust in the Solar Wind

    Interstellar dust grains in the outer solar system have been sampled during the last 10 years by the Ulysses and Galileo spacecraft. Landgraf et al. (p. 2319) examined the dust detector data and noticed a deficit of grains within a specific mass range at distances between 2 to 4 astronomical units (the Earth-sun distance) from the sun. In a model study, they show that this deficit could arise through solar radiation pressure overwhelming solar gravitation for these specific grains, thus removing them from this narrow region. The authors' model is consistent with grains composed of astronomical silicates, magnetite, or graphite.

  5. Flat Colloidal Crystals

    The assembly of colloidal particles of one charge and surfactants of the opposite charge normally creates three-dimensional fractal aggregates. Ramos et al. (p. 2325) examined the interaction of negatively charged latex particles with vesicles made from neutral and cationic surfactants. In certain composition ranges, the latex particles assembled into two-dimensional (2D) crystals of several hundred particles that were robust against dilution or shearing. In the authors' model for crystal formation, the latex particles adsorb onto a vesicle until charge is neutralized and then attract to form a “raft.” The bilayers present on the particles eventually fuse and form the final crystal.

  6. Pocketing the Template

    RNA polymerase from T7 bacteriophage (T7RNAP) is a single subunit enzyme that binds to specific promoter DNA sequences and initiates transcription. During the initiation phase, short RNA products are repeatedly synthesized and released until a transition occurs to give a stable elongation complex that can transcribe the complete T7 phage DNA. Cheetham and Steitz (p. 2305) present a 2.4 angstrom structure of a transcribing T7 RNA initiation complex that provides insight into how the enzyme selects for synthesizing RNA and how RNA products up to about 8 nucleotides long can be formed without breaking promoter contacts. The authors provide evidence for a “scrunching” model in which the DNA template accumulates in the active site pocket of T7RNAP during the early stages of transcription: Once the pocket is filled, either the RNA product will be abortively released or T7RNAP will dissociate from the promoter to allow entry into the processive elongation phase.

  7. Two for Nun

    In transcribing DNA template into RNA, either sequence elements or protein factors can signal the end to transcription. The phage factor Nun stops transcription of phage template by binding to the nascent transcript and blocking the progress of bacterial RNA polymerase (RNAP). What are the molecular interactions between Nun and the transcription machinery that enable the phage factor to stop the large bacterial RNAP? In the presence of zinc, the Nun COOH-terminus inhibits its own RNA binding by blocking the amino-terminal binding domain. However, another phage protein, NusA, can stimulate the binding of Nun to RNA. Watnick and Gottesman (p. 2337) studied these interactions further with photochemical cross-linking analyses and found that Nun directly contacts the nascent RNA, RNAP, and double-stranded DNA. Contact with DNA occurs at the DNA binding site of RNAP. Nun may arrest transcription by acting to anchor the RNAP to DNA. Thus, Nun, along with RNAP, contacts both the nascent RNA and template DNA.

  8. DNA Helicase Defects

    Bloom's syndrome and Werner's syndrome are two human disorders that are characterized by growth retardation and a high incidence of cancers. The genes reported to be mutated in these disorders, the Bloom's syndrome gene BML and Werner's syndrome gene WRN, encode DNA helicases. Two reports focus on action of homologs of BML and WRN, the yeast gene SGS1 and Neurospora gene qde-3, respectively. Cogoni and Macino (p. 2342) cloned the qde-3 gene and show that it participates in post-transcriptional gene silencing, which suppresses the expression of a foreign gene when introduced into a cell. Lee et al. (p. 2339) showed that mutation of SGS1 and another helicase homolog SGS2 resulted in defects both in transcription by the RNA polymerase I molecule and in DNA replication. The combination of gene silencing, transcription, or replication effects exhibited by homologs of the Bloom's and Werner's syndrome genes may contribute to the various genetic defects observed in the human disorders.

  9. A Protein Version of tRNA

    The translation process that synthesizes peptides is ended by release factors that liberate the completed peptide and a ribosome recycling factor (RRF) that participates in the release of the messenger RNA and the fission of the two ribosomal subunits. Selmer et al. (p. 2349) now provide the crystal structure of RRF and find that it looks remarkably like a molecule of transfer RNA (tRNA). The two molecules each contain two arms of the same dimensions that are set at the same angle. The only major difference is that RRF does not extend to the 3′-CCA terminus of the tRNA, the spot where the amino acid is attached. The shape similarity suggests that RRF binds to the tRNA entry site on the stationary ribosome during disassembly.

  10. Inflammation in Alzheimer's Disease

    Activation of microglial cells in the brain may be responsible for the inflammatory component of Alzheimer's disease. Tan et al. (p. 2352) now show that cultured microglial cells exposed to the pathogenic amyloid-β peptide (Aβ) exhibit an increase in surface expression of CD40, a receptor that is important in the inflammatory response. Exposure of Aβ-treated cells to the ligand for CD40 resulted in activation of microglial cells and an increase in their production of the inflammatory cytokine tumor necrosis factor α. These findings suggest that Aβ can promote the interaction of microglial cells with CD40 ligand that leads to inflammation.

  11. Development's Role in Cognitive Deficits

    Normal adults who suffer brain damage can exhibit a cognitive performance pattern in which some abilities are spared while others are significantly diminished. These phenotypes often are used as models for cognitive deficits observed in various genetic disorders, one of which is Williams syndrome (WS). Adult WS individuals perform relatively poorly on visual-spatial and numerical tasks while displaying approximately normal verbal skills. Paterson et al. (p. 2355; see the Perspective by Bishop) study a group of WS infants and find that they do well on number but less well on language. This dissociation suggests that cognitive development may not occur in strictly modular fashion and that cognitive profiles in genetic disorders may change during development.

  12. Antibiotic Action

    Expanded usage of antibiotics has led an alarming increase in resistance of bacteria to the currently available store of antibiotics and spurred the search for new classes of antibiotics. Breukink et al. (p. 2361.) have examined the mechanism of action for a promising peptide antibiotic called nisin Z. Nisin is derived from Lactococcus lactis and is a common food preservative. Like vancomycin, nisin Z targets the Lipid II bacterial membrane component. However, unlike vancomycin, nisin Z performs its bactericidal effect by poking holes in the bacteria. Insights into the actions of nisin may assist in developing a new class of highly efficient antibiotics.

  13. A Working Light Saver

    Optically based circuitry will need a method for storing and retrieving optical signals similar to the static and dynamic memory devices currently used in electronics. Lundstrom et al. (p. 2312) report a storage medium based on two coupled quantum dots. A photon creates an electron-hole pair, which separates and then is stored separately on the two quantum dots. After several seconds, an applied bias brings the electron and hole together again. This recombination event re-emits a photon, but the overall delay in recombination brought about by storage is about ten orders of magnitude longer than the natural decay time of an electron-hole pair. [See the Perspective by Kotthaus.]

  14. Rapid Water Movement

    The concentration of different isotopes of oxygen in rocks can be used to determine the amount, rate of flux, and type of fluid that may have moved through the rock during its formation and during any subsequent alteration. Mora et al. (p. 2323) have used in situ secondary ionization mass spectrometry to determine the oxygen isotopic composition of two different plagioclase grains (anorthite and andesine) from the Boehls Butte anorthosite. They found the anorthite and adjacent andesine grains to be extremely depleted in the heavy oxygen isotopes, whereas the andesine grains showed a steep gradient from depleted to normal oxygen isotopic concentrations over the length of the grains (micrometer-scale distances). These unusual depletions and steep gradients record the distinctive heterogeneous history of the rock environment. In the anorthosite, the oxygen isotopic concentrations record a rapid influx of a relatively large amount of meteoric water at temperatures greater than 500°C, followed by the rapid loss of water through channels or cracks formed by the rapid uplift of the rock, the rapid cooling of the rock, a negative volume change related to fluid-rock reactions, or a combination of all three processes.

  15. Estrogen Receptors and Development

    Estrogen is a steroid hormone produced by the ovaries that binds to the estrogen receptors ERα and ERß. Knockout studies have used homologous recombination to eliminate either version of ER from the mouse to determine the roles of these receptors, but the interpretation of these studies has not been straightforward because ERα and ERßmay have compensatory roles in development and because the remaining receptor could interact with maternal estrogen. Couse et al. (p. 2328) have now used homologous recombination to produce the double-knockout mouse αβERKO. The resultant males were infertile and resembled αERKO males. The prepubertal females displayed a relatively normal reproductive tract, but the adult females displayed an ovarian phenotype in which redifferentiation was taking place: The follicles degenerated into seminiferous tubule-like structures like those of the testis and contained cells with a Sertoli cell phenotype. Hence, both ERs are required for ovarian function and oocyte survival in the adult.

  16. A New Strain for the United States

    In August and September of 1999, an outbreak of human encephalitis occurred in New York that was shown to be caused by West Nile Virus, a flavivirus, and that was also associated with deaths of a variety of birds. Lanciotti et al. (p. 2333) and Anderson et al. (p. 2331) did independent phylogenetic analyses of viral genomic sequences obtained from infected birds, mosquitoes, and humans in the New York City area and in Connecticut and found that the problem resulted from infection by a single strain of virus of a particular lineage. Although this virus has been found in many parts of Africa, Asia, and Europe, these isolates closely resembled sequences found previously in Israel and in Romania. The virus has not previously been seen in the United States, and the future impact on public health is not yet known. [See the news story by Enserink.]

  17. What It Takes to Be an Enzyme

    The ability of protein enzymes to accelerate reactions has been ascribed to sequestration from solvent, the geometric arrangement of amino acid side chains, and stabilization of the transition state relative to the reactants. However, how an enzyme evolves to achieve this aptitude has been elusive. Xu et al. (p. 2345) describe the crystal structure of an antibody that catalyzes the Diels-Alder reaction and compare its structure to those of less capable antibodies derived from the same germ-line precursor. The improved chemical efficacy derives from a tight complementarity between the active site and the transition state—so tight that both polar and nonpolar interactions between the protein and substrate are fit precisely.

  18. Getting Up the Nerve

    During development, sympathetic neurons require nerve growth factor (NGF) to survive. Riccio et al. (p. 2358) describe a mechanism by which signals initiated by NGF promote cell survival. They find that the transcription factor CREB (cyclic AMP response element-binding protein) is required for the effect of NGF and sufficient by itself to promote cell survival. They also identify a target gene that appears to participate in the survival response. Expression of the gene encoding Bcl-2, a protein that acts to prevent apoptosis, was increased in cells exposed to NGF. Furthermore, overexpression of Bcl-2 prevented cell death caused by inhibition of CREB function. Thus, CREB appears to have an essential role in promoting neuronal survival, at least in part through controlled expression of survival factors.

  19. Visual Mechanisms

    Lee and Blake (Reports, 14 May, p. 1165), designed an experiment to test whether the visual system could resolve larger geometric objects using only unpredictable but synchronized changes among local features. Their results led to the suggestion that there may be a new visual mechanism sensitive to temporal structure. Adelson and Farid comment that these results “can be explained with well-known mechanisms” because in their experiments, which involved changes in the temporal structure of an object and background, “there will be moments by chance when one region's contrast is high while the other's is low…”Lee and Blake respond that “to assert that these infrequent, hypothetical events explain the perception of form seems conjectural” but “agree that there is no need to posit the existence of new visual mechanisms.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/286/5448/2231a

Navigate This Article