This Week in Science

Science  07 Dec 2007:
Vol. 318, Issue 5856, pp. 1519
  1. Satellites Take Shape


    Saturn's smallest moons were thought to be the remnants of collisions between larger moons. These collisions would also have created debris that became further refined and ultimately filled in the rings that circle the planet. Porco et al. (p. 1602) instead show that the small moons grew out of accreted ring material. Their analysis of the shapes and densities of moons in images taken by the Cassini spacecraft indicates that the moons grew to a maximum size governed by the balance of local gravity. The sizes were also limited by availability of additional ring material once those regions were cleared out. Modeling by Charnoz et al. (p. 1622) reveal that this process results in the characteristic elongated and bulging shapes of two of the moons, Pan and Atlas.

  2. Optical Quantum Computing

    There are currently several experimental routes being pursued in the goal of realizing a working computer. One approach is linear optics, in which the bits of quantum information (qubits) are stored in the polarization state of single photons and the logic gates are formed from simple elements such as beam splitters, mirrors, and waveplates. O'Brien (p. 1567) reviews recent progress in this area and points out the challenges that remain to be addressed.

  3. Evidence of Cosmic Texture

    After the Big Bang, the expanding universe progressed through a series of phase changes in which various forces and fields became decoupled and symmetries were broken. According to theory, these phase changes spread at the speed of light across patches of the universe. However, they may have done so irregularly and left behind cosmic defects similar to those seen in crystals, although none has yet been seen by astronomers. Cruz et al. (p. 1612, published online 25 October; see the Perspective by Brandenberger) propose that the remnant of a cosmic defect known as a texture has the right properties to explain an unusual cold spot in the cosmic microwave background, the frozen map of the universe at the point when photons and the first atoms decoupled hundreds of thousands of years after the Big Bang. If this feature is a texture, then its existence constrains the fundamental symmetry-breaking energy scale.

  4. Modeling Electron Correlation


    Capturing the essential physics underlying highly correlated electron systems is a huge challenge in contemporary condensed matter physics. However, the shear complexity in describing all the interactions that can take part has meant a full understanding has been lacking. Shim et al. (p. 1615, published online 1 November; see the Perspective by Fisk) introduce a theoretical effort to understand the angle-resolved photoemission spectra and optical spectra of the prototypical heavy fermion compound CeIrIn5 in terms of dynamical mean-field theory combined with local density approximation (DMFT+LDA) calculations. Their calculations show how the electrons evolve with decreasing temperature from a localized high-temperature state to a delocalized fluid of quasi-particles that have masses many times greater than that of a free electron.

  5. Making the Right Cut

    Regulated intramembrane proteolysis (RIP) represents an important signaling mechanism that is conserved from bacteria to humans. A notable example of RIP is the activation by cleavage of the transcription factor Sterol Regulatory Element Binding Proteins by site-2 protease (S2P), a key event in regulating cellular cholesterol levels. Feng et al. (p. 1608) now present the crystal structure of the transmembrane core of an archaebacterial S2P metalloprotease, which provides insight into how S2P functions. The structure shows the mechanism of cleavage at an active site, containing a catalytic zinc ion that is embedded deep in the membrane. Two conformations observed in the crystals suggest that a helical gating mechanism controls substrate access.

  6. Oil-Repelling Surfaces

    Several approaches have been used to make superhydrophobic materials that excel at repelling water. These methods typically combine a material with a low surface energy with a form of surface roughness that keeps the water as buoyant droplets on the surface with high contact angles. It is much more difficult to use this route to create superoleophobic materials because organic liquids typically have much lower surface tensions. Indeed, calculations indicate that it might not be possible to achieve such a surface from just these two design criteria. Tuteja et al. (p. 1618) show that by considering a third factor, the use of reentrant surface structures (which include concave surface features), surfaces can be created that repel a wide range of organic materials, including octane and decane.

  7. Avoiding Resistance

    One of the most commonly used classes of pesticides are toxins produced by the bacteria Bacillus thuringiensis, known as Bt toxins. The primary threat to the efficacy of Bt toxins is the evolution of resistance by pests. In major pests, resistance to Bt toxins in the Cry1A family is linked to alterations in cadherin proteins that act as primary toxin receptors in the midgut of susceptible insects. Now Soberón et al. (p. 1640, published online 15 November; see the Perspective by Moar and Anilkumar) show that engineered Bt toxins can overcome insect resistance. Furthermore, in investigating the effectiveness of these engineered proteins, the authors provided evidence for the means by which these toxins cause mortality in insects.

  8. Bacterial Pilus Structure Revealed


    Bacterial pili, filamentous adhesive structures that extend from the cell surface, are important virulence factors and potential vaccine targets. Pili from Gram-negative bacteria have been structurally characterized. Now Kang et al. (p. 1625; see the Perspective by Yeates and Clubb) describe the structure of the major pilin subunit from a Gram-positive human pathogen, Streptococcus pyogenes. In the crystal, the subunits associate in columns reminiscent of the likely arrangement in native pili. The structure also reveals intramolecular isopeptide bonds that may stabilize the structure and contribute to protease resistance. This could be a more general mechanism of protein stabilization in Gram-positive organisms, which lack the disulfide bond formation machinery of Gram-negative bacteria.

  9. Dissecting X Inactivation

    One of the two X chromosomes in mammalian females is randomly inactivated early in development to match the single active X chromosome of males. This process is regulated through the X-inactivation center (Xic). The two Xics interact in trans at the beginning of X-inactivation, presumably to allow reciprocal activation/inactivation. So far, single copies of elements from the Xic have not been able to recapitulate X inactivation, suggesting additional elements must be required. Augui et al. (p. 1632) find that a region ∼200 kilobases upstream of the Xic—the X-pairing-region (Xpr) —is sufficient in a single copy to allow a transient interaction between the two Xics at a time before the beginning of X inactivation. This pairing is cell cycle dependent, can occur from an ectopic location, and may activate the expression of Xist, a noncoding RNA that coats the inactive X chromosome.

  10. Dealing with DNA Damage

    Like railway tracks severed clean through, a break in both strands of genomic DNA can result in disaster. To avoid potential cellular chaos in the face of such damage, a complex DNA damage response has evolved. In eukaryotes, phosphorylation of histone H2AX and polyubiquitination of proteins at sites of damage recruit DNA-repair proteins, forming cytologically visible foci. Kolas et al. (p. 1637, published online 15 November) show that the ubiquitin ligase RNF8 is responsible for polyubiquitination at double-strand breaks in yeast. RNF8 is recruited to the damaged sites though its interactions with phosphorylated MDC1, and acts downstream of MDC1 to promote the formation of at least two distinct classes of repair foci. RNF8 binds the E2 conjugating enzyme UBC13 to drive polyubiquitination at the site of the break, and also helps regulate the G2/M cell-cycle checkpoint.

  11. Learn from Your Mistakes

    Human experience is based on learning that our actions affect subsequent positive or negative outcomes. Rewards strengthen associations between contextual stimuli and actions thereby reinforcing and maintaining successful behavior; whereas punishments induce avoidance of erroneous actions. While we usually learn from both positive and negative reinforcement, the relative amount of learning from success or errors varies between individuals. Klein et al. (p. 1642) investigated a human genetic polymorphism associated with the density of brain dopamine D2 receptor. Reduced D2 receptor density was associated with less efficient learning from errors. In people with lower D2 receptor density, the reduced capacity to learn from errors was accompanied by reduced feedback-related activity in the posterior medial frontal cortex, an area known to monitor for negative action outcomes.

  12. Identification of a Deubiquitinating Enzyme

    Modification of proteins by the covalent attachment of ubiquitin chains has emerged as a major regulatory mechanism in cells. The enzymes that oppose this reaction, the deubiquitinating enzymes are relatively poorly understood. Kayagaki et al. (p. 1628, published online 8 November) used an siRNA screen to search for deubiquitinating enzymes that influenced expression of the interferon I (IFN-I) gene in response to activation of Toll-like receptor 3 (TLR3), a central response by which the innate immune system detects viral RNAs and protects organisms from infection. They detected the deubiquitinase DUBA, and went on to show that depletion of the enzyme enhanced the expression of IFN-I and overexpression of the protein inhibited the response. Other experiments identified the ubiquitin ligase TRAF3, which is required for induced expression of IFN-I, as a potential target of DUBA. Thus DUBA restrains production of IFN-I, which, in excess, is associated with autoimmune disease.

  13. Drugs, Psychosis, and Schizophrenia

    It is well established that the psychosis-inducing effects of the dissociative anesthetics phencyclidine (PCP) and ketamine, when used at sub-anesthetic doses such as in drug-abuse, are produced by blockade of neurotransmission at NMDA-type glutamate receptors. However, their mechanisms of action at the brain circuitry level are poorly understood. Behrens et al. (p. 1645) found that exposure of mice to sub-anesthetic doses of ketamine triggers an early and profound increase in neuronal superoxide production that is specifically due to induction of the inflammatory enzyme complex, NADPH oxidase-2 (Nox2). This in turn leads to the loss of phenotype of a specific subset of GABAergic interneurons, the parvalbumin-positive (PV) fast-spiking interneurons. Prevention of superoxide effects in the brain using a brain-permeable superoxide dismutase mimetic, as well as apocynin, a Nox2 inhibitor, strongly attenuated the ketamine-induced loss of parvalbumin and GAD67 immunoreactivity.

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