This Week in Science

Science  28 Jan 2000:
Vol. 287, Issue 5453, pp. 545
  1. Immunodeficiency Virus Transmission Across Species

    The human immunodeficiency viruses (HIV-1 and HIV-2) entered the human population as viruses that crossed over from nonhuman primates. Hahn et al. (p. 607) review the evidence for the origin of the viruses that led to the AIDS epidemic. They also sound a cautionary note—26 different nonhuman primates are known to harbor simian immunodeficiency viruses and could give rise to new viral crossover events.

  2. Transoceanic Communications

    Two of the major climate cycles with tropical origins that can have global effects are the El Niño-Southern Oscillation, or ENSO, and the Asian and African monsoons. Whether they are connected has been uncertain; one problem has been that the instrumental record, particularly of sea surface temperatures (SSTs), is short. Cole et al. (p. 617) now provide a long-term coral record of SSTs from the western Indian Ocean. Comparison with other coral records and ENSO records implies that the climate in the Pacific, which is heavily influenced by ENSO, strongly affects the climate in the Indian Ocean.

  3. Emerald Trails

    Some of the most valuable gems throughout history have been emeralds, yet in many cases the origins of the most precious emeralds have been obscure. Giuliani et al. (p. 631; see the cover and the news story by Stokstad) analyzed the oxygen isotopes of some of the world's most famous emeralds, including the central jewel of the crown of France, to trace the origins of emeralds worldwide since antiquity. Ion microprobe analyses allow analysis without harming the quality of the gems. Because emerald deposits worldwide show a huge variation in oxygen isotopes, specific origins can be identified. The data show that trade in early emeralds involved, surprisingly, sources in Pakistan, and reveal the rapid infiltration of emeralds from Columbia soon after their discovery in the early 1500s.

  4. Nanotube Sensors

    Sensors for gaseous molecules such as NO2 and NH3 are used in environmental monitoring, industry, medicine, and many other fields. Traditional solid-state sensors require relatively high temperatures to achieve significant sensitivity for these molecules. Kong et al. (p. 622) show that single-wall carbon nanotubes can be used as miniaturized chemical sensors at room temperature. The electrical resistance of individual semiconducting nanotubes can change by up to three to four orders of magnitude upon exposure to NO2 or NH3. Chemical selectivity can be achieved by adjusting the electrical gate voltage.

  5. Glasses in 3D

    In the supercooled fluid and glassy states, particle motions become restricted and structural relaxation (rearrangements of cages of particles) is thought to occur through the cooperative movements of clusters of particles. Direct evidence for this hypothesis is difficult to obtain for molecule fluids, but the motion of particles in colloidal systems is within the resolution of confocal microscopy. Weeks et al. (p. 627; see the Perspective by Ediger) present three-dimensional images of particle motions in colloidal glasses. They find that the fastest particles move cooperatively and that the size of mobile particle clusters grows in supercooled liquids as the glass transition is approached, but then drops sharply at the glass transition.

  6. Putting Nanotubes on the Rack

    Most studies of carbon nanotube strength have examined sideways bending. Yu et al. (p. 637) looked at what happens when multiwall carbon nanotubes (MWNTs) are pulled along their length until they break. They attached MWNTs to two opposing atomic force microscope tips, which themselves were mounted in a scanning electron microscope. Thus, they could measure stress-strain curves while recording images of the fracture processes. The outermost wall broke and the coaxial tubes pulled apart like a sword from its sheath—the length of the two ends often exceeded the initial length. Subsequent transmission electron microscopy of the fragments revealed a variety of deformation processes.

  7. Epothilone Expression

    Epothilone is a microbial compound that appears to possess efficacy in the treatment of certain types of cancer. Pharmaceutical development of specific and useful derivatives has been hampered by availability; the chemical synthesis is lengthy and laborious, and the natural source is not amenable to biochemical or genetic manipulation. Tang et al. (p. 640) describe the cloning of the gene cluster responsible for the biosynthesis of epothilone and its expression in a suitable host where molecular tinkering is feasible.

  8. I Know It's Right Along Here...

    Searching for a familiar book in one's library evokes a variety of neural responses in the brain. Jiang et al. (p. 643) show that the early stages of visual processing are activated less and less as one scans across bookshelves because the same type of visual scene is treated as a repetitive and familiar stimulus. However, frontal cortex areas that subserve working memory display an enhanced activity, which serves as a marker of top-down specification of a particular and precise visual pattern. Taken together, these changes make it possible to ignore irrelevant, distractor books and to recognize the target book efficiently.

  9. Epidemic Dynamics--Past

    Epidemiological investigations can provide important insights into population biology because such studies can rely on long-term public health records involving large population sizes. Using a simple nonlinear model, Earn et al. (p. 667; see the Perspective by May) explain a series of remarkable global changes in patterns of measles epidemics in which transitions occurred between regular synchronized cycles and irregular unsynchronized patterns. They accurately reproduced changes in patterns of epidemics on the basis of changes in birth and vaccination rates. This work establishes that the source of the dynamical transitions is extrinsic, rather than any intrinsic property of the dynamics themselves. Slow changes in extrinsic factors can stimulate quite sudden shifts in measles population dynamics.

  10. Epidemic Dynamics--Future

    Unlike diseases such as heart disease or cancer, in which treatment only affects the individual patient, treatment of an infectious disease can affect an entire population. There has been uncertainty regarding the effects of antiretroviral therapy (ART) in light of (i) the continued ability of some treated individuals (especially for monotherapy) to transmit the virus, (ii) the development of resistant strains, and (iii) the possibility that risky behaviors could increase. Blower et al. (p. 650) have modeled these possibilities for the San Francisco gay community during the next 10 years. They found that increasing the use of ART would decrease the death rate and prevent a substantial number of new infections, even in the presence of drug resistance and risky behavior.

  11. Targeting HIV Integration

    Integration of human immunodeficiency virus-type 1 (HIV-1) DNA into the host genome is a key step in virus replication that makes it an attractive target in designing antiretroviral drugs. However, the search for an inhibitor that would be an effective antiviral agent has proven frustrating. Hazuda et al. (p. 646) determined that certain 1,3-diketo acids can inhibit HIV-1 integrase activity at a step in the integration process, called strand transfer, that has not been affected by previous inhibitors. Finding that the active site of HIV-1 integrase can be recognized in different ways during integration should help promote the identification of medically relevant integrase inhibitors.

  12. Prying Open Prion Secrets

    The prion proteins of yeast, although unrelated to those of mammals, show similar characteristics in that they can undergo a conformational change that alters the phenotype of the cell, and this altered phenotype can be passed on to daughter cells. Hence, the conformation of yeast prions (as is the case with mammalian prions) should encrypt heritable information without the help of a nucleic acid intermediate. Li and Lindquist (p. 661; see news story by Balter) now elegantly show that prion-like properties can be transferred from a prion to another totally unrelated protein. The investigators fused the amino-terminal and M regions of the Sup35 yeast prion protein to the rat glucocorticoid receptor (a transcription factor that is regulated by steroid hormones). When endogenous Sup35 acquired an altered conformation, this conformational change was conferred on the Sup35 portion of the fusion protein. Not only Sup35 activity but also that of the glucocorticoid receptor was altered, as shown by the reduced ability of the fusion protein to activate transcription.

  13. Southern Ocean CO2 Storage

    Approximately half of the CO2 produced anthropogenically remains in the atmosphere, while the other half is absorbed by the oceans and terrestrial biosphere. Recent observational estimates of anthropogenic CO2 inventories in the Southern Ocean suggest that storage there is low, implying that fluxes into the Southern Ocean also are low, but modeling studies have indicated that those fluxes should be high. Caldeira and Duffy (p. 620) present an ocean-climate model that reconciles the apparent contradiction of high uptake and low storage in the Southern Ocean. Their results imply that reduced uptake of atmospheric CO2 by the Southern Ocean could occur if climate change were to reduce the density of surface water there.

  14. Singled In and Out

    Even in a narrow channel, diffusing particles normally can pass by one another in different directions, but in some situations the channel is barely wider than the particles. Single-file diffusion (SFD) should then occur in which the highly correlated particle motions no longer obey Fick's laws of diffusion. Verifying SFD for molecules is difficult, so Wei et al. (p. 625) examined the situation for colloidal particles moving in circular tracks. The particles are paramagnetic, so their interaction strength could be varied with an applied magnetic field. The authors verify that the particle displacement distributions follow a Gaussian function.

  15. A Detailed View of Quantum Phase Transitions

    Quantum phase transitions can occur in systems in electrons that are highly correlated, such as magnetically ordered materials, high-temperature superconductors, and in metal-insulator transitions. Despite the intensive studies, the essential aspects of the transition have not been conclusively established. Looking at the simple system of niobium-doped silicon with composition spanning the metal-insulator transition regime and probing the frequency-temperature behavior of the conductivity, Lee et al. (p. 633) provide a comprehensive study of the quantum phase transition and evaluate the essential scaling laws necessary to describe the transition, thus giving further insight into this intriguing phenomenon.

  16. Reducing Plan

    Sunlight provides the energy for plants to carry out photosynthesis in which carbon dioxide is reduced to carbohydrates. The reducing equivalents produced by the membrane-embedded photosystem complexes are carried by ferredoxin to enzymes that generate NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate) for use in the carbohydrate biosynthetic pathways. These reducing equivalents also are used to signal the presence of light; here, too, ferredoxin is the carrier but thioredoxin is the intermediate that is then used to turn on the activities of the biosynthetic enzymes by reducing their disulfides. Dai et al. (p. 655) have determined the structure of the enzyme that enables transfer of the reducing equivalents from ferredoxin to thioredoxin. The biconcave ferredoxin:thioredoxin reductase interposes an iron-sulfur cluster between the electron-carrying iron of ferredoxin and the to-be-reduced disulfide of thioredoxin.

  17. Electromotility Origins

    The membrane of outer hair cells in the mammalian cochlea is involved in electromotility, the voltage-dependent change in cell length. The mechanisms underlying this phenomenon are still poorly understood. Oghalai et al. (p. 658) used the recovery kinetics of fluorescence photobleaching to study the fluidity of the membranes of outer hair cells. Cell shortening by several different procedures caused a significant reduction in membrane fluidity. These results may help to explain the electromechanics of outer hair cells and the mechanical signal amplification in the cochlea.

  18. Responding to ER Stress

    The presence of misfolded proteins in the endoplasmic reticulum (ER) is sensed by cells and initiates a stress response. In yeast, the transmembrane protein IRE1p senses misfolded proteins and produces signals that lead to appropriate changes in gene expression. Mammalian cells have related transmembrane proteins known as IRE1α and IRE1β that also appear to act as receptors for improperly folded proteins. Urano et al. (p. 664) report that IRE1 appears to activate the cJun NH2-terminal kinase (Jnk) in much the same way as do tumor necrosis factor α receptors on the cell surface–that is, IRE1 appears to bind to a TRAF protein (TNF receptor-associated factors). Such interaction with TRAF2 appears to initiate a cascade of activation of protein kinases that leads to activation of Jnk. Cells can apparently use similar signaling mechanisms to sense and transmit both intracellular and extracellular signals.

  19. Questions About Magnetic Lineations on Mars

    Using inverse modeling of data from the Mars Global Surveyor, Connerney et al. (Reports, 30 Apr., p. 794) interpreted linear magnetic features on Mars as reminiscent of patterns associated with sea floor spreading on Earth. In a comment, Harrison argues that the results of the Connerney et al. model do not support Earth-type field reversals as the cause of the lineations because the effective inclinations do not fall into groups separated by 180°, as might be expected from a field-reversal mechanism. Connerney et al. respond that Harrison's calculations of field directions may be inappropriate in light of the 200-kilometer model resolution, and that the statistical tests proposed by Harrison do not adequately account for the potential complexities of hundreds of millions of years of Mars tectonic history. The full text of these comments can be seen at

  20. Neck Posture of Sauropod Dinosaurs

    Stevens and Parrish (Reports, 30 Apr., p. 798) presented modeling results that suggested that diplodocid sauropod dinosaurs were adapted mainly to ground feeding or low browsing, in contrast to the conventional view of these sauropods as high browsers. Upchurch, while generally concurring with that conclusion, suggests that Stevens and Parrish may have insufficiently considered the role of anterior trunk vertebrae in shaping the feeding envelope, and also questions the reliance of their Apatosaurus analysis on a single specimen known to “have suffered severe damage at the base of the neck.” Stevens and Parrish acknowledge that anterior trunk flexibility, while beyond the scope of their study, probably contributed to the diplodocids' feeding movements, and provide additional details on how the modeling study dealt with potential for error due to the neck damage in the Apatosaurus specimen. The full text of these comments can be seen at