This Week in Science

Science  10 Aug 2001:
Vol. 293, Issue 5532, pp. 1009
  1. Solar Cell Organization

    In organic solar cells, absorbed photons create charge carriers (electrons and “holes”) that are taken up by n-and p-type materials that send them to the electrodes. The efficiency of the cell will depend on how closely the n-and p-type materials can be brought into contact. Schmidt-Mende et al. (p. 1119; see the Perspective by Nelson) have developed a simple solution-based process in which self-organization creates a nanoscale interface in the organic layer. The hole-accepting material is incorporated as a discotic liquid crystal, which creates columnar structures within the film that are surrounded by an electron-accepting organic dye. This structure leads to efficient charge separation of carriers between the discotic material and the dye.

  2. Perturbed Planet Formation

    In the standard model for terrestrial planet formation in our solar system, enhanced accretion of planetesimals occurred in a protoplanetary disk to form much larger planetesimals that eventually formed planets. Kortenkamp et al. (p. 1127) show that this process could be efficient if drag from solar nebular gas and perturbations from a larger companion (such as a giant planet, a brown dwarf, or another star) were included. This different mode of “runaway growth” might be useful to explain the formation of extrasolar planetary systems and to estimate whether terrestrial-like planets may exist deep within opaque accretionary disks.

  3. Structure of the Intergalactic Medium

    The intergalactic medium is the gaseous material between luminous structures such as galaxies and clusters. Kriss et al. (p. 1112; see the Perspective by Miralda-Escudé) have probed the ionized structure of this gas at intermediate epochs of the universe using the Far Ultraviolet Spectroscopic Explorer (FUSE). Many of the early structures that formed were quasars, and some structures were starburst galaxies. These structures could be found even in low-density regions, which is consistent with the current model of structure formation by gravitational instability.

  4. Precocious Land Plants

    The colonization of land by plants, typically put at about 400 million years ago, allowed the establishment of terrestrial ecosystems and the evolution of life on land. Heckman et al. (p. 1129; see the news story by Pennisi) pooled molecular data on nuclear protein coding genes drawn from across the spectrum of diversity of living organisms in an attempt to estimate the time of origin of land plants and fungi. The results indicate a substantially earlier origin—perhaps 700 million years ago—for land plants and fungi than is commonly held.

    CREDIT: HECKMAN ET AL.
  5. Missing a Supermassive Black Hole in M33

    Recent work has shown a correlation between the mass of a supermassive black hole at the center of a spiral galaxy and the velocity dispersion in the bulge of the galaxy. Merritt et al. (p. 1116; see the 20 July news story by Seife) analyzed spectra from the Space Telescope Imaging Spectrograph (STIS) for the spiral galaxy M33, a member of the Local Group that has almost no central bulge. They estimated an upper limit to the mass of any candidate central black hole to be 3000 solar masses, which is orders of magnitude too low for a supermassive black hole. Thus, either the mass-to-dispersion relation changes for intermediate-mass black holes or these less-massive black holes form by a different mechanism.

  6. Getting Polarization by Design

    The integration of photonic band gap (PBG) structures with active lasing media could lead to the development of lasers with superior features such as single-mode emission over large areas, high power, surface emission with a very narrow divergence angle, and controlled polarization. Noda et al. (p. 1123) now present a combined theoretical and experimental study that suggests that such devices may not be so far away. The optical properties were computed for a two-dimensional PBG crystal with a square lattice array of holes, and the device was then fabricated. The polarization mode of the emitted light can be controlled in structures with elliptically shaped holes.

  7. Making Proteins in the Nucleus

    Translation, in which the ribosome decodes messenger RNA into protein, has been thought to occur exclusively in the cytoplasm of eukaryotic cells and not in the nucleus. However, some aberrant RNAs containing premature termination codons are recognized and degraded in the nucleus, a phenomenon most easily explained if ribosomes are scanning messages in the nuclear compartment. Iborra et al. (p. 1139; see the Perspective by Hentze), using three different protein-labeling methods, show that up to ∼15% of cellular translation actually occurs in the nucleus. Transcription of the RNA is coupled to translation, as has been found in prokaryotes. One function of these nuclear ribosomes may be to “proofread” newly synthesized RNA transcripts for errors.

  8. A Tale of Histone's Tails

    The DNA of all eukaryotes is packaged into chromatin, which is composed largely of histone proteins. Covalent modifications of the tails of these proteins play an important role in both in chromosome organization and in the specific regulation of individual genes. Litt et al. (Science Express, 9 August) and Noma et al. (p. 1150) show that methylation of Lys4 in the amino-terminal tail of histone H3 is specific to euchromatic domains (where genes are generally active) and methylation of H3 Lys9 is specific to heterochromatic domains of chromatin (where genes are generally inactive). Inverted repeats flanking the heterochromatin act as boundary elements and preventing its spread into the surrounding euchromatic regions. At the level of individual genes, Lo et al. (p. 1142) show that phosphorylation of Ser10 on histone H3 by Snf1 kinase facilitates acetylation of Lys14 by Gcn5 (but not vice versa), and that both are needed for the activation of the INO1 gene in vivo.

  9. Cardiovascular Health Gone in a WNK

    High blood pressure (hypertension) is one of the principal causes of heart attack, stroke, congestive heart failure, and kidney failure. Wilson et al. (p. 1107; see the news story by Marx) identify two related genes that are responsible for a rare form of hypertension called pseudohypoaldosteronism type II, an autosomal dominant trait characterized by abnormalities in renal electrolyte handling. The two genes, hWNK1 and hWNK4, are expressed in the kidney and encode members of a recently described family of serine-threonine kinases. The disease-causing alterations in the WNK genes are likely to be gain-of-function mutations that would enhance the expression or activity of the kinases. Because hWNK4 lies at a locus on chromosome 17 that has been linked to bloodpressure variation in the general population, this gene may also play a role in the most common forms of hypertension.

    CREDIT: WILSON ET AL.
  10. Pointing the Finger

    One difficulty in developing a vaccine against human immunodeficiency virus-type 1 (HIV-1) is that only three isolated human antibodies are potent enough to neutralize this virus in vitro and in vivo. Saphire et al. (p. 1155) offer the crystal structure of one of these antibodies, b12, as a clue that may point vaccine developers in a productive direction. Docking this structure onto that of gp120, the envelope protein of HIV-1, suggests that a precisely angled Trp residue inserts into a narrow crevice of gp120 in a fashion that resembles that of a critical Phe residue of CD4, an HIV-1 receptor.

  11. Acetylation Flips the Transcription Switch

    Cells respond to external challenges such as viral infection by adjusting gene expression. Viral infection activates the expression of interferon-β (INF-β) via a protein complex, termed the enhanceosome, that forms on the gene's promoter. The interferon enhanceosome recruits two acetyltransferases, PCAF and CBP, that not only acetylate the histone components of nucleosomes but also acetylate the enhanceosome itself. Munshi et al. (p. 1133; see the Perspective by Struhl) report that PCAF acetylation of the enhanceosome protein HMG-I at the Lys71 residue stabilizes the enhanceosome and protects it from CBP acetylation. However, acetylation of HMG-I by CBP at Lys65 leads to the dissociation of the enhanceosome from the DNA molecule. Therefore, the enhanceosome is a dynamic complex that is acted upon by competing acetylations to create a switch that turns transcription on or off.

  12. Coincidence Detection in Pyramidal Neurons

    It is not known whether the timing of an action potential in pyramidal cells reports the timing of the afferent activity that triggered the spike. Pouille and Scanziani (p. 1159) compared the temporal precision with which firing is induced in hippocampal neurons by afferent stimuli when synaptic inhibition was functional and when it was suppressed. Inhibition limited the temporal variability of pyramidal cell discharge, and this effect of synaptic inhibition is stronger near the soma than at dendritic sites.

  13. Unraveling the Placebo Effect

    The placebo effect has long intrigued medical and pharmacological researchers. De la Fuente-Fernández et al. (p. 1164) examined the neural basis of the placebo effect and its role in the treatment of Parkinson's disease (PD) using [11C]raclopride binding as measured by positron emission tomography. The placebo effect in PD is mediated by the specific improvement in dopaminergic neurotransmission through an increase in the release and the synaptic levels of dopamine in the striatum. Although patients receiving an active drug in a placebo-controlled study benefited from the placebo effect, this effect did not synergistically augment the effect of the active drug.

  14. The Fermi Level and High-Temperature Ferromagnetism

    The observation of weak ferromagnetism in lanthanide-doped hexaborides with transition temperatures (Curie temperatures) in excess of 700 K spurred additional work aimed at synthesizing materials with higher Curie temperatures or stronger ferromagnetism. The failure of these attempts suggested that the threefold degeneracy associated with the cubic structure of the hexaboride was a key feature. Akimitsu et al. (p. 1125) now show that a material without any magnetic constituents, CaB2C2, that crystallizes in a tetragonal structure also exhibits weak ferromagnetism similar to the La-doped hexaborides. Band-structure calculations show similarities between the hexaborides and these materials, which suggests that it may be the molecular orbitals near the Fermi level that give rise to the effect. The results may provide a route for the preparation of other high-temperature ferromagnets.

  15. Sources of Receptor Specificity

    The cytokine interleukin-5 (IL-5) induces the proliferation and differentiation of various blood cells through its receptor, IL-5R. Although the β subunit of the IL-5R is similar to other receptors, the α subunit is specific for IL-5 signaling. Geijsen et al. (p. 1136) have explored IL-5 specificity and report that IL-5R-α interacts with the PDZ-domain-containing protein syntenin, and that syntenin interacts with the transcriptional activator Sox4. In cytokine-receptor signaling, syntenin is critical for IL-5-mediated activation of Sox4. Thus, a mechanism of transcriptional activation by cytokine-specific receptor subunits has been elucidated. The IL-5R-induced activation of Sox4 may play a role in the regulation of early B cell proliferation and may function in the differentiation of other hematopoietic cells.

  16. Opening Up the RNA Killing Machine

    Double-stranded RNA can direct the destruction of homologous messenger RNAs and thus silence gene expression, a process known as RNA interference (RNAi). RNAi involves two steps. Double-stranded RNAs are processed into short ∼21- to 23-nucleotide fragments by the Dicer nuclease. These fragments are incorporated into the RNA-induced silencing complex (RISC) nuclease complex and guide it to the RNA to be destroyed. Hammond et al. (p. 1146) report the purification of the RISC complex and identify one of its components as the protein Argonaut2. The interaction of Dicer and Argonaut2 may facilitate the incorporation of the ∼21- to 23-nucleotide RNA fragments generated by Dicer into RISC.

  17. The Effectiveness of Parks

    Bruner et al. (Reports, 5 Jan. 2001, p. 125) surveyed 93 protected areas in 22 countries to test whether parks “are an effective means to protect tropical biodiversity.” They found that park effectiveness correlated with “basic management activities such as enforcement, boundary demarcation, and direct compensation to local communities”—a result, they suggested, that argues for increased park funding. Vanclay, in a comment, takes issue with the statistical measure used by Bruner et al. to gauge effectiveness, presents alternative analyses suggesting that only two variables (boundary demarcation and the involvement of local educators) are relevant to park effectiveness, and argues that Bruner et al. presented no evidence to support the conclusion that increased funding will improve park effectiveness. Bruner et al. respond that Vanclay's analyses “largely support rather than detract from our findings,” present additional details in support of the original study's methodological soundness, and note that Vanclay's analysis, like theirs, suggests that “increased support for management (financial and otherwise) will increase effectiveness.” The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/293/5532/1007a

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