This Week in Science

Science  26 Sep 1997:
Vol. 277, Issue 5334, pp. 1901
  1. The rise and fall of the oceanic crust

    Accurate measures of the depth of the ocean floor (bathymetry) are essential for understanding the structural and chemical evolution of the oceanic crust, the interaction of oceanic and continental plates or plate components, the dynamics of ocean circulation, and effects of all of these factors on marine biota. Smith and Sandwell (p. 1956; see the cover and the news story by MacKenzie, p. 1921) have combined the most comprehensive collection of ship-depth soundings with satellite-derived (Geosat and ERS-1) gravity data to produce a high-resolution map of global sea floor topography. From the distribution of sea floor depth, area, and age, they conclude that global bathymetry cannot be explained by a simple lithospheric cooling model in which newly formed oceanic crust moves away from mid-ocean ridges, but requires some randomly placed reheating events to raise the sea floor in some places.

  2. Growth from within

    Dendrimer molecules are monodisperse, ordered polymers with a hierarchy of branched structures grown from a central core. Their surfaces and internal voids provide sites for encapsulation of molecules and for molecular recognition. Galliot et al. (p. 1981) show how an existing dendrimer can be chemically modified so that new dendrimer units can be grown from well-defined sites in the network. The synthesis route should allow a wide range of chemical species to be grown inside the dendrimer voids.

  3. Small but strong

    Most materials never approach their theoretical strengths because defects cause fractures that lead to mechanical failures; however, small structures, such as metal whiskers, can have low defect densities and can be much stronger. Wong et al. (p. 1971) have studied the strength of silicon carbide nanorods and carbon multiwall nanotubes by immobilizing tubes at one end and then measuring forces and deflections along their exposed length with an atomic force microscope. The silicon carbide nanorods were found to be significantly stronger than their larger cousins, and the bending of multiwall nanotubes occurred through an unusual elastic buckling process.

  4. Snuggling up

    Ordered arrays of small particles offer the prospect of designing materials with optical or electrical properties that can be tuned by changing the interparticle separation, the particle size, or the particle stoichiometry. Collier et al. (p. 1978) show that when monolayers of silver particles capped with alkylthiol layers suspended on a water surface are compressed, they undergo an insulator-to-metal transition that can be observed visually as the formation of a mirror. The optical response is a characteristic of quantum interactions between the particles, that is, an overlap between the wave functions of the different particles.

  5. Familiar words to a child

    How generalizable are early learning mechanisms? Jusczyk and Hohne (p. 1984) show that infants as young as 8 months reliably identify words from lists as either familiar or novel depending on whether they were used frequently in children's stories heard 2 weeks earlier. The authors suggest that this long-term storage of words reflects not only early abilities in segmenting speech but also the development of a lexicon that associates the acoustic properties of words with their visual or conceptual representations.

  6. Clues to Huntington's disease

    Although the expansion of a polyglutamine repeat in the gene that encodes huntingtin is known to be associated with Huntington's disease, it is unclear how the gene is linked to the neurodegeneration that occurs in the striatum and cortex. DiFiglia et al. (p. 1990) may have a clue to this process that is a direct outgrowth of work done in a mouse model of the disease. Mice transgenic for a fragment of huntingtin have previously been shown to have intraneuronal nuclear structural features or inclusions and dystrophic neurites (representing axons of degenerating cortical neurons) that contain aggregates of the transgene product. The authors now show that these inclusions and dystrophic neurites are also present in the brains of individuals affected with Huntington's disease. The longer the polyglutamine repeat, the more huntingtin accumulated. The presence of ubiquitin in these structures suggests that mutant huntingtin is resistant to degradation by normal proteolytic cleavage mechanisms and that the abnormal accumulation is related to the development of disease.

  7. Telling helpers apart

    T helper cells have different functions and participate in different types of immune responses. The type 1 helper (TH1) cells are found in inflamed tissues and activate macrophages by interferon-γ production, whereas the type 2 helpers (TH2) produce interleukin-4 (IL-4) and -5 and are found at sites of allergic reaction, along with eosinophils and basophils. How the TH2 cells find the appropriate tissues has been unknown. Sallusto et al. (p. 2005) found that human TH2 cells, but not TH1, express the CCR3, a chemokine receptor that binds eotaxin. CCR3 was previously detected primarily on eosinophils and basophils, thus providing a possible explanation for the selective arrival of the major cell types that contribute to an inflammatory allergic response. CCR3 is also the only molecule shown thus far that can distinguish TH2 cells from other classes of T helper cells.

  8. Enzymes, smoke, and emphysema

    The main risk factor for emphysema, which affects 14 million Americans, is cigarette smoking. The initial accumulation of leukocytes in the airways ultimately leads to the characteristic destruction of the elastic fibers of the lung, which could be caused by the activation of a number of enzymes. Hautamaki et al. (p. 2002) found that, unlike normal mice subjected to cigarette smoke, mice that were deficient for macrophage elastase did not have increased numbers of macrophages in their lungs or develop emphysema.

  9. Oxygen origins in plants

    The generation of oxygen from water in plants and algae occurs through photosystem II, which is known to contain a tetranuclear manganese cluster and a tyrosyl radical. Hoganson and Babcock have reexamined recent data and propose a different model for water oxidation in which the tyrosyl radical acts as abstractor of hydrogen atoms, rather than simply transferring electrons, and dioxygen itself forms from water bound as terminal ligands to manganese, not from bridging oxygen species between the metal atoms. This mechanism bears similarities to other enzymatic reactions involving amino acid radicals.

  10. Solar output and climate

    Direct measurement of solar radiance have only been made over parts of the last two solar cycles, so it has been difficult to relate long-term records, such as those of sunspots, to actual changes in solar irradiance. Willson reports the latest measurements of solar irradiance from the ACRIM satellite that cover the minimum of solar cycle 22. The data imply that irradiance increased from the minimum in solar cycle 21 by 0.03 percent (or 0.5 watts per square meter), which would provide some additional solar forcing over the past decade. [See the news story by Kerr.]

  11. A handle on hydroxyl radicals

    Hydroxyl radical (OH) is perhaps the most important oxidant in Earth's atmosphere; it plays key roles in both the troposphere and in ozone-destroying reactions in the stratosphere. Summers et al. analyze recent data on the distribution of OH and ozone in the stratosphere using the data from the MAHRSI and CRISTA satellite measurements, along with recent measurements of water levels from the HALOE satellite study. The data imply that ozone loss due to OH chemistry is much less than losses predicted from models and that OH levels had been overestimated. [See the Perspective by Crutzen.]

  12. Understanding deep disorder

    In many common minerals, such as feldspars and pyroxenes, the cations, such as magnesium, iron, aluminum, silicon, and calcium, can fit in different structural sites in the crystal while maintaining a constant bulk chemical composition. A crystal is considered ideally ordered if each cation is always found in one structural site, while the crystal is ideally disordered if each cation is distributed equally and thus randomly between all of the structural sites. Hazen and Yang used in situ x-ray diffraction to measure the effect of disorder on the compressibility of pseudobrookite-type MgTi2O5 samples with various degrees of order. The samples were compressed to pressures up to 7.51 gigapascals in a diamond-anvil cell. They found by extrapolation that the ideal disordered structure was 6 percent less compressible than the ideal ordered structure, which represents an order of magnitude greater difference than that predicted by theory. Their work suggests that the amount of order-disorder in a crystal structure may be significant in determining the structure and composition of Earth's mantle; earth scientists rely on experimental and theoretical understanding of the compressibilities of minerals based on structural transformations and compositional differences, but rarely on the degree of order in the crystal structure.

  13. Controlled growth

    Protein crystallization represents a major experimental challenge; many proteins have not yet been crystallized because the conditions for forming large crystals are not known. Recent theoretical efforts to understand protein crystallization have focused on the range of interactions between the proteins and how this affects their phase diagram. Ten Wolde and Frenkel performed simulations on model protein systems and found that the free energy barrier for crystal nucleation is significantly lowered, over a narrow temperature range, as a dense fluid droplet is formed prior to the formation of a critical crystal nucleus. As the effective range of interactions of proteins, which is crucial for this observation, can be tuned through addition of non-ionic polymers, it should be possible to selectively speed up the crystal nucleation rate without also speeding up the growth rate.

  14. Actively at rest

    Mature T cells emerge from the thymus in a resting state. Once they meet the antigen for which they have a specific receptor, they become activated, proliferate, and most of them eventually die. The unstated assumption has been that the resting state is the default state. Kuo et al. now show that not only is this assumption incorrect, but that a particular transcription factor, LKLF, is responsible for actively maintaining this state of quiescence. Mice made deficient in LKLF lacked mature T cells in the blood stream and the few T cells in the spleen and lymph nodes responded as if they were spontaneously activated and were undergoing activation-induced cell death, without exposure to antigen. [See the Perspective by Freitas and Rocha.]

  15. Enzymatic teamwork

    An unusual fusion protein in coral that is specialized for the metabolism of a fatty acid hydroperoxide is described by Koljak et al. The protein consists of a lipoxygenase that forms the hydroperoxide substrate and a catalase-related heme peroxidase that catalyzes the next step of enzymatic transformation. The reaction end product is an allene oxide, a putative prostaglandin precursor.

  16. A complex about DNA

    DNA-(cytosine-5) methyltransferase (MCMT) methylates newly replicated DNA in a tightly controlled manner. Chuang et al. show that MCMT binds to proliferating cell nuclear antigen (PCNA), an auxiliary factor for DNA replication and repair. The PCNA binding domain was required for localization of MCMT to DNA replication sites, and the interaction with PCNA could be disrupted by the cell cycle inhibitor p21WAF1. The authors propose that PCNA recruits MCMT to newly replicated DNA, and p21WAF1 prevents DNA methylation by blocking access of MCMT to PCNA. [See the Perspective by Baylin.]

  17. Spinal cord splice

    Damage to the spinal cord is particularly serious because the axons damaged normally do not rebuild their connections. Li et al. have found that, by transplanting olfactory ensheathing cells into the site of a spinal cord lesion, the spinal cord axons can be encouraged to regenerate. For the very specific motor skill tested in these rats, such regeneration correlated with recovery of the motor skill. This holds some promise for lesions of the human spinal cord, but caution is in order, as those lesions are not often of the transection type tested here.

  18. Entering bacterial hideouts

    Many major bacterial pathogens live within the cells of the host to avoid some of the host's immune protections. If the bacterial genes that are vital for their survival were known more options would exist for designing potential antimicrobial therapies. Valdivia and Falkow have developed a method to select for such genes. They constructed a selection plasmid that contains the fluorescent green protein gene and pieces of the Salmonella typhimurium genome. After transfection into S. typhimurium, the bacteria were allowed to invade macrophages. Bacteria from fluorescent macrophages were grown in agar and then only those that did not fluoresce were used to reinfect mammalian cells. This simple system selects for genes preferentially expressed while the bacterium is intracellular and has the potential to be used with bacteria that have as yet intractable genetic systems.

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