This Week in Science

Science  22 Dec 2000:
Vol. 290, Issue 5500, pp. 2209
  1. Toward Silicon-Based Optoelectronics

    Silicon dominates semiconductor microelectronics, but the fabrication of silicon-based optical devices, which would allow for a seamless and fully integrated optoelectronics technology, is still challenging. The indirect band gap of silicon, a property shared with other group IV elements such as germanium, has presented a considerable barrier to its use in optical applications (and other materials) such as GaAs, are typically used instead. Using a quantum-cascade design that involves a series of coupled and carefully deposited Si and SiGe quantum wells, Dehlinger et al. (p. 2277) demonstrate electroluminescent behavior from such a structure. This single-band design overcomes the limitations of indirect gap materials.

  2. Single Photons on Demand

    For the successful implementation of quantum cryptography, a light source capable of reliably producing at most a single photon per pulse is a key requirement. Previous work has shown that photon emission from a nonlinear radiator, such as a single molecule or a quantum dot, shows antibunching behavior. However, antibunching is not a sufficient condition to produce single photons, and further mechanisms are required to ensure single photons on demand. Michler et al. (p. 2282; see the Perspective by Benjamin) present one such mechanism involving the excitation of an InAs quantum dot embedded in a microdisk cavity with pulsed laser light. The subsequent relaxation pathway of the quantum dot is such that the emission of a single photon per excitation pulse is ensured.

  3. Progress in Niemann-Pick C Disease

    Patients with Niemann-Pick C (NPC) disease have a genetic defect that causes excessive accumulation of cholesterol in the liver, spleen, and other vital organs. At the cellular level, the cholesterol becomes trapped in the endolysosomal compartment rather than being transported to the plasma membrane. In almost all patients, the disease is caused by disruption of a transmembrane protein, NPC1, whose precise function has remained elusive. Further insights into the relation of NPC1 to cholesterol transport are the subject of two reports (see the news story by Marx). Davies et al. (p. 2295) show that NPC1 has sequence and functional homology with a family of prokaryotic permeases called the resistance-nodulation-division (RND) proteins, which transport lipophilic drugs and other target molecules out of the bacterial cytosol. When expressed in Escherichia coli, NPC1 transported acriflavine and fatty acids, but not cholesterol, from the culture medium into the cytosol. These results suggest that NPC1 is a member of an ancient family of multidrug efflux pumps that in mammalian cells have assumed housekeeping functions in cellular cholesterol homeostasis. In independent work, Naureckiene et al. (p. 2298) show that a rare subset of NPC patients with normal NPC1 function have a genetically determined deficiency in HE1, a cholesterol-binding protein localized in the lysosomes. These findings may lead to a better understanding of the mechanisms regulating cholesterol transport.

  4. Picked Out of a Lineup

    Electron energy loss spectroscopy (EELS) can provide chemical fingerprints of specific elements in a sample without the need for the atoms to be present in the topmost surface layer, as would be the case for scanning probe techniques. Suenaga et al. (p. 2280) now show that spatial resolution rivaling that of scanning probe and electron microscopy techniques can be obtained in EELS. They can identify gadolinium (Gd) atoms, which were encapsulated in fullerenes that were then aligned within a single-wall carbon nanotube. Individual Gd atoms, which are spaced about a nanometer apart in this environment, can be identified.

  5. Atlantic Cooling

    During the past several million years, Earth's climate has cooled significantly. This cooling may have been the result of a change in the ocean's large-scale circulation caused by the closure of the Central American Seaway. The blocking by the Isthmus of Panama began to affect deep ocean circulation 4.5 million years ago and was essentially complete in another 1 to 2 million years. Marlow et al. (p. 2288) present a 4.5-million-year record of sea surface temperature off the coast of southwest Africa, which shows that temperatures there have decreased by an extraordinary 10°C during the past 3.2 million years. This drop is thought to be the result of a combination of global cooling, increased wind-driven upwelling, and a radical change of surface Atlantic Ocean circulation.

  6. An Arid Amazon?

    Reconstructing the precipitation history of the Amazon Basin since the last glacial period is important for evaluating claims that arid conditions reduced or fragmented the rainforests, for constraining the role of the tropics in the budget of atmospheric methane, and for understanding how tropical climate may have affected the global water cycle (see the Perspective by Betancourt). Maslin et al. (p. 2285) have taken an obvious but previously unused approach to clarify this vigorously debated issue. By measuring the calcite oxygen isotopic composition of surface-dwelling foraminifera, they capitalize on the isotopic difference between freshwater and ocean water to determine how much discharge has flowed from the Amazon River and mixed with the ocean just downstream of the river's mouth during the past 14,000 years. Their results show that the Younger Dryas was extremely dry, and that the present period is the wettest time in the record. In a separate study, Mayle et al. (p. 2291) use pollen analysis to examine the long-term dynamics of forest-savanna boundaries in southern Amazonia, which depend intimately on climate patterns. Consistent with the results of Maslin et al., they find that the present southern boundary of forest in Bolivia represents the greatest southern extent of forest for the past 50,000 years, and that the forest at its southern limit may be less than 3000 years old.

  7. A Hotspot for AD Genes

    The principal genes that are mutated in the rare, early onset familial form of Alzheimer's disease (AD) were identified and characterized some time ago. However, pinpointing genes associated with the more common, late onset form of AD has proved much more difficult. Three reports by Bertram et al. (p. 2302), Myers et al. (p. 2304), and Ertekin-Taner et al. (p. 2303) now present evidence from genetic linkage studies that highlight a hotspot on the longarm of human chromosome 10 that may contain one or more genes associated with late onset AD.

  8. Divider's License

    When eukaryotic cells divide, they must replicate their genome once and only once. A process known as licensing makes sure that cells must finish dividing before being allowed to replicate DNA again. In addition to the inhibition of further replication by cyclin-dependent kinases, geminin is thought to block the binding of mini-chromosome maintenance (MCM) complexes to the origins of replication. Wohlschlegel et al. (p. 2309; see the Perspective by Lygerou and Nurse) now show in human cells that geminin interacts with human homolog of Cdt1, a replication factor that was recently identified as being needed for MCM loading. Cdt1 is present in the G1 and S phases, whereas geminin is present in the S and G2 phases, thus suggesting that geminin helps prevent inappropriate firing of replication that might be induced by Cdt1.

  9. Maintaining Early Development

    Although much is known about the transcription machinery for the three eukaryotic polymerases, relatively little is known about the role of the TATA-binding protein (TBP) or TBP-related factors in the early development of vertebrates. Veenstra et al. (p. 2312) have used antisense oligonucleotides to examine the role of TBP or the TBP-like factor (TLF) during early Xenopus embryogenesis. The two factors have different roles, in that TBP-deficient embryos undergo the initial phases of gastrulation but fail to complete subsequent stages of development, and TLF is required for the initiation of zygotic transcription. Hence, TBP and its related factor TLF are both necessary for Xenopus development, with each factor displaying different functions in early embryogenesis and transcription.

  10. Binding Assays by the Genome

    In the cell, proteins bind to DNA elements to regulate such processes as transcription, replication, and chromosome condensation and cohesion. Many recent studies have tried to elucidate the various mechanisms involved in these processes, including specific protein-DNA interactions. For these analyses, researchers have widely used the techniques of chromatin immunoprecipitation analysis and DNA microarrays to examine protein-DNA interactions and gene expression levels, respectively. Ren et al. (p. 2306) have now combined these two methods to monitor in vivo protein-DNA interactions across the entire yeast genome. This genome-wide location analysis method was used to characterize two transcriptional activators Gal4 and Ste12. These techniques should allow for a more detailed understanding of global regulatory networks.

  11. Hold Those Thoughts

    Working memory (the active representation of information in one's mind) is mediated by a widely distributed network of neuronal circuits in the brain. In a functional magnetic resonance imaging study, Furey et al. (p. 2315; see the Perspective by Robbins et al.) analyzed how the brain cholinergic system interacts with the different components of the central nervous system involved in a visual working memory task. During encoding of faces, a change in cholinergic activity induced by a drug that inhibits the breakdown of acetylcholine enhanced the selectivity of responses in the ventral occipital cortex and simultaneously reduced the activity in prefrontal regions. These results show how neurochemical processes regulate the neural systems involved in working memory function.

  12. Simplifying Surfaces

    In a variety of scientific disciplines, it is often desirable, although not always feasible, to transform high-dimensional data into a low-dimensional space. For example, the black-and-white image of a face contains light and dark areas whose appearance may depend on the direction of illumination. As the head is rotated, the mix of light and dark areas changes in a continuous but very complicated fashion because of the presence of discrete features, such as cheekbones and eyebrows. Nevertheless, there is an underlying simplicity defined by a single parameter—rotation. Tenenbaum et al. (p. 2319) and Roweis and Saul (p. 2323) present two algorithims that are able to compute this transformation, and the implications of being able to make this computation are discussed by Seung and Lee in a Perspective.