This Week in Science

Science  07 Jul 2000:
Vol. 289, Issue 5476, pp. 9
  1. Mapping Out Potential

    The quantum Hall regime in a two-dimensional electron gas is robust against defect densities that decrease the electron mean free path to the order of the electron wavelength. The disorder potential induced by these defects (ionized impurities) is thought to play a crucial role in the quantum Hall effect. Finkelstein et al. (p. 90) used a scanning probe to “pull-up” a small bubble of electrons. They then mapped out the electrostatic potential over the two-dimensional plane by scanning the bubble across the sample and looking at the number of electrons that fall out of or into the bubble as it passes regions of high or low potential. The technique offers the ability to investigate and understand a wide variety of submerged electronic structures on nanometer-length scales.

  2. Subducted Seamount

    Active subduction in the Philippine Sea Plate beneath the Eurasia Plate has formed the Nankai trough off the coast of southwest Japan and has led to the recurrence of large earthquakes every 100 to 200 years. Of these, the 1946 Nankaido earthquake was unusual because long-period seismic data indicated a large rupture, but short-period data indicated a small rupture. Kodaira et al. (p. 104), using ocean-bottom seismometers to image the velocity structure of the Nankai trough, found evidence for the subduction of a large seamount (about 13 kilometers thick by 50 kilometers wide). The presence of this relatively intact seamount would explain the anomalous seismic data from the 1946 earthquake. Quicker and shorter rupture could occur along the plate boundary where the seamount is not coupled to the subducting plate. Slower and longer rupture could occur where the seamount was coupled to the plate, which would cause the rupture to propagate along a new and longer fracture through the seamount structure.

  3. Stellar Carbon and Oxygen Production

    Carbon and oxygen in the interstellar medium may be produced in the hydrogen- or helium-burning phases of red giant stars when these elements can move up to the surface of the star and be released. Carbon is produced by a three-alpha-particle process and includes secondary production of oxygen during the third alpha-particle interaction. Oberhummer et al. (p. 88) found that variations of the nucleon-nucleon strong interaction by more than 0.5% and of the Coulomb interaction by more than 4% would inhibit the formation of carbon-12 and oxygen-16 in models of the hydrogen- and helium-burning phases of low-mass stars (1.3 to 20 solar masses). Thus, there is a narrow range of nuclear forces within which red giant stars can contribute carbon and oxygen to the interstellar medium. Understanding these subtle forces will help to establish more accurate abundances of carbon and oxygen in the universe.

  4. Toward Mechanical Carbon Nanotube Memories

    Molecular-based electronics offer the potential of higher integration densities, and much effort has focused on identifying viable molecular units that can be used for switching devices. Rueckes et al. (p. 94) propose that junctions of a crossed pair of nanotubes in which the upper tube is suspended above the lower one could operate as nonvolatile, reversible memory devices. Application of an appropriate bias to each of the wires would produce a stable kink in the upper wire; thus, the separation distance between the tubes and hence the junction resistance could be changed in a manner analogous to a mechanical relay. Preliminary results have been obtained with thicker nanotube ropes. Integration densities approaching 1012bits per square centimeter could be achieved by using many tubes to form a mesh of these junctions.

  5. Positive About C60

    The fullerene C60 readily forms anions up to C606−, and thus, forming positively charged C60 species is a particular synthetic challenge. Reed et al. (p. 101; see the Perspective by DesMarteau) report a route to C60+ via the formation of HC60+ through the use of the superacids H(CB11H6X6), where X is Cl or Br. Unlike other superacids, their conjugate bases are poor nucleophiles and do not decompose C60. Subsequent reaction of HC60+with a strong oxidant based on a triarylaminium radical cation yields C60+. These results show how the usually interrelated properties of acidity, redox potential, and nucleophilicity can be separated and exploited in separate steps.

  6. The Core of Transposition

    Transposases catalyze the movement of a double-stranded DNA segment from one location in the genome to another: the sequence of events comprising double-stranded cleavage of both ends of the segment, binding to the target DNA site, and then insertion of the segment into that site. Davies et al. (p. 77; see the cover) have solved the structure of a tranposase-DNA complex, which offers insights into the mechanisms of recognition and DNA cleavage as well as how the dimerization of the enzyme leads to initial strand scission and subsequent excision (see the Perspective by Williams and Baker). The similarity of this reaction to retroviral integration signifies an important advance in understanding a fundamental aspect of evolution and of contemporary disease.

  7. Blueprints for Bigger Tomatoes

    Genetic studies of tomato have identified a quantitative trait locus (QTL) responsible for a good portion of the evolutionary change in fruit size that has characterized the domestication of the original wild relatives of the modern tomato into plants that produce large red fruits. Quantitative trait loci characteristically affect morphology in a graded, rather than all-or-none, fashion. Now, Frary et al. (p. 85; see the Perspective by Doebley) have cloned and analyzed the tomato fruit size QTL fw2.2, which contains several open reading frames. Surprisingly, one gene, ORFX, is responsible for the changes in fruit size—cultivated strains expressing the wild species gene exhibited small fruit size. The gene is transcribed early in floral development and may effect fruit size through regulation of cell division.

  8. The Ups and Downs of Tumor Cell Growth

    Epithelial cells normally have a “top” and a “bottom,” but as they become malignant, this important architectural feature is lost. Using the fruit fly Drosophila as a model, Bilder et al. (p. 113; see the Perspective by Peifer) identify a trio of membrane-associated proteins that act in a common pathway to regulate epithelial cell polarity and cell growth. The authors speculate that two of the proteins, Scribble and Discs-large, bind transmembrane proteins that organize the cell surface while the third, Lethal giant larvae, plays a role in the protein targeting system that preserves this cell surface organization.

  9. How Sweet It Is

    The receptors in the nose that detect odors are members of an extremely large family of G protein-coupled transmembrane receptors. Ishimoto et al. (p. 116) present evidence which suggests that some taste receptors may also be members of this family. In the fruit fly Drosophila, the gene Tre1 specifically controls the sensitivity to the sweet taste of trehalose but not to other sugars; replacement of a mutant Tre1 gene restores the fly's ability to taste trehalose. The expression of the gene in the cells that sense taste adds to the evidence that this gene may code for a bona fide taste receptor.

  10. High Blood Pressure and Pregnancy

    Hypertension is a common complication of pregnancy. An important clue to its underlying cause is provided by Geller et al. (p. 119; see the news story by Wickelgren), whose studies take them from patient to molecular causation to a plausible mechanism at atomic resolution. The authors identify a family with inherited early-onset hypertension that is exacerbated in pregnancy and show that the causative mutation lies in the gene encoding the mineralocorticoid receptor (MR), a protein that regulates salt reabsorption in the kidney. The mutation changes one amino acid in the hormone binding domain, which causes MR to become constitutively activated. This aberrant activity is enhanced further by progesterone, a hormone that is produced at high levels in pregnancy and that normally acts as an MR antagonist. The inherited mutation appears to facilitate molecular interactions in MR that normally require binding of its natural ligands.

  11. Going in the Side Door

    Voltage-dependent potassium (K+) channels open in response to a depolarization of the membrane potential and conduct K+ across the plasma membrane. These channels are then inactivated by a peptide that is thought to physically insert into and obstruct the ion-conducting passageway. Gulbis et al. (p. 123) have determined the crystal structure of a complex of the cytoplasmic β subunit with the cytoplasmic domain of the integral membrane α subunit. Although the fourfold symmetric β subunit is axially aligned with the channel pore, it appears that ions actually enter through a spacer region between the membrane and cytoplasmic domains of the α subunit, and that the inactivation peptide also penetrates in this lateral fashion.

  12. Microsculpting

    Electrochemistry is a widely used technique for the deposition of thin films and can be used to enhance the etching rate of some materials. Schuster et al. (p. 98) introduce a technique for controlling local electrochemical reactions that may allow the fabrication of three-dimensional structures with submicrometer resolution. A series of nanosecond-long voltage pulses are applied to the tip of the scanning probe microscope, which is used as the tool piece, in order to deposit or etch material locally at the workpiece.

  13. A More Intricate Clock in Fungi

    The transcriptional feedback loop that controls the fundamental daily rhythm of biological organisms has proven to be more intricate than expected. Now Lee et al. (p. 107) show that the fungus Neurospora joins the fruit fly and mouse in having two interlocked feedback loops at the core of the circadian clock. The clock component frequency has a positive effect on another clock component, white collar-1 and -2 levels, in addition to its previously described inhibitory effect on its own transcription. This extension of the two-loop pattern to a fungus indicates the generality of this organization.

  14. Paleodatabase Management

    Paleontology in the past two decades has increasingly focused on “taxic paleobiology”—an effort to document the patterns of organismal diversity through time. Much argument, however, has focused on the accuracy of the fossil-derived taxonomic databases from which these patterns have been assembled. Critics have contended that a lack of taxonomic specialist knowledge among database researchers has resulted in large errors in compilation that reduce the meaningfulness of the databases. Adrain and Westrop (p. 110), using a detailed assessment of trilobite diversity for the Ordovician and Silurian periods, have carried out a rigorous test of the accuracy of taxonomic database procedure. They find that the level of error in existing databases is indeed large, but that the error is effectively random. The patterns of diversity through time appear to be vindicated.

  15. R/M/N Focus Formation and the Presence of Intact BRCA1

    Zhong et al. (Reports, 30 July 1999, p. 747) found that formation of irradiation-induced immunoreactive foci related to the Rad50/Mre11/NBS1 (R/M/N) protein complex, which assemble in normal cells after DNA damage, was “dramatically reduced” in the tumor cell line HCC1937, which synthesizes a defective form of the BRCA1 gene product. Reconstitution of the mutant cells with wild-type BRCA1 restored normal focus formation. The results, Zhong et al. concluded, suggest that BRCA1 is important for the cellular responses to DNA damage mediated by the R/M/N complex. Wu et al. present data that “run counter” to these claims. Using two antibodies specific to the NBS1 protein (the monoclonal EE15 and the polyclonal D29), they found no diminution of focus formation after irradiation, and no change in the focus formation pattern after reconstitution with wild-type BRCA1. These data, argue Wu et al., “make it difficult to conclude that BRCA1 is responsible for organizing radiation-induced R/M/N foci.”

    Zhong et al. respond that their own experiments using the EE15 and D29 antibodies yielded different results: EE15-immunoreactive foci could be detected in both untreated and irradiated cells, and D29 foci failed to appear in any of the cell lines studied by Zhong et al. Experiments with other commercially available antibodies sensitive to NBS1 and Mre11, however, supported the results in the original study, according to Zhong et al.: R/M/N irradiation-induced foci were reduced in the BRCA1-mutated HCC1937 cells. “Clearly,” they conclude, “BRCA1 plays a role in DNA damage repair.” The full text of these comments can be seen at