This Week in Science

Science  13 Jun 2003:
Vol. 300, Issue 5626, pp. 1617
  1. Controlling Schottky Barriers

    When two semiconductors or a metal and semiconductor are brought together, band realignment and charge transfer result in an electrical barrier. However, Schottky theory does not always describe the electronic properties of the barrier very well. McKee et al. (p. 1726) propose that structural, chemical, and electrical properties of the interface need to be included for a more precise description. Their data suggest that the height of the Schottky barrier can be tuned by controlling the properties of the interface region, an approach that should be useful for other oxides grown on silicon.

  2. Spectroscopic Characterization of a Fermi Gas

    Radio-frequency (RF) spectroscopy has long been used to characterize the energy spectra of atomic ensembles. The frequency of resonant transitions can be used as the timekeeper in atomic clocks, but their accuracy and precision is limited by the broadening of lines caused by interactions and collisions between atoms. Gupta et al. (p. 1723; see the Perspective by Inguscio) performed RF spectroscopy on an ensemble of fermionic lithium atoms and show that Pauli exclusion between fermions suppresses the scattering rate between atoms, thus narrowing the resonance peak. The result provides the prospect of using cold fermionic gases for improved atomic clocks.

  3. Impact and Extinction?

    The causes of global extinctions can be difficult to identify, particularly in attempting to link events that occurred early in Earth's history. Ellwood et al. (p. 1734) have found evidence for an impact ejecta layer in mid- Devonian (380 million years ago) marine carbonate rocks in the Moroccan desert. The impact ejecta are associated with the global Kacák/otomari extinction event and may indicate that a bolide impact caused that extinction.

  4. Changing the Monsoon

    The Indian Ocean monsoon (IOM) is known to have been stronger 10,000 years ago. It is important to understand whether it became less intense suddenly as well as how the IOM varies on decadal to centennial time scales in order to link its behavior to transient climatological variability. Fleitmann et al. (p. 1737) present an oxygen isotope record that shows variations in the hydrological cycle over Southern Arabia for most of the past 10,000 years. This reveals a change in the forcing mechanisms of the IOM during the Holocene, shifting from the influence of glacial boundary conditions to that of Northern Hemisphere solar insolation, which caused a gradual drop in IOM intensity over 5000 years.

  5. Silverish Bullet

    Hydrogen has been hailed as the perfect fuel for many applications, and it has been suggested that a significant fraction of our fossil fuel consumption can be replaced by hydrogen-burning fuel cells. Might there be consequences? Tromp et al. (p. 1740) investigated a hydrogen economy with an atmospheric model, assuming that some H2 would escape from fuel cells without being burned. They then computed the distribution of H2 in the stratosphere that would result from a fourfold increase in atmospheric H2 and the impact on stratospheric water and ozone. The lower stratosphere would become wetter and cooler, and more ozone would be destroyed.

  6. Shaping Up on the Treadmills

    In animal and fungal cells, microtubules are nucleated from a centralized organelle that organizes the polymers into polarized arrays. Shaw et al. (p. 1715; see the Perspective by Wadsworth) provide an analysis of single microtubule dynamics and behavior in a plant cell system—a system that lacks a centralized microtubule-organizing center. New polymers are nucleated at distributed sites on the plant cell cortex, and the microtubules then detach from initiation sites and use motility to organize into arrays. Thus, a hybrid form of polymer treadmilling represents the principle mechanism of microtubule motility in the cortical arrays of plant cells and provides an effective means for repositioning microtubules into new organizational states.

  7. Fly Sexual Dimorphism

    In a comparative microarray analysis, Ranz et al. (p. 1742) have discovered that sex determines how genes are expressed in two closely related species of fruit fly. For example, males of Drosophila melanogaster have up-regulated olfactory functions, whereas males of D. simulans have up-regulated visual functions, hinting at the evolution of differences in mating behavior. Furthermore, male-biased and female-biased genes have specific chromosomal locations, suggesting that gene location may be functionally more important than expected.

  8. Interference Between Channels

    The simple picture of chemical reactions is that reactants with enough energy go through the transition state and form products. However, energy in molecules is quantized, and the opening up of different rotational-vibrational states of similar energies could lead to interference effects that would cause the reactivity of some state-to-state channels to decrease. Dai et al. (p. 1730) studied the reaction of hydrogen atoms with deuterium molecules in a molecular beam apparatus and with high-level calculations, and they observe oscillations in reactivity.

  9. Find Your Partners

    Distinct intracellular organelles communicate via vesicular carriers that must fuse only with the correct target membranes, and so-called v- and t-SNAREs on the vesicle and target membranes are thought to control the specificity of these reactions. In order to confirm that SNAREs are the arbiters of intracellular membrane fusion reactions, Hu et al. (p. 1745) engineered SNARE proteins so that their interacting domains were expressed on the cell surface. These “flipped” SNAREs could indeed cause efficient, spontaneous cell-cell fusion.

  10. Entering at the Start

    Recent reports of leukemia in children undergoing gene therapy have highlighted the importance of understanding the basis for retroviral integration. Wu et al. (p. 1749; see the Perspective by Trono) have shown that HIV or HIV-based vectors preferentially target genes, but integration of murine leukemia virus near the promoter region was favored.

  11. Making Sense of It All

    There are three times as many codons as there are amino acids. Some transfer RNAs (tRNAs) recognize more than one codon, and some codons are recognized by more than one tRNA. Amino acids differ in how frequently they appear in proteins, and codons that specify the same amino acid differ in how frequently they appear in genes. Elf et al. (p. 1718) model the relative proportions of charged tRNAs (those tRNAs that carry an amino acid) as a function of amino acid availability. They identify which tRNA species will be most sensitive to changes in amino acid concentration, and the corresponding codons are in fact the ones that are used in the upstream regions of genes encoding amino acid biosynthetic enzymes.

  12. The Substrate for Protein Kinase C in LTD

    Cerebellar long-term depression (LTD) requires protein kinase C-regulated interactions between glutamate receptor subunit GluR2 and other postsynaptic proteins. Chung et al. (p. 1751) found that LTD could not be elicited in cultured Purkinje cells from mice that were deficient in GluR2. LTD in cells from these mice could be rescued with GluR2, but not by mutant GluR2 lacking its protein kinase C phosphorylation site.

  13. Outrageous Offers

    What is fair to one person may seem unfair to another. Sanfey et al. (p. 1755; see the Perspective by Camerer) examine this type of evaluative decision-making by performing a brain imaging study of participants in the Ultimatum Game, in which a pot of money is divided only if one player agrees with the other's offer. A 50:50 offer is usually acceptable, and smaller slices of the pie are less appealing. The likelihood of rejection was found to correlate with the relative brain activity in the anterior insula (involved in emotional responses) versus the dorsolateral prefrontal cortex (cold-blooded reasoning).

  14. Knowledge and Neurons

    Neuronal responses in visual cortex can be modulated when information on the future target is acquired. Sharma et al. (p. 1758) used a target fixation paradigm in which information about stimulus location could be acquired progressively from successive trials or varied randomly. In both monkeys and humans, fixation latency correlated with the probability of the target appearing in a prespecified location. Electrophysiological recordings revealed that the increase in the number of spikes evoked during fixation followed closely the changes in probability of target appearance.

  15. Designing an Anti-SARS Drug

    Anand et al. (p. 1763) present two crystal structures: the protease from human coronavirus 229E (one of the causative agents of the common cold) and the protease from a pig coronavirus in complex with a peptide inhibitor. They use these data to build a model of the homologous protease from the recently identified and sequenced SARS coronavirus, and they confirm that the expressed recombinant SARS coronavirus protease displays the predicted enzyme activity. From the modeled structure, they suggest that the existing small molecule AG7088, currently in clinical trials as a treatment for rhinovirus (another causative agent of the common cold), may be a good starting point for developing SARS therapeutics.

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