This Week in Science

Science  11 May 2001:
Vol. 292, Issue 5519, pp. 1017
  1. Nanotube Crystals

    Most synthesis methods for carbon nanotubes produce entangled, poorly ordered mats, and nanotubes with different chirality are usually found within one sample. Directed growth methods show more success for creating uniform, ordered nano- and microstructures, but usually the ordered assemblies that form contain only tens of nanotubes. Schlittler et al. (p. 1136) report the synthesis of micrometer-scale nanotube crystals containing up to several thousand individual nanotubes. Thermolysis of nanopatterned precursors was used to generate the crystals, each of which consists of an ordered array of nanotubes with identical diameter and chirality.

  2. Making a Full Recovery

    Industrial catalysts generally fall into two categories—soluble compounds (homogeneous catalysts) that are often well defined but can be difficult to recover from solution, and insoluble materials (heterogeneous catalysts) that are readily recovered but whose surface reactions can be difficult to understand. Xi et al. (p. 1139) have used a tungsten oxide cluster to catalyze olefin epoxidation as a soluble species, at least as long as the oxidant, hydrogen peroxide (H2O2), holds out. When the oxidant is used up, the catalyst precipitates for ready recovery. The H2O2 can be generated in situ from air with a secondary catalytic system. This approach, when applied to the synthesis of propylene oxide, a major commodity chemical, could avoid the formation of unwanted by-products made in current industrial production.

  3. Spiraling into a Photonic Band Gap

    Photonic band gap (PBG) crystals are artificial materials designed with periodic fluctuations of the refractive index on length scales of optical wavelengths. In these materials, energy gaps open up that prohibit the propagation of light of particular wavelengths and thus can be used to guide light. To date, most designs have been based on the popular diamond-like or woodpile structures, but fabrication techniques have not been amenable to spatially extended three-dimensional structures. Toader and John (p. 1133) present a blueprint for an alternative PBG crystal structure based on a square-spiral structure that could be deposited over large scales. Their calculation also indicates that the structure should be functional even if defects are present.

  4. Complete Archean Ophiolite

    Ophiolites consist of layers of magnesium- and iron-rich rocks formed by a magmatic intrusion associated with extensional tectonics, such as an oceanic spreading center. Although ophiolites are relatively rare, complete sequences have been noted through much of Earth's history from the Proterozoic to Phanerozoic, and a few incomplete ophiolites were found in older rocks of the Archean. Kusky et al. (p. 1142; see the Perspective by Karson) found a complete ophiolite sequence of Archean age (about 2504 million years old) in the northern part of the Chinese craton near the village of Dongwanzi. The complete Dongwanzi ophiolite indicates that the fundamental processes of plate tectonics were operating earlier in Earth's history than had been previously recognized.

  5. A Dominant Population

    One theory for the origins of modern humans is that they emerged from Africa about 100,000 years ago and replaced extant populations even as far away as northern Europe, Asia, and Indonesia. A second theory is that there were large degrees of interbreeding with extant populations that had a much larger role in future generations. Ke et al. (p. 1151; see the news story by Gibbons) provide strong support for the first theory in an extensive analysis of three Y chromosome markers characteristic of African origins in more than 12,000 males from 163 populations in East Asia. All of the individuals in their survey carried at least one of the three markers. Thus, there seems not to be even a minimal contribution to the gene pool from previous Asian hominids.

  6. Fighting Foot-and-Mouth Disease

    In early February 2001, an epidemic of foot-and-mouth disease began in northern England. By early April, more than 1000 farms across Great Britain had been infected, and hundreds of thousands of cows, sheep, and pigs were slaughtered. Ferguson et al. (p. 1155) use data supplied to several groups of epidemiologists by the U.K. Ministry of Agriculture, Fisheries and Food as a basis for modeling the future spread of the disease under various different scenarios of treatment, including selective culling and vaccination. They conclude that the best option for controlling the current epidemic is extensive culling of animals in the immediate vicinity of infected farms.

  7. Rock Cake for Bugs

    The New Albany Shale is a remnant of Late Devonian mud that is rich in carbon in the forms of natural gas and resistant organic matter, or kerogen. When such sedimentary rocks are exposed, the carbonaceous material is weathered by several chemical and physical processes. Petsch et al. (p. 1127) now show that bacteria also process kerogen. They have isolated microorganisms from cores and monitored their consumption of kerogen in powdered, sterilized rock by measuring the incorporation of 14C versus 13C isotopes (14C is absent in ancient organic matter). At least three-quarters of the carbon assimilated by their bacterial cultures originated from the rock. Until now, kerogen was not thought to be bioavailable, but this work shows that bacterial consumption of organically rich sediments may be a significant factor in weathering and, over geological time scales, may have made a significant contribution to global biogeochemical carbon cycles.

  8. Sterically Stopping Cholestrol Synthesis

    Statins are drugs that are widely used to lower cholesterol levels by inhibiting the enzyme HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase (HMGR) that catalyzes the committed step in cholesterol biosynthesis. Istvan and Deisenhofer (p. 1160) have determined structures of the catalytic portion of human HMGR bound to six different statins at resolutions of 2.3 angstroms or better. The statins bind to the enzyme active site and thus sterically prevent substrate binding. The binding pocket rearranges to accommodate statin molecules, and residues near the carboxyl terminus become disordered.

  9. Synergy Between Selector and Signaling Proteins

    Throughout development, complex structures such as organs and appendages form as directed by spatial and temporal cues. Studies have begun to define these cues, but we currently do not know how the various cues are integrated. Using the Drosophila wing imaginal disk, Guss et al. (p. 1164; see the Perspective by Affolter and Mann) examined the relation between the selector proteins and signaling factors that influence wing-specific genes. Wing development requires DNA binding by both selector proteins and signaling proteins for the specification of a developmental field.

  10. Fertility Factor

    It had been suggested at one time that all transcription machinery included the TATA-binding protein (TBP). Subsequently, TBP-related factors (TRF) were found whose function was unclear but might represent tissue-specific factors. Recent studies in Xenopus and Caenorhabditis elegans showed that TRF2 is required for early development, and that elimination of this factor leads to embryonic lethality. In contrast, Zhang et al. (p. 1153) now show that when mice lack functional TRF2, the animals are generally healthy but the resultant males are sterile because of a defect in spermatogenesis. Hence, TRF2 in mammals may represent a tissue-specific factor for sperm development.

  11. Let's Get Organized

    Cellular architecture is governed by a network of intracellular fibers known collectively as the cytoskeleton, and one its major components is the microtubule network. Surrey et al. (p. 1167) examined the types of structures that microtubules together with microtubule-based motors could construct in the absence of other cellular structures. They observed the self-assembly of several types of complex structures at steady state, including asters and vortices whose formation depended on the ratio and type of motors and microtubules. They also developed a computer simulation that faithfully predicted the observed structures.

  12. Cap in Hand

    In ciliated protozoa, the ends of chromosomes, or telomeres, are bound to capping proteins that are thought to protect the telomeres from degradation. Baumann and Cech (p. 1171; see Perspective by de Lange) have identified long-sought homologs of these proteins in fission yeast and in humans. These proteins, called Pot1 (for protection of telomeres), bound to telomeric DNA in a sequence-specific manner. Deletion of the pot1 gene in fission yeast caused immediate loss of telomeric DNA and chromosome circularization—a phenotype more dramatic than that caused by loss of telomerase, the enzyme that synthesizes new telomeres.

  13. The Roots of Drug Cravings

    Even after long periods of being drug-free, persons addicted to cocaine can exhibit craving behavior. In an animal model, Vorel et al. (p. 1175; see the news story by Holden) observed that electrical theta burst stimulation of the ventral subiculum but not the cerebellum or the medial forebrain bundle could elicit strong cocaine-seeking behavior. The stimulated ventral subiculum fibers are glutamatergic, and these results suggest that glutamate receptor antagonists could block the cravings and help in treating addiction.

  14. Turning Up the Heat on Uniform Thin Films

    One of the key issues in nanotechnology is the structural stability of thin films and devices. An ideal test system for studying the thermal stability of thin films is that of silver deposited on the (100) face of iron, in which atomically uniform films form whose thickness can be controlled at the monolayer level. Luh et al. (p. 1131) report that films comprising 1, 2, and 5 monolayers are thermally stable (up to 900 K), whereas films comprising any other number N monolayers bifurcate into composite films of N + 1 and N - 1 films. Because of the confinement of electrons in the film between the barrier at the substrate interface and at the top surface, the system forms a quantum well that exhibits quantized energy levels. They show that the electronic energy associated with the quantized levels can explain the results.

  15. Two Ruptures for One Event

    On 18 June 2000, a magnitude 7.8 earthquake occurred along the Wharton Basin in the Indian Ocean. This event occurred within the Indo-Australian plate, but the epicenter was near the southern region of deformation associated with some proposed microplates. Robinson et al. (p. 1145) took advantage of some well-recorded seismograms for this remote earthquake and found that the event was actually two ruptures, one trending about north to south along a fossil transform fault and one trending about east to west from a second fault. These results indicate that earthquakes may be more complex and, for the Wharton Basin event, the earthquake may help define the structure of the microplates.

  16. Plunges in Productivity

    The Triassic-Jurassic mass extinction, although not as famous as the Permian-Triassic extinction before it or the Cretaceous-Tertiary extinction after it, had a large impact on Earth's biota and evolutionary history. Most of Earth's species became extinct, and dinosaurs emerged as the dominant land animals in its aftermath. However, the details in timing and causes of this large extinction are uncertain. Ward et al. (p. 1148) now demonstrate that the Triassic-Jurassic extinction, like these two other large extinctions, evidently was characterized by a major disruption of Earth's carbon cycle that was reflected in a drop at the extinction boundary in the 13C/12C ratio of the oceans. Their analysis of a section in British Columbia also implies that the extinction was relatively abrupt and occurred in less than 1 million years.

  17. Retinal Regulation

    The horizontal cells in the outer plexiform layer of the retina generate the surround inhibition for the bipolar cells, but the underlying mechanism has remained elusive. Kamermans et al. (p. 1178) show that there are gap junction hemichannels at synapses between horizontal cells and cone photoreceptors. Blockade of these hemichannels by carbenoxolone causes horizontal-cell hyperpolarization and disrupts feedback responses in cones and horizontal cells. This finding suggests the existence under physiological conditions of a negative-feedback mechanism from horizontal cells to cones through an electrical or ephaptic mechanism.