This Week in Science

Science  15 Jan 1999:
Vol. 283, Issue 5400, pp. 297
  1. Mass-Independent Isotope Fractionation

    Exchange of isotopes as part of a chemical process (reaction or diffusion) almost always reflects the relative masses of the isotopes involved. However, several gases—notably ozone, carbon dioxide, nitrous oxide, and carbon monoxide—and some meteorite grains and minerals seem to preserve a mass-independent relation among the isotopes. As discussed by Thiemens (p. 341) in a review, this effect can be used as a tracer or to provide information on the origins or sources and sinks of the gases, even though the mechanism responsible for the mass-independent fraction may remain obscure. One example of such an effect is the enrichment in the atmosphere of the heavier isotopes (17O and 18O) in ozone relative to the background levels. Asymmetric molecules (such as 16O17O18O) carry the highest enrichments, whereas symmetric molecules (such as 17O17O17O) are slightly depleted. Most explanations for this observation invoke kinetic effects induced by molecular symmetry. Mauersberger et al. (p. 370) now report rate coefficients for a range of ozone selection channels with different isotopic compositions. The results suggest that the collision process itself, rather than molecular symmetry, determines the enrichment.

  2. Neutral Hydrogen on Io

    Io, Jupiter's closest large satellite, creates an enigmatic oxygen- and sulfur-rich plasma torus around Jupiter through the satellite's active volcanism and interaction with Jupiter's strong magnetic field. Roesler et al. (p. 353) used the Hubble space telescope imaging spectrograph to obtain far-ultraviolet images of neutral oxygen, sulfur, and hydrogen Lyman-α emission of Io's extended atmosphere. The temporal and spatial variations of the oxygen and sulfur suggest that volcanic emissions must be altered or accelerated by the jovian magnetic field, the plasma torus, or both. The hydrogen signal localized near each pole of Io was unexpected and may be produced by hydrogen-rich polar frost, dissociation of hydrogen concentrated at the poles by the jovian magnetic field, or reflection of the solar hydrogen Lyman-α radiation from Io's surface.

  3. Still Waters Fix Least

    One commonly predicted effect of climate change induced by greenhouse gases is an increase in the stratification of the Southern Ocean, a major area of primary production. Arrigo et al. (p. 365) show that this change may further hinder the ability of the plankton communities to decrease atmospheric CO2by fixation. Their studies of the plankton dynamics in the Ross Sea off Antarctica show that diatoms (which form silicate, not carbonate bodies) dominate in highly stratified waters, whereas Phaeocystis antarctica assemblage, which consume a much greater amount of CO2, dominate in mixed regions.

  4. Ordered Porous Polymers

    Unusual morphologies often result when two different polymer types are joined in one molecule. Rod-coil diblock copolymer, for example, join a rigid polymer based on conjugated monomers (such as polyphenylquinoline, PPQ) with a flexible polymer that forms coils (such as polystyrene, PS). Jenekhe and Chen (p. 372) show that PPQ-PS diblock copolymers, when placed in solvent (carbon disulfide) that dissolves only the flexible coil, form micelles with diameters of a few micrometers. Solvent evaporation produces a well-ordered porous material that may have several applications, including use as a photonic band gap material.

  5. An Omnivorous Hominid?

    Early hominids were thought to forage mainly in wooded areas for fruits and C3 plants (for example, trees, bushes, and herbs). Sponheimer and Lee-Thorp (p. 368; see the Perspective by Kohn and the news story by Vogel) present carbon isotopic evidence from the enamel of teeth from Australopithecus africanus, which suggests that this 3-million-year-old ancestor ate more than fruits and leaves. The enamel shows enrichment in 13C consistent with a diet that includes C4 plants (for example, grasses and sedges) found in more open grasslands, animals that consumed C4plants, or both. Thus, A. africanus may have had a more diverse diet that included ranging into open grasslands or preying on small, vulnerable mammals without the apparent use of any tools.

  6. Interbase Electron Transfer in DNA

    Studies of electron transfer through DNA have produced conflicting results, from ultrafast “molecular wire” behavior over many base pairs [characterized by β values of 0.1 per angstrom (Å−1) or less] to strongly distance-dependent transport more typical of proteins (β values nearer to 1 Å−1). One confounding issue in several previous studies showing ultrafast behavior is that the donors and acceptors have been species intercalated into DNA. Kelley and Barton (p. 375) now study electron transfer in DNA molecules that incorporate fluorescent analogs of adenine (1,N6-ethenoadenine and 2-aminopurine). Electron transfer rates deduced from fluorescence quenching by guanine or deazaguanine bases yielded β values ranging from 0.1 to 1.0 Å−1. They conclude that stacking interactions and reactant energetics strongly influence the distance dependence and that electrons are preferentially transported between bases on the same strand.

  7. Freezing Epitopes for HIV-1 Vaccines

    The initial interactions between the human immunodeficiency virus-type 1 (HIV-1) envelope protein gp120 and the host cell result in a series of conformational changes in the viral envelope that result in cell-virus fusion. LaCasse et al. (p. 357; see the Perspective by Montefiori and Moore) used formaldehyde fixation of cells expressing viral envelope protein co-cultured with cells expressing CD4 and the coreceptor CCR5 to capture an intermediate structure. This epitope complex, which has been “frozen” on the expressing cell, was used as an immunogen in a mouse model. Unlike the currently available recombinant gp120 vaccine candidates, the “fusion-intermediate” immunogen stimulated production of antibodies that neutralized a broad array of primary HIV-1 isolates.

  8. Putting Pathways Together

    Although the biochemical signaling pathways through which cells respond to various stimuli have generally been elucidated as linear chains of reactions, such pathways interact to form a complex network. Bhalla and Iyengar (p. 381; see the Perspective by Lisman and Fallon) used computation methods to model networks that comprised up to four individual signaling pathways. All of the model parameters were derived from experiment. The results show that such networks can have properties not present in the individual pathways that may be critical for proper cellular regulation. Combinations of common signaling pathways showed, for example, the capacity to remain activated after an initial stimulus was removed and had specific requirements for the amplitude and duration of a signal to reach a threshold for activation. Such coordination may provide a mechanism for storing information.

  9. Change for the Better

    Mutations, generally deleterious and occasionally advantageous, provide the grist for adaptive evolution. De Visser et al. (p. 404) have investigated the complex relation between the rate at which new mutations arise and the course of adaptive evolution in bacterial populations evolving in the laboratory. In small or well-adapted populations, their evolution is limited by the rate at which they can produce new mutations. In large or poorly adapted populations, the rate of new mutations has little effect on the rate of evolution. Thus, a hypermutable phenotype can be advantageous in the right circumstances.

  10. Immune-Deficiency Link

    Mice and humans who lack Bruton's tyrosine kinase (Btk) have an immunodeficiency that primarily affects B cell development and production of serum immunoglobulin. Suzuki et al. (p. 390) and Fruman et al. (p. 393) generated mutant mice, or mice in which only lymphocytes were affected, that were deficient in the p85α regulatory subunit from phosphoinositide-3 kinase (PI3K). These mice had a phenotype similar to that of the Btk deficient mice.

  11. Hyperactivity Paradox

    Psychostimulant drugs have been used for many years to treat children with attention-deficit hyperactivity disorders (ADHD). This therapy was introduced after empirical observations without an understanding of this paradoxical effect. Gainetdinov et al. (p. 397; see the news story by Marx) analyzed mice lacking the dopamine transporter gene. These animals showed striking similarities in their behavior to ADHD children and could also be calmed down by psychostimulants. The paradoxical effect depends on normal serotoninergic neurotransmission. These findings may serve as a useful model to develop new therapies to improve these widespread childhood behavioral defects.

  12. Cannabis, Opiates, and Reward Mechanisms

    Much debate has centered around the linkage between cannabis use and subsequent use of opiates. Ledent et al. (p. 401) analyzed mutant mice that were lacking the central cannabinoid receptor CB1. These animals did not respond to any of the cannabis derivatives, and none of the typical CB1 receptor-mediated effects could be observed. Although opiates had no unexpected behavioral effect, mutant animals showed less addiction and also less severe withdrawal symptoms. It seems as if CB1 receptors are involved in the manifestation of physical dependence in opiate addiction. This finding may further our understanding of the processes that lead to drug addiction and may also help to develop new strategies to combat drug addiction.

  13. Slow but Steady Mixing

    Diffusion rates, although generally small, still can have important impacts on chemical processes and heterogeneity on large scales by controlling mineral transformations and element partitioning. Chakraborty et al. (p. 362) investigated cation diffusion in olivine and its high-pressure equivalent, wadsleyite, both of which are major constituents of Earth's upper mantle. The data imply that diffusion in wadsleyite is much more rapid than in olivine; thus, chemical homogenization is more likely in Earth's transition zone (depths of about 400 to 660 kilometers) than at shallower depths.

  14. Fostering Cell Transformation

    The fos proto-oncogene encodes a transcription factor that, when inappropriately expressed, leads to cell transformation. In a screen for target genes of Fos that function as effectors of cell transformation, Bakin and Curran (p. 387) identified dnmt1, the gene encoding DNA 5-methylcytosine transferase. Expression of dnmt1 was shown to be necessary and sufficient for cell transformation. Thus, Fos may regulate gene expression both directly and indirectly through alterations in DNA methylation.

  15. Investigating the Possible Use of Fire at Zhoukoudian, China

    S. Weiner et al. (Reports, 10 July, p. 251) reexamined “the oldest reliable evidence for the controlled use of fire by humans” in Layer 10 at Zhoukoudian (the Peking Man site). They found “burned and unburned bones … present in the same layer with stone tools,” but “no ash or charcoal remnants.” They concluded that there was only “indirect evidence for burning.”

    X. Wu comments that Weiner et al. “do not fully address the wealth of data collected at the entire site since the 1930s, and they do not adequately account for problems of material transport by water that apply to the particular sample they studied.” He states that other studies do “show that fire was used in this site.”

    In response, S. Weiner et al. state that the “demonstration of free carbon” in earlier studies “does not necessarily imply that the organic matter was burned.” They agree with Wu that there is “evidence in support of fire use” and that “many open questions” remain, but state that there is not “conclusive, direct evidence for the use of fire by Homo erectus.

    The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/283/5400/299a

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