This Week in Science

Science  09 Apr 1999:
Vol. 284, Issue 5412, pp. 221
  1. Tiny Switch

    As the demand for faster computers with larger memory grows, there is a need to reduce the size of the devices that perform logic operations. However, the extent to which the dimensions of transistors can be reduced and operate conventionally is limited by leakage currents. One approach for creating nanometer-scale devices is to make use of properties other than charge. Amlani et al. (p. 289; see the Perspective by Smith) demonstrate one such approach based on quantum-dot cellular automata, in which polarization and not charge determines the state of the device. Logical AND and OR gates were fabricated whose state at very low temperatures was determined by the position of just two electrons.

  2. Don't Crowd the Fermions

    Particles that obey Bose-Einstein statistics (such as photons) can occupy the same energy level—bosons like to be together. More than 40 years ago, Hanbury Brown and Twiss demonstrated experimentally one of the fundamental properties of these particles: When they are emitted from a thermal boson source, they do so in bunches (see the Perspective by Büttiker). Henny et al. (p. 296) and Oliver et al. (p. 299) now verify in two different and independent experiments that, in contrast to the bosons, particles obeying Fermi-Dirac statistics (such as electrons) exhibit antibunching. This behavior was predicted theoretically, but up to now had evaded experimental verification.

  3. Cooler Ceramic Synthesis

    For many device applications, single-crystal films or materials are favored because the grain boundaries of polycrystalline materials adversely effect their properties. Switzer et al. (p. 293) show that electrochemical methods can produce single-crystal films of cubic bismuth oxide at room temperature on gold substrates. This material, which has excellent ion transport properties for fuel cell applications, is normally unstable below 730° Celsius and is formed as a kinetically stabilized product.

  4. About Face

    Hominid phylogenies are defined in many cases by differences in characters of the face and cranium (the most commonly preserved and diagnostic fossils). McCollum (p. 301; see the news story by Morell) examines, for the case of the robust australopithecines (an enigmatic group of hominid fossils), the degree to which many of these characters can be attributed to relatively simple changes in dental proportions that have a developmental basis. Thus, the cladistic “definition” of this group may not be indicative of the phylogeny.

  5. Waving in the Distance

    When waves are triggered in a medium—by throwing stones in a pond or sending electrical impulses to the heart—they usually emanate from the point of stimulation. Christoph et al. (p. 289) describe an electrochemical system where triggering initiates a wave at a point distant from the trigger. The oxidation of formic acid at platinum electrodes has passive (OH poisoned) and active (high current) states, and an appropriate voltage pulse can initiate two circularly traveling wave fronts that activate the passive state. However, a pulse of an opposite sign, which might be expected to reinforce the passive state, instead initiated activation waves but from a point opposite the trigger pulse. Such action at a distance occurs because the coupling function for ion migration that allows wave propagation reverses its sign at longer distances.

  6. Solar Cascade

    Several climate indices and environmental or ecological phenomena show an approximate 11-year cycle that seems to be similar to the solar cycle. However, the irradiance change associated with the solar cycle is very weak, so it has been very difficult to demonstrate a mechanistic connection. Shindell et al. (p. 305; see the news story by Kerr) now show, using a climate model that includes ozone variability and wavelength-dependent irradiance, that the solar cycle may have an effect on tropospheric climate. The connection is that changes in stratospheric ozone abundance, driven by the solar irradiance cycle, may affect stratospheric circulation, which affects tropospheric energy and thus regional surface temperatures.

  7. Leaving the Fold

    Protein structures usually react to single mutations by relatively small adjustments in their structure; significant changes to secondary and tertiary structural elements are generally believed to require more severe sequence changes, such as large insertions or deletions. Cordes et al. (p. 325) show that in the small Arc repressor protein dimer, a switch between just two residues results in a local change from a β-sheet to a helical structure. Within the constraints imposed by the connectivity between the secondary structure elements, such a change can be understood from the sequence pattern. Even relatively small numbers of mutations can thus alter a protein fold.

  8. Controlling NF-κB

    The activity of NF-κB, a master transcription factor for many genes essential for inflammatory and immune responses, is held in check by inhibitory proteins known as IκBs. In cells exposed to tumor necrosis factor (TNF) or interleukin-1 (IL-1), IκBs are phosphorylated and degraded. The phosphorylation of IκBs is thought to be mediated by IκB kinase complexes that include similar IKKα and IKKβ (or IKK1 and IKK2) protein kinase subunits. Four reports, which describe the phenotypes of mice that lack either IKKα or IKKβ, provide new insight into the physiological roles of these enzymes. Li et al. (p. 321) show that cells from mice lacking IKKβ indeed have reductions in the responsiveness of NF-κB to TNF and IL-1. The mice themselves die before birth because of increased apoptosis in the liver. Loss of IKKα, however, produced a quite distinct phenotype. Hu et al. (p. 316) and Takeda et al. (p. 313) find that these mice die shortly after birth and show abnormalities in proliferation and development of skin and in morphogenesis of the limbs and skeleton. In cells from mice lacking IKKβ, activation of NF-κB in response to TNF and IL-1 remained intact. Delhase et al. (p. 309) provide additional evidence that IKKα is the enzyme that mediates signaling from TNF-α and IL-1 from an analysis of mutant forms of the IKKs that lack sites for activation by phosphorylation. Furthermore, their results indicate that IKKβ may be inactivated through autophosphorylation of a cluster of serine residues at the carboxyl terminus of the protein, thus limiting the duration of inflammatory signaling. In a Perspective, May and Ghosh outline the current understanding of the signaling mechanisms that control activity of NF-κB in light of the findings that define distinct biological roles for IKKβ.

  9. Balancing Nutrition

    Plant seeds contain stored protein, lipid, and starch in various combinations that contribute to that seed's value as a food or oil source. Lin et al. (p. 328) identified a protein, SSE1, in the small oilseed plant Arabidopsis, and found that mutations in SSE1 can disrupt the storage of protein and lipid in favor of starch. Thus, the diverse metabolic pathways that assemble the storage materials are intricately interlinked, and the ultimate balance of components may be amenable to readjustment.

  10. Dads and Daughters

    In mammals, although each parent contributes genetic material to its offspring, parent-specific genomic imprinting allows for the favored expression of one parent's allelic contribution over the other. Li et al. (p. 330) examined the imprinted mouse gene Peg3. Although this gene is expressed exclusively from the paternal allele, it is responsible for regulating maternal behavior, nurturing, and growth of the offspring. Mutation of Peg3 yielded a decreased level of oxytocin neurons. Because oxytocin has been shown previously to affect maternal nurturing and lactation, the regulation of maternal behavior by the paternally expressed gene Peg3 is now better understood physiologically.

  11. Patterns in Nature

    Harte et al. (p. 334; see the Perspective by Rosenzweig) examine the properties of self-similar populations and communities. They demonstrate that several fundamental ecological generalities—the species-area curve, the species-abundance distribution, and a new area-endemism relation—can be linked conceptually into a single framework. The work also shows that two widely accepted patterns—the lognormal species-abundance distribution and the power-law species-area curve—conflict with one another.

  12. Injurious Ions

    The role that ions play in cell death is the subject of two reports. During stroke, neurons are thought to die in part because of the overactivity of the NMDA class of neurotransmitter receptors. These receptors form channels for a variety of ions, including potassium. Yu et al. (p. 336) explored the hypothesis that potassium efflux from cells may contribute to their demise. Under conditions that mimic those found in stroke, cultured neurons were found to undergo apoptosis in response to NMDA-receptor-mediated potassium efflux. One of the classic examples of cells dying an apoptotic death is by calcium influx into neurons, which activates a phosphatase called calcineurin. How calcineurin evokes death has not been clear. Now Wang et al. (p. 339) report that calcineurin can associate with and remove an inhibitory phosphate from the pro-apoptotic protein BAD. This process relocates BAD from the cytosol of the neurons into the mitochondria, where it dimerizes with and inactivates the anti-apoptotic protein Bcl-xL. Thus, brain insults and conditions that increase Ca2+ can modulate the phosphorylation state of a key protein in the apoptosis pathway.

  13. Commons Problems

    In the 30 years since Hardin published his “The Tragedy of the Commons,” the question of how to solve the apparently inevitable overuse of commonly held resources has received increased attention. Ostrom et al. (p. 278) review local cases of failure (and success) in managing commonly held resources, and discuss strategies for global resources, such as biodiversity and the atmosphere and oceans.

  14. CCR5 Promoter Alleles and Specific DNA Binding Factors

    M. P. Martin et al. (Reports, 4 Dec. p. 1907) performed “genetic association analysis of five cohorts of people with acquired immunodeficiency syndrome (AIDS) [which] revealed that infected individuals homozygous for a multisite haplotype of the CCR5 regulatory region containing the promoter allele, CCR5P1, progress to AIDS more rapidly than those with other CCR5 promoter genotypes, particularly in the early years after infection.”

    J. H. Bream et al. (including several co-authors of the original report) comment that they “have found a distinction in specific binding affinity for separate CCR5P allele sequence motifs to nuclear binding (potential transcription) factors, which suggests a possible mechanism for CCR5P1/P1 epidemiologic consequences.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/284/5412/223a

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