This Week in Science

Science  19 May 2000:
Vol. 288, Issue 5469, pp. 1133
  1. Up from the Deep

    Deep samples from the mantle provide direct evidence of mantle composition and processes. Most samples are diamonds and rock fragments known as xenoliths brought up in volcanic eruptions, or recovered along subduction zones, from depths of less than 150 kilometers (km). The deepest samples, perhaps even extending into the lower mantle (depths of 660 km or more), have been represented by diamonds and diamond inclusions or altered xenoliths. Collerson et al. (p. 1215) discuss evidence which suggests that xenoliths may have been brought to the surface from depths greater than 470 km on the island of Malaita, Southwest Pacific. The mineral assemblage in these xenoliths contain majorite (a phase thought to be present in the transition zone between the lower and upper mantle), diamond, and other phases.

  2. Io Poses for a Close-Up

    At the end of 1999 and the beginning of 2000, the Galileo spacecraft completed three close fly-bys of Io. Five reports describe high-resolution observations from this mission, as well as some made with the Hubble Space Telescope (HST), and provide new insight into the volcanic processes that continue to shape Io (see the cover). McEwen et al. (p. 1193) present images at a resolution of 5 to 500 meters per pixel of active lava lakes, an erupting curtain of lava and oddly shaped mountains influenced mainly by gravitational collapse rather than volcanism. They conclude that the active volcanism is related to fluid silicate magmas rather than sluggish sulfur magmas, although some of the bright flows might be sulfur-rich. Photopolarimeter-radiometer data presented by Spencer et al. (p. 1198) and near-infrared mapping by Lopes-Gautier et al. (p. 1201) suggest that the caldera floors are relatively cool and that the reddish-deposits surrounding active volcanoes represent dustings of exotic sulfur deposits (more polymerized species such as S3 and S4). The HST observations of Spencer et al. (p. 1208) provide evidence for gaseous S2 in the plume of Pele. Estimates of the SO2/S2 ratio indicate that the silicate magmas may be buffered by an oxygen fugacity similar to conditions on Earth. Finally, Kieffer et al. (p. 1204) model the movement of Prometheus' plume, which has moved between 75 to 95 kilometers in 20 years. They suggest that lava flows moving over a sulfur-rich snowfield causes enhanced vaporization of the sulfur species.

  3. Zipping Up Nanotubes

    Theoretcial studies suggest that two single-walled nanotubes can coalesce to form a single nanotube with a wider diameter, but direct experimental evidence for such a process has been lacking. Terrones et al. (p. 1226) have now captured the coalescence process using high-resolution transmission electron microscopy. They also investigate coalescence through molecular dynamics and Monte Carlo simulations and suggest that a zipper-like mechanism triggered by vacancies in adjacent tubes is responsible for the large-scale reorganization that ultimately leads to the formation of a wider nanotube.

  4. A Hairpin Turn for DNA Computing

    Computation is possible with DNA sequences because certain logical operations can be represented by the success or failure of particular hybridization events within a large pool of molecules. As the problems become more complex (for example, performing larger searches), the number of DNA molecules needed and time for hybridization increases rapidly and appears to limit the practicality of this approach. Sakamoto et al. (p. 1223; see the news story by Cho) suggest an alternative implementation that may prove more efficient. Sequences are encoded within single-stranded DNA molecules such that incorrect solutions create “hairpins,” or internal double-stranded regions. Digestion of hairpin DNAs or amplification of normal strands yielded the correct solution to a nontrivial search problem.

  5. Smoothing Out Turbulence

    Turbulence invariably hinders movement of an object through a fluid. Typically, streaks and eddies are generated near a boundary. Du and Karniadakis (p. 1230), using both models and an experimental system, demonstrate a way to suppress such eddies through generation of a traveling wave oriented transverse to the main flow. In practice, they show that eddy suppression can be accomplished with arrays of vibrating electromagnetic tiles or other smart skins on a surface.

  6. Surviving Without Sex

    Sexual reproduction and genetic exchange are thought to be essential to long-term evolutionary success, yet there have long been puzzling hints that a small number of organisms persist without any exchange at all. The bdelloid rotifers, small translucent bag-like animals that inhabit freshwater, are all females that reproduce parthenogenetically; males have never been seen. Any suspicions that these organisms might indulge in sex at barely detectable levels have now been laid to rest. Mark Welch and Meselson (p. 1211; see the Perspective by Judson and Normark) analyzed DNA sequence data from representative rotifer species and find a wide divergence between allelic sequences within single individuals of the asexual species. The level of divergence is consistent with the view that the bdelloids have abstained from sex for many millions of years. With this result, the mystery of sexual reproduction is deepened.

  7. Class-Conscious TBPs

    Despite years of intense research, there are many unanswered questions about the protein machinery that ensures accurate transcription of genes in the eukaryotic cell. One of these questions centers on whether a key component of this machinery, called TBP (TATA-binding protein), always requires the assistance of proteins called TAFs (TBP-associated factors) in order to activate transcription. Two reports provide evidence from chromatin immune precipitation studies in yeast cells that TAFs are required for transcription of some, but not all, genes. Kuras et al. (p. 1244) show that TBP exists in two distinct transcriptionally active forms, one associated with TAFs and one free of TAFs, and that these two forms display promoter-selective binding. Li et al. (p. 1242) describe two distinct classes of gene promoters, one that recruits TBP in the absence of TAFs and another that recruits both TBP and TAFs. These results challenge conventional views on how TBP is assembled at eukaryotic gene promoters.

  8. Staying a Step Ahead

    Patients with cystic fibrosis (CF) produce a thickened form of mucus that stagnates in their lungs, which makes them highly susceptible to chronic colonization by the ubiquitous bacterium Pseudomonas aeruginosa. In vitro studies have previously shown that P. aeruginosa is highly adaptable under changing conditions. Oliver et al. (p. 1251; see the Perspective by Rainey and Moxon) have shown that naturally occurring isolates of the bacterium from CF patients not only show an extremely high frequency of “mutator” phenotypes but that the mutators also display high levels of antibiotic resistance. Unusually, mutators do not revert to wild type, but appear to become fixed in the population and persist in CF, probably because of the constant selection pressure from the antibiotic armamentarium given to these patients. The news that P. aeruginosa can accelerate its evolution under naturally occurring circumstances presents a significant challenge both to clinical scientists and to evolutionary biologists.

  9. Synaptic Modifications

    In the hippocampus, pyramidal neurons receive multiple excitatory stimuli through thousands of synapses. Accumulation of translation machinery in postsynaptic cells has been observed following stimulation, but the function has remained elusive. Huber et al. (p. 1254) examined the role of localized protein synthesis in generating changes in synaptic strength with stimulation. They found that protein synthesis in the postsynaptic cells modified synaptic transmission within minutes to create long-term depression of the synaptic contact. The finding has implications for the cellular mechanisms of information storage and memory.

  10. Scrapie Strategy

    Scrapie is carried by an abnormally folded, infective form of the prion protein, which then directly induces the abnormal form in previously normal prion protein molecules to cause disease in sheep, mice, and other animals. When animals are infected, the abnormal prions replicate in the spleen and then move into the central nervous system, a process that requires an intact immune system. Montrasio et al. (p. 1257) have now shown that dendritic cells are the key cellular element necessary for prion replication and neuroinvasion. They eliminated follicular dendritic cells, an antigen-presenting cell in the spleen by injecting a soluble lymphotoxin-β receptor that inhibited formation of functional dendritic cells. The similarity between the clinical course of scrapie and vCJD (the new variant of Jacob-Creutzfeld Disease seen in humans and possibly derived from bovine spongiform encephalopathy) suggests that interfering with the lymphotoxin-β receptor system may also delay the progress of the human disease.

  11. Observing Complex Wave Propagation

    The propagation of seismic waves through sedimentary basins has been difficult to model because the wave interactions can be complex and depend on the geometry of the earthquake relative to the crustal structure. Koketsu and Kikuchi (p. 1237) used the densely instrumented region around the Kanto Basin, Japan, which includes Tokyo, to model the propagation of seismic waves from a magnitude 5.7 earthquake observed by an unprecedented 384 instruments that monitored strong ground motion. They found that a refracted wave, created by the interaction of the faster traveling waves in the adjacent mountain range relative to the slower traveling waves in the basin, was formed and propagated in a direction oblique to the edge of the basin. This previously unrecognized refracted wave explains the intensity and direction of the observed strong ground motions and will help to predict where the strongest ground motions will occur in the future.

  12. Not All Cuprates Earn Their Stripes

    Great effort has been made in trying to understand the mechanism of their high-temperature superconductivity in doped perovskites. Recent work on the strontium-doped lanthanum cuprate has revealed the existence of a stripe, or spin-ordered, phase near the onset of superconductivity. As the consensus is that most of the conduction is confined to the cuprate (CuO2) layers, it has been argued that the stripe phase is the driving mechanism for superconductivity in all of the cuprate materials. Now, Bourges et al. (p. 1234) present results from a high-resolution neutron scattering study of yttrium barium copper oxide, another high-temperature superconductor, which indicate that the stripe phase is not a compatible model for the description of the onset of superconductivity in this compound. The results suggest that stripes are not a universal feature or driving force of high-temperature superconductivity.

  13. All According to Body Plan

    There is a remarkable range of skeletal forms in modern and extinct animals. Of the available options, which designs were exploited by early metazoans, and at what rate did the range of designs expand into the available theoretical morphospace? Thomas et al., (p. 1239) in a study of the of the famous early Cambrian Burgess Shale fauna, show that the number of potential designs exploited increased as rapidly as the number of higher taxa, and much more quickly than diversification within phyla. They estimate that four-fifths of the available design options were exploited over a relatively short (15 million years) period in evolutionary history.

  14. A Pig Tale

    The dominant RN mutation in pigs causes accumulation of glycogen in skeletal muscle. Although RN pigs grow large, the meat from these animals gives a low yield of cured cooked ham and the trait is therefore economically undesirable. By a positional cloning strategy, Milan et al. (p. 1248) show that the RN mutation occurs in the PRKAG3 gene, which encodes a muscle-specific isoform of the regulatory γ subunit of adenosine monophosphate- activated protein kinase (AMPK). These results indicate that AMPK plays a crucial role in energy metabolism. Further analysis of the AMPK signaling pathway may provide insight into the pathogenesis of type II diabetes, a disease associated with defective glycogen storage in muscle.

  15. Diversity of Human alpha beta T Cell Receptors

    Arstila et al. (Reports, 29 Oct., p. 958) estimated the lower limit of diversity of the human T cell receptor (TCR) repertoire at 2.5 × 107 different TCRs. The upper limit, they maintained, hinges on “the number of different α chains that each of the 106 β chains can pair with”—an average, they suggested, of 100 different α chains for each β chain. Noting that this implies an upper limit of some 108 different αβ combinations, Kesmir et al. comment that the actual upper bound could be considerably higher, as the same β chain may appear repeatedly and, each time, garner an alternative set of 100 different α chains. Thus, Kes, mir et al. calculate, the upper bound for TCR diversity could be as high as 1011, a figure that “would allow almost every T cell in the naïve repertoire to have a unique TCR.” Arstila et al. respond that the alternative calculation of Kes, mir et al. may rest on incorrect assumptions regarding the total turnover of naïve T cells, and may be less compatible than the Arstila et al. model with data regarding cell cycle length and the frequency of antigen-specific T cell precursors. “The phenomenon that Kes, mir et al. postulate,” they conclude, “although in principle possible, has little impact on the total diversity.” The full text of these comments can be seen at http://www.sciencemag.org/cgi/content/full/288/5469/1135a