This Week in Science

Science  25 Jan 2008:
Vol. 319, Issue 5862, pp. 381
  1. Dissecting Stardust Origins


    Stardust recently returned the first samples collected from a known comet. It was thought that these samples might resemble interplanetary dust particles (IDPs), which are also thought to come from comets and have been collected in Earth's stratosphere and elsewhere on Earth, such as in polar ice. Ishii et al. (p. 447; see news story by Kerr) directly compare silicate grains from Stardust and IDPs and show that this is not the case. Instead, the Stardust samples resemble grains from meteorites. These findings imply that there is a continuum between asteroids and comets, that at least this comet does not have much material from the outer solar system, and that the IDPs may be the most primitive remaining material in the solar system.

  2. Spinning Odds and Evens

    The need for a net change in electronic spin along a reaction pathway usually leads to substantial slowdown of the overall transformation. Burgert et al. (p. 438) show that this spin-conservation principle extends straightforwardly to a series of small anionic metal clusters comprised of ∼10 to 20 aluminum atoms. Previous studies had revealed a puzzling alternation in the reactivity of odd and even-numbered Alnclusters with oxygen. By varying the spin state of both the clusters (through addition of H atoms) and the O2 (through excitation to the singlet), the authors obtain mass spectrometric data that correlate reactivity with spin-conserving pathways.

  3. Dynamics of a Dangling Bond

    The development and optimization of functional materials and devices depend on thoroughly characterizing the carrier transport properties of the material. As device structures decrease in size, macroscopic characterization techniques may no longer be valid. Berthe et al. (p. 436, published online 13 December) investigate the transport of inelastic tunneling electrons through a localized state in Si, a dangling bond, and look at how the transport properties are modified by the local microscopic environment.

  4. Core Problem

    A major goal of mantle geochemistry has been to find isotopic or chemical signatures of the outer core in mantle-derived materials. Unusual osmium isotope ratios in Hawaiian rocks have been used to argue that such material could rise to the surface from the core through deep mantle plumes. However, this interpretation has been contentious, and other evidence to back it up has been sparse. Luguet et al. (p. 453; see the Perspective by Meibom) demonstrate instead that anomalous Os ratios need not arise from material leaked from the core. Sulfides can affect the fractionation of the Pt-Os and Re-Os systems and result in Os heterogeneities in the mantle itself.

  5. A Look Inside Reactors


    Many industrially significant chemical reactions rely on flowing gaseous compounds through packed solid catalysts. Optimization in these heterogeneous environments would benefit from the capacity for detailed mapping of flow patterns and reactive site distributions, but probes of sufficient sensitivity are often lacking. Bouchard et al. (p. 442) show that magnetic resonance imaging, which typically requires liquid samples, can be applied to the much more diffuse gas-solid interface of a microreactor by signal enhancement from the para nuclear spin isomer of H2. They probe the hydrogenation of propylene to propane, and find that use of the ρ-H2 in tandem with precisely timed pulse sequences allows direct visualization of flow velocities and active-catalyst density profiles.

  6. Short-Wavelength Photonics

    Reductions in the operational wavelength of laser diodes into the blue and violet has made possible the development of high-density recording media such Blu-ray optical discs. Generally, the laser diodes at the heart of such technology are formed from bulk crystals of GaN. Matsubara et al. (p. 445, published online 20 December) now show that photonic crystal technology, already demonstrated for surface-emitting lasers at infrared wavelengths, can be scaled down to emit in the blue-violet regime. The possibility to engineer the emission wavelength and polarization mode may enable even higher optical storage densities.

  7. Soft Mantle Signature

    Earth's lower mantle extends from depths of 600 to about 2900 kilometers, and for much of this depth the mantle contains two major minerals, perovskite and ferropericlase. Both minerals contain some iron, and at depths below about 1300 kilometers, it has been shown that changes in the spin pairing of the iron affects the properties of these minerals. Crowhurst et al. (p. 451) measured the stiffness of ferropericlase at high pressures across this transition. With increasing depth across the transition, the mineral becomes progressively softer to the propagation of seismic waves, and more so as the iron content increases. These data may help explain the lack of a clear seismic signal of a phase transition at depth and may help explain the seismic signatures of the deeper lower mantle.

  8. The Inconstant Female

    Female choice is thought to drive evolution through sexual selection. It has been assumed that females over time would show consistent preferences for the same male traits. However, Chaine and Lyon (p. 459; cover) found in a long-term study of sexual selection in lark buntings that females have flexible patterns of choice for male traits over several years. This finding explains both the stability of traits under sexual selection and the evolution of multiple male sexual signals. Analyses of phenotypic selection with short time frames can lead to incorrect predictions about the trajectory of sexual selection, which might explain earlier contradictory findings.

  9. Fast and Slow

    During gene transcription, some activator proteins bind cyclically to their promoters, with periodicities of ∼30 seconds (fast cycling) or ∼30 minutes (slow cycling). Karpova et al. (p. 466) now show that these different cycles are distinct, but that the same transcription activator can engage in both cycling activities on the same promoter at the same time. It seems that the fast cycle is involved in transcription initiation, whereas the slow cycle modulates the number of promoters accessible for initiation.

  10. Homering in on T Cell Activation


    In the immune system, T cells are stimulated by signals that emanate from the T cell receptor (TCR) and co-stimulatory cell surface receptors, most notably CD28. CD28 signals profoundly influence the ensuing immune response—TCR stimulation in the absence of CD28 results in anergy, a state of permanent inactivation. Huang et al. (p. 476) provide evidence that the alternate pathways for a T cell are coordinated by two cytoplasmic scaffolding proteins, Homer2 and Homer3. Absence of these proteins caused unchecked activity of the central cytokine transcriptional regulator NFAT (nuclear factor of activated T cells), up-regulation of cytokine expression, and signs of overt T cell reactivity. Thus, a dynamic decision mechanism dictates whether a T cell will become activated or anergized.

  11. Activating Aurora

    Cell division in eukaryotes requires tight spatial and temporal control of its many components. Aurora B kinase, as part of the chromosomal passenger complex (CPC), plays a critical role in regulating chromosome segregation to daughter cells. How is the activity of Aurora B itself controlled? Rosasco-Nitcher et al. (p. 469) show that Aurora B is regulated at several levels by the protein teleophase disc 60-kD (TD-60), which, like Aurora B, is also found at inner centromeres during metaphase. Interaction with TD-60 brings the CPC to the centromere, and, in combination with microtubules, activates Aurora B. Furthermore, Aurora B can only act on previously phosphorylated substrates, whose phosphorylation is also enhanced by TD-60. Thus, TD-60 may function to ensure that high levels of Aurora B activity occur only at centromeres.

  12. How Yeast Responds to Change

    The origin of the rapid adaptive response of yeast cells to changes in environmental osmolarity has been unclear. Mettetal et al. (p. 482; see the Perspective by Lipan) now show that increases in extracellular osmolarity activate the high-osmolarity glycerol signaling pathway, which changes transcription of particular target genes. By measuring the cellular response to pulses of medium with increased ionic strength, the authors were able to develop a predictive model of the dynamics of this regulatory system. Rather than changes in gene expression, which have often been suggested to be at the core of the response to osmotic shock, the fast response is actually dominated by a nontranscriptional response that probably involves altered glycerol transport.

  13. Root of the Problem

    Soils contain more than three times as much carbon as the atmosphere and four times as much carbon as terrestrial biomass above ground level. One of the most abundant types of below-ground carbon is that of fine roots. Beside the obvious importance within the area of plant-physiology of understanding how long those structures persist, it also is critical for understanding how effective carbon sink soils may be in a warmer, higher CO2 world. Strand et al. (p. 456) examine two different techniques used to determine the residence time of fine-root carbon. One method, based on the analysis of the isotopic composition of carbon, systematically underestimates carbon turnover, and the other, based on direct observation of roots with cameras, overestimates turnover. The difference is irreconcilable as these two approaches are measuring different processes.

  14. Silence, Repeat, Silence

    Repeated sequences in the genome can promote potentially hazardous recombination, as can active transposons, which can also cause mutations by disrupting genes they insert into. Such undesirable elements are inactivated by DNA methylation and packaging into inert heterochromatin. Saze et al. (p. 462) have conducted a screen in thale cress (Arabidopsis thaliana) to identify mechanisms that prevent the encroachment of DNA methylation and heterochromatin into nearby genes. A gene, increase in BONSAI methylation (IBM), acts in this pathway, and is likely to represent a histone H3 lysine 9 (H3K9) demethylase. IBM may counter spreading of silencing marks by removing H3K9 methylation, which may direct the underlying DNA methylation.

  15. Knowing the Unknowns

    It can be difficult to compare one gene with another in order to decide whether the two genes are derived from the same ancestor. It is harder still when the comparison involves thousands of genes at a time or even entire genomes, as the outcomes for individual genes could vary widely. Wong et al. (p. 473; see the Perspective by Rokas) analyzed large, automated, genomic evolution studies and suggest that the underlying comparison protocols suffer by excluding an estimate of the uncertainty of gene alignments. Genome-wide comparisons could be more informative if measures of the uncertainty could be included as part of the analysis.

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