This Week in Science

Science  24 Apr 1998:
Vol. 280, Issue 5363, pp. 493
  1. Eyes on the nucleus

    As the site of DNA synthesis, repair, transcription, and RNA processing, the nucleus is a virtual hub of cellular activity. Relatively little is known about how many of these nuclear processes are organized in space and time. Femino et al. (p. 585; see cover) developed a quantitative fluorescence in situ hybridization (FISH) technique that allowed them to monitor the activity of single RNA molecules within the cell. Using this technique, they generated a kinetic profile of individual transcription events (initiation, elongation, termination, and messenger RNA transport) at a single b-actin allele in serum-stimulated fibroblasts. Nelms et al. (p. 590) used ultrasoft x-rays to induce DNA double-strand breaks in defined subnuclear regions of human fibroblasts and then visualized over time the relative positions of the damaged DNA and specific repair proteins. In contrast to other nuclear processes, repair does not appear to involve movement of DNA through the nucleus; rather, the repair proteins relocalize directly to the sites of DNA damage. Lamond and Earnshaw (p. 547) review current evidence on the role of distinct nuclear structures in a variety of cellular processes.

  2. Inside a hurricane

    Damage from a hurricane can vary greatly over a few tens to hundreds of meters, indicating that the intensity of hurricanes can vary on these short length scales. Observational evidence linking hurricane dynamics to these damage patterns is difficult because few measurements have high enough spatial resolution to pick up the dynamic features. Wurman and Winslow (p. 555) used a high-resolution mobile weather radar to investigate hurricane Fran in 1996 and show that intense, sub-kilometer-scale boundary layer rolls significantly modulated the near-surface wind speed and extreme wind gusts.

  3. Hierarchical switching

    Self-organization of polymeric materials can result in the formation of distinct phases, such as lamellar or tubular structures; the characteristic length scale of these structures depends on various parameters such as composition and temperature. By switching between different phases, the physical properties of the material can be changed. Ruokolainen et al. (p. 557) show that by using an appropriate diblock copolymer and by altering the chemical composition of one of the block copolymer phases with additional compounds, phase switching can be achieved on two different length scales, one at around 5 nanometers and the other at about 30 nanometers. The conductivity of the materials changes with temperature, indicating that changes in the dimensionality of the phases (such as from one-dimensional tubules to two-dimensional lamella) lead to anisotropy in the conductivity.

  4. Atomic Kondo effects

    When a single magnetic impurity is introduced into a nonmagnetic metal, at sufficiently low temperatures the host's electrons form a many-body ground state that screens the spin of the local impurity. This screening cloud exhibits a dense set of low-energy excitations known as the Kondo resonance. Madhavan et al. (p. 567) present evidence for a Kondo resonance for a single cobalt atom on a gold surface. Spectra obtained with a scanning tunneling microscope at cryogenic temperatures reveal a narrow resonance localized over the cobalt atom that the authors interpret as a Kondo resonance.

  5. Dry landings

    The fate of the Lost Colony, England's first attempt at settling North America, has been uncertain. Most settlers survived the first year, but the colony was not heard from again. By using a tree-ring climate index developed from cyprus trees, Stahle et al. (p. 564) now show that the colonists arrived just before one of the worse droughts in the region in the last 800 years beginning in 1587. The next settlement at Jamestown in 1607 was just barely successful: Only 38 of 108 colonists survived the first year. The record shows that Jamestown was settled near the beginning of a prolonged 7-year drought.

  6. Direct oxidative activation of methane

    Conversion of low molecular weight alkanes, such as methane, to liquids, such as methanol, would allow reserves of natural gas to supplement petroleum fuels and chemical feedstocks. The carbon-hydrogen bonds in these compounds are quite unreactive, and oxidation routes to generate derivatives directly must avoid total oxidation to products such as CO and CO2. Periana et al. (p. 560; see the news story by Service, p. 525) report that a platinum complex with nitrogen-bearing ligands, dichloro (η - 2-{2,2'-bipyrimidyl})platinum(II), converts methane to methyl bisulfate with SO3 as an oxidant in concentrated sulfuric acid at 220° Celsius. The yield for one pass of methane through the reaction is 72 percent, and the product can be hydrolyzed to methanol.

  7. Gender bias

    How are the various aspects of words retrieved during speech? Van Turennout et al. (p. 572) use event-related potentials and a two-stage task to suggest that the syntactic characteristics (gender of nouns) are retrieved from the brain approximately 40 milliseconds prior to phonology (the initial phoneme of nouns).

  8. Heart receptors

    Myocardial hypertrophy occurs in human diseases that place increased demand on the heart and can ultimately lead to ventricular failure. Various cell surface receptors such as α1-adrenergic receptors or angiotensin II receptors that couple to the Gq class of heterotrimeric guanine nucleotide binding proteins (G proteins) can contribute to ventricular hypertrophy. Akhter et al. (p. 574) made transgenic mice that expressed a fragment of the Gq α subunit specifically in heart tissues. This peptide interfered with signaling by all receptors coupled to Gq. When such transgenic mice were manipulated to cause pressure overload of the heart, less ventricular hypertrophy was observed in animals expressing the Gq peptide than in control animals. The results suggest that inhibition of multiple receptors that signal through Gq might be an effective strategy to prevent progression of myocardial hypertrophy to heart failure.

  9. Making a commitment

    A key aspect of control of the cell division cycle is appropriate initiation of DNA synthesis once and only once per cell cycle. Zou et al. (p. 593) show that in budding yeast, part of such control is mediated by regulated binding of the protein Cdc45 to chromatin at origins of DNA replication. Binding of Cdc45 is shown to be dependent on the activity of the cyclin-cyclin-dependent kinase complex known as S phase-promoting factor (SPF). Cdc45 interacts with other proteins (Cdc6p and Mcm2p) that associate with DNA as part of the pre-replication complex. Loading of Cdc45 onto the replication origin appears to provide a critical event by which SPF activity brings about commitment to the initiation of DNA replication.

  10. Secretory architecture

    Salmonella typhimurium is one of several diverse bacterial pathogens that has evolved a specialized secretion system (type III) for delivery of proteins into the host cell. Kubori et al. (p. 602) have visualized this secretory complex by electron microscopy. The complex spans both inner and outer bacterial membranes, and consists of a cylindrical base that resembles flagellar basal bodies and a slender, needle-like domain that projects outward from the cell surface.

  11. Clouds on Uranus

    In many images, Uranus resembles a soft blue sphere that offers little understanding of the possible dynamic motions of the atmosphere. Karkoschka (p. 570) took near-infrared images of Uranus with the Hubble Space Telescope in July and October 1997 and observed 10 clouds with high contrast compared to the atmosphere. By tracking the motions of these clouds and comparing these observations with Voyager 2 images taken in 1986, he concludes that the rate of rotation of the clouds in the southern hemisphere has not changed between 1986 and 1997. In addition, clouds in the northern hemisphere appear to be circling the planet at a slower rate than the southern clouds.

  12. B cell survival

    B lymphocytes need to bind to their antigen and to gain access to the germinal center (GC) of the spleen to become antibody-secreting plasma cells. Fischer et al. (p. 582) have observed that high-affinity antigen alone cannot ensure the survival of the B cell in the GC. Through the use of mice genetically deficient in the type 2 complement receptors (Cr2), they have determined that the GC survival signal is provided by the binding of a fragment of activated complement to the Cr2. This system may act as a safeguard against autoreactive B cells, which would fail to survive because active forms of complement are primarily abundant during infection and inflammation.

  13. Sorting amino acids

    Loading an amino acid onto its transfer RNA (tRNA) accurately is an essential step in decoding genetic information. The two-stage reaction involves recognition and activation of the amino acid, followed by recognition and attachment to the appropriate tRNA. Nureki et al. (p. 578; see the commentary by Fersht, p. 541) describe the structure of isoleucyl-tRNA synthetase and how it implements a two-sieve mechanism. First, amino acids larger or more polar than isoleucine are repulsed from the amino acid recognition site. Second, aminoacyl-tRNAs containing an amino acid smaller than isoleucine fit into and are hydrolyzed by the editing site which appears to utilize a hydrolytic module reminiscent of an aspartate protease.

  14. Conductin and wnt signaling

    Signaling by wnt is involved in diverse developmental processes that include cell fate decisions and morphogenesis. The protein β-catenin regulates gene expression in response to wnt signaling. Behrens et al. (p. 596) report the identification of a mouse axin homolog, conductin, that forms a complex with the tumor suppressor gene product APC (adenomatous polyposis coli) to direct the degradation of β-catenin. It is shown that conductin controls the stability of β-catenin. Thus, conductin represents a newly identified protein that regulates wnt signaling, a pathway that is involved in both development and carcinogenesis.

  15. Gene capture

    A structural and functional relation between the Vibrio cholerae repeated sequences (VCRs) and a class of mobile genetic elements (integrons) that have previously been associated with antibiotic resistance have been found by Mazel et al. (p. 605). The VCRs may have acted during the evolution of Vibrio as a “gene-capture” system for acquiring heterologous genes.

  16. Checkpoint role for p38

    To prevent errors in cell division, cells have a checkpoint mechanism that monitors proper assembly of the spindle (the machinery that moves the chromosomes to daughter cells). If all is not well, a signal is sent that arrests the process of cell division. In the meiotic cycle of Xenopus oocytes, mitogen-activated protein (MAP) kinase is required for proper function of the spindle checkpoint. Takenaka et al. (p. 599) report that in the mitotic cycle of somatic cells, a different MAP kinase family member, p38, is necessary for checkpoint function and can arrest cells in M phase in the absence of spindle defects. The results reveal an additional role for the p38 protein, known to be activated in cells exposed to various stressful stimuli.

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