This Week in Science

Science  08 Jan 2010:
Vol. 327, Issue 5962, pp. 123
  1. Cradle of Diversity

    Is the biological diversity of reefs a result of attracting species that originated elsewhere, or are they particularly important as cradles of evolution? Kiessling et al. (p. 196; see the cover) examine a large database of fossil benthic marine organisms dating back to the Cambrian to test these questions. It seems that reefs, even in comparison to other shallow marine environments, were indeed important during the origination of new species, including ones that migrate elsewhere.

  2. Nematic Electronic Order in Iron Superconductors

    The properties of many high-temperature superconductors vary strongly as the composition of a doping element changes, and at sufficient under- or overdoping, other phases with different types of electronic ordering can form. Chuang et al. (p. 181; see the Perspective by Fradkin and Kivelson) use scanning tunneling microscopy techniques to probe the electronic structure of an underdoped compound in the iron superconductor family, Ca(Fe1−xCox)2As2. They observed periodic nanostructures oriented along Fe–Fe bonds that exhibit an electronic ordering related to ordering seen in nematic liquid crystals.

  3. Hidden Symmetry Revealed

    CREDIT: COLDEA ET AL.

    It is not often that an exact theory can describe a many-particle quantum-mechanical system, but one of the few exceptions is the behavior of a string of ferromagnets—an Ising chain—at magnetic field strengths that separate different types of ordering. Its excitations were predicted 20 years ago to be governed by the symmetry group E8, one of the most intriguing objects in mathematics. Now, Coldea et al. (p. 177) report direct experimental confirmation of this result in a quasi-one-dimensional Ising ferromagnet CoNb2O6, which they probed by neutron scattering. Two of the eight predicted excitations could be observed. Moreover, the ratio of the two lowest excitations is in quantitative agreement with the so-called “golden ratio” predicted by theory.

  4. Stars Going Quietly or with a Bang

    Stars with masses seven to ten times the mass of the Sun, which can burn carbon in their cores at their end of their lives, may end up as oxygen-neon core white dwarfs or explode as core-collapse supernovae. The defining line between these two end products of stellar evolution is not well understood. Gänsicke et al. (p. 188; published online 12 November) identified two white dwarfs whose photospheric oxygen-to-carbon abundance ratio exceeds unity. Their low carbon abundance and their large quantity of oxygen imply that they are oxygen-neon white dwarfs that lost their hydrogen envelopes. As such, they may have evolved from stars at the borderline between stars that explode as supernova and stars that form white dwarfs.

  5. Smooth Space Pebble

    In September 2008, on its way to meet comet 67P/Churyumov-Gerasimenko, the Rosetta spacecraft flew by asteroid Steins, a member of a very rare class of asteroids that had never been observed closely by spacecraft. Keller et al. (p. 190) analyzed the images to generate a reconstruction of the asteroid's shape. Steins is oblate with an effective spherical diameter of 5.3 kilometers, and it lacks small craters, which may have been erased by surface reshaping. Indeed, Steins's shape resembles that of a body that was spun-up by the YORP effect—a torque produced by incident sunlight, which can alter the rotation rate of a small body—that causes material to slide toward the equator. This effect may have produced Steins's distinctive diamond-like shape.

  6. CRISPR Defenses

    Prokaryotes can be infected by parasites and pathogens and, like eukaryotes, have evolved systems to protect themselves. Horvath and Barrangou (p. 167) review a recently discovered prokaryotic “immune system” characterized by CRISPR—clustered regularly interspaced short palindromic repeats—found in most archaeal and many bacterial species. CRISPR loci harbor short sequences captured from viruses and invasive genetic elements. These sequences are transcribed, and the RNA is cleaved into short CRISPR RNAs (crRNAs) by one of a family of CRISPR-associated (cas) proteins. These crRNAs direct other cas family proteins to homologous nucleic acid targets to effect their destruction. Through its ability to impede the spread of specific nucleic acid sequences, the CRISPR/Cas systems might be exploited to block the dissemination of antibiotic-resistance markers.

  7. Recruiting the Components for Cell Division

    A complete chromosome set must be apportioned to each daughter cell during cell division. A number of molecular mechanisms check that chromosome pairs or homologs are correctly aligned and attached to microtubules just before they separate to the two daughters; their spatial orientation ensures an even inheritance of the genome. One critical component of this system is the protein kinase Bub1. Kawashima et al. (p. 172, see the Perspective by Javerzat; published online 19 November) now show that the main substrate for Bub1 kinase activity in fission yeast is the chromatin protein histone H2A. Phosphorylation of H2A recruits the shugoshin proteins to chromatin, and especially to centromeres, where they also act to ensure correct chromosome segregation. This link between Bub1, histone H2A phosphorylation, and shugoshin is conserved in budding yeast and mammalian cells.

  8. Niche Metamorphosis

    CREDIT: DIVYA MATHUR

    The gut epithelium is continually renewed by cells generated from intestinal stem cells. In the fruit fly Drosophila, cells of the adult gut are derived from cells set aside before metamorphosis. Mathur et al. (p. 210) now show that an early asymmetric cell division in larval stages establishes a niche that encloses a proliferating group of undifferentiated stem cells. At metamorphosis, the niche breaks down and the stem cells are freed. Most of them differentiate into adult gut progenitors, but a minority takes on the task of establishing a new, adult, stem cell niche with more liberal operating rules.

  9. Anti-MicroRNA Antiviral

    MicroRNAs (miRNAs) are small noncoding RNAs found in eukaryotes and viruses. They are critical regulators of a wide range of cellular processes. The highly conserved miRNA miR-122 is required for infection by hepatitis C virus (HCV), a leading cause of liver disease in humans. Present HCV treatment regimes can have serious side effects and are effective in only 50% of cases. In order to try to tackle HCV infection, Lanford et al. (p. 198, published online 3 December) targeted miR-122 using a complementary locked nucleic acid (LNA) oligonucleotide. Treatment of chimpanzees infected by HCV with the LNA antagonist resulted in a long-term reduction of disease symptoms without the concomitant appearance of resistant strains of the virus.

  10. Dissecting TFIIB Mechanics

    CREDIT: LIU ET AL.

    Eukaryotic RNA polymerase II (Pol II) requires five protein cofactors for promoter recognition and initiation of transcription. The factor TFIIB is implicated in start site selection and stabilization of the initial transcript. The co-crystal structure of Pol II and TFIIB showed an N-terminal “finger” region located in the RNA exit channel, but the core C-terminal region of TFIIB was disordered. Now Liu et al. (p. 206; published online 12 November) present a structure of the same complex determined under different conditions in which the C-terminal structure is well localized but the finger is disordered. Docking DNA into the structure suggests that the C-terminal region stabilizes initial promoter melting. After transcription of a few bases, TFIIB probably switches to the alternate conformation where the C-terminal region is released and the finger region stabilizes the initial transcript.

  11. RNA Structural Principles Revealed

    The thermodynamic principles that link RNA primary and secondary structure are well understood, but the relation to tertiary structure is unclear. To gain insight, Bailor et al. (p. 202) analyzed all available three-dimensional structures of an important RNA motif, the two-way junction, and found that flanking helices sample only a small percentage of possible interhelical orientations. They identified a set of general rules for the relative orientation of helices as a function of the size of the interconnecting junction. The results also rationalize how ligands stabilize specific conformations. Understanding the topological constraints that define RNA global conformation and dynamic adaptation provides guiding principles for rational manipulation of RNA structure.

  12. Cocaine Addiction and Histone Methylation

    Long-lasting behavioral syndromes associated with chronic cocaine exposure may result from dysregulation of the global transcriptional machinery. Maze et al. (p. 213) observed that histone lysine methylation in the nucleus accumbens plays a critical role in mediating the regulation of gene expression in response to repeated cocaine self-administration. Chronic cocaine was linked to overall reductions in dimethylation of lysine 9 of histone 3 (H3K9) in this brain region. Repressing H3K9 after chronic cocaine administration facilitated reward-related changes in behavior. The authors identifed the methyltransferase G9a as an essential mediator and an important regulator of dendritic spine plasticity. Downregulation of G9a was linked to the transcription factor ΔFosB.

  13. Silicon Microwires as Photocathodes

    Solar hydrogen generation will require the development of photocathodes with high surface area, durability, and efficiency. Silicon microwire arrays, which allow for greater light penetration, could achieve this goal if the carrier mobilities are sufficiently high so that surface reactions occur before charges recombine. Boettcher et al. (p. 185) report the electronic properties on positively doped silicon microwire arrays that were grown with copper catalysts and used in a methyl viologen redox system. Although equivalent efficiencies for normal solar fluxes were only 2 to 3%, the high internal efficiencies and low use of the available optical flux suggest that further improvements are possible.

  14. Spin Transitions and Mantle Phases

    The extreme temperatures and pressures at depth can cause mantle minerals to undergo phase transformations. For example, it is thought that an electronic spin transition that occurs at high pressures in the lower mantle leads to the partitioning of various iron mineral phases. Using a multianvil apparatus to study a synthetic material similar in composition to the lower mantle, Irifune et al. (p. 193, see the Perspective by Hirose; published online 3 December) demonstrate that the spin transition occurs at lower pressures, and thus shallower depth, than suggested by earlier studies based on simpler compositions. As verification of the material's applicability in lower mantle studies, its density profile with pressure overlaps very well with those predicted by seismological models.

  15. Ratting Out a Diabetes Gene

    Inbred animals with inherited susceptibility to disease can be especially informative regarding pathogenetic mechanisms because they carry naturally occurring genetic variants of the same type that cause disease in humans. This principle is illustrated by Rosengren et al. (p. 217; published online 19 November), whose analysis of an inbred strain of rats prone to develop type 2 diabetes led to the discovery of a gene whose aberrant overexpression suppresses pancreatic insulin secretion in both rats and humans. The culprit gene, ADRA2A, encodes the alpha2A adrenergic receptor and is potentially a valuable lead for diabetes therapy because it can be targeted pharmacologically.