This Week in Science

Science  30 Apr 2004:
Vol. 304, Issue 5671, pp. 645
  1. Seeding Subtropical Cirrus Clouds


    The anvil cirrus clouds that form in the tropics have less influence on climate per unit area than do stratocumulus clouds, but changes in sea surface temperatures may greatly expand their extent and climate impact, as could changes in aerosol nucleating particles created by pollution sources. In order to better characterize what factors can control the distribution and characteristics of anvil cirrus clouds, Fridlind et al. (p. 718) analyzed data from the CRYSTAL-FACE field campaign over southern Florida in July 2002. Most anvil cirrus cloud crystals are not formed on aerosols originating at cloud base in the boundary layer between Earth's surface and the lower troposphere, as has been assumed, but on aerosols entrained at much higher mid-tropospheric altitudes. Distant sources of pollution may have more effect on the properties of these clouds than do local sources.

  2. Cuprate Chameleon

    The different optical properties of high-temperature cuprate superconductors above and below their critical temperature can provide insights into the behavior of the carriers that form the superconducting condensate. Boris et al. (p. 708) provide a new set of data which show that the optical properties do not behave as previously reported. These data suggest that an exotic pairing mechanism involving decreases in kinetic energy does not need to be invoked to describe these materials.

  3. An Ancient Invader

    The obligate intracellular Chlamydia species that are notorious pathogens in humans have genome sizes in the 1-megabase range, and it has long been speculated that these have evolved from an ancestor with a larger genome. The sequence of a Chlamydia-related symbiont provides evidence that the ancestor indeed contained a larger genome. Analyses by Horn et al. (p. 728) suggest that Chlamydia has been intracellular for several hundreds of millions of years, probably before multicellular hosts were available. Nevertheless, the same mechanisms used to achieve infection by these ancient symbionts have persisted in the human pathogens. The type III secretion system, in particular, seems to have been in use for more than 700 million years.

  4. One Size Fits the Black Hole


    At the center of the Milky Way Galaxy lurks a supermassive black hole called Sagittarius A-Star (Sgr A*). Its mass has been estimated based on gravitational effects and radiative emissions near the source. Bower et al. (p. 704; see the Perspective by Morris) have determined the size of the black hole from radio emissions near the event horizon. The mass combined with the size suggests a superdense object that can only be a black hole. In addition, the observations support jet models for the outflowing plasmas and low-accretion rate models for the inflowing plasmas.

  5. Sites Under Fire

    When humans began to use fire in a cultural setting has been uncertain. There is widespread evidence of hearths and burned bones dating back to about 300,000 years ago, and several earlier sites dating to about 500,000 appear to be hearths or provide other evidence for the controlled use of fire. Goren-Inbar et al. (p. 725; see the news story by Balter) now provide evidence from Gesher Benot Ya'aqov of an intimate association between humans and fire since perhaps about 800,000 years ago. The site shows a series of layers with burned wood and flint artifacts; in one studied layer, the burned flint is highly concentrated, consistent with controlled fire use.

  6. Hollowed-Out Nanocrystals

    Almost 60 years ago, Smigelkas and Kirkendall showed that when two competing diffusion-controlled reactions occur at an interface, the motion of atoms is not limited to atom-atom swapping but involves the motion of vacancies. The resulting difference in diffusion rates of the atoms can create porosity. Yin et al. (p. 711; see the Perspective by Buriak) exploit this effect at the nanoscale to make hollow nanocrystals of cobalt (Co) oxides or chalcogenides. When Co nanocrystals react with oxygen, sulfur, or selenium, vacancies form within the metal, but given their small size, they diffuse to the core and form a large single void. The oxidation of encapsulated platinum (Pt) inside a Co shell creates a porous, loose fitting CoO casing for the Pt particle. The “yolk-shell” particles show significant catalytic activity and may be useful in preventing the Pt from inducing secondary reactions during chemical conversions.

  7. Wrapping Up the Axon

    The regulation of myelin sheath thickness is extremely important for neural function because it regulates nerve conduction velocity. Myelin thickness is proportional to axon size—small axons have little myelin, whereas large-caliber axons have a thick myelin sheath. Schwann cells deliver the correct number of myelin wraps, but how do Schwann cells “sense” the diameter of the engulfed axon? Michailov et al. (p. 700; see the cover and the Perspective by ffrench-Constant et al.) now suggest that Schwann cells respond to an axonal growth factor, Neuregulin-1, and integrate this cell surface signal to create a biochemical measure of axon caliber. Altering the extent of Neuregulin-1 signaling alters myelin thickness, and reduced Neuregulin-1 gene dosage in mice “misleads” the Schwann cell as to the true caliber of the axonal segment, which results in reduced myelination and slowing of nerve conduction velocity.

  8. Rethinking RNA Helicase Activity

    One of the questions raised when the double-stranded nature of DNA became clear was how the strands were separated during replication and transcription. The identification of proteins exhibiting DNA helicase (unwinding) activity answered the question, and this line of thinking was readily applicable to the tendency of RNA strands to form double helices. Fairman et al. (p. 730; see the Perspective by Linder) show, however, that a family of proteins that has been thought of as possessing RNA helicase activity is, in fact, better described as displacing binding partners (possibly processively) from single-stranded RNA.

  9. Plague Only a Hop Away

    Plague occurs naturally in wild burrowing rodents and remains a medical threat from central Asia through Africa into the southern United States and South America because of transmission via fleas. The gerbil plague focus in Kazakhstan has been under long-term public health scrutiny because the reservoir host lives in stable, delineated burrow systems and colonies. Davis et al. (p. 736; see the news story by Stone) have analyzed 40 years of data and discovered a threshold phenomenon that produces a clear cyclic pattern of re-emergence of plague about 2 years after a gerbil population peak. Predictions from models built on the Kazakhstan data will facilitate the development of cost-effective public health control strategies.

  10. Brain Map Plasticity


    What are the cellular mechanisms underlying experience-dependent development of cortical circuitry? Petersen et al. (p. 739) investigated synaptic connectivity of the developing layer 2/3 barrel cortex in rats after sensory input had been deprived by trimming a defined subset of the whiskers. Both sensory responses and intercolumnar synaptic connectivity within nondeprived cortex were strengthened through increases in the connection probability. Morphological analysis revealed a concomitant reorientation of axonal, but not of dendritic, arbors toward the nondeprived barrel columns.

  11. Cofilin in Locomotion

    Cofilin is a cytoskeletal protein that binds and severs actin filaments. But is cofilin simply required to regenerate free actin, while the Arp2/3 complex regulates actin polymerization at the leading edge of migrating cells? Ghosh et al. (p. 743) now suggest that cofilin plays a more direct role in cell motility. Cofilin is regulated by phosphorylation, and, using an inactive phosphocofilin mimic that could be switched on by a brief burst of light, global activation produced increased cell movement. Local activation instead induced cell surface protrusions that set the direction of cell migration.

  12. Information Transmission Battles HIV Transmission

    Unlike other African countries, Uganda has shown a deep reduction in human immunodeficiency virus-type 1 (HIV) incidence since the 1990s. This difference largely appears to be the result of population-wide reduction of casual sexual contact. Stoneburner and Low-Beer (p. 714) suggest that widespread behavioral changes occurred in Uganda because of the way in which information about HIV was transmitted. Information flow through personal networks, together with a high level of acquaintanceship with victims, has led to widespread knowledge of the cause—and alarm about the consequences—of HIV infection.

  13. Missing Sinks

    Isoprene and other terpenes are part of the biogenic volatile organic compounds (BVOCs) emitted from vegetation. These compounds can react with oxidants in the air such as the hydroxyl radical (OH). Di Carlo et al. (p. 722) report measurements made in forests which show that the reactivity of OH there is greater than the sum of that which should be caused by all identified reactants, indicating that additional, unknown species are present and reacting with OH. The difference between the measured and expected OH reactivity increases with temperature in the same manner as do BVOC emission rates, which suggests that the unknown reactants are biogenic in origin.

  14. Viral miRNAs

    Micro-(mi)RNAs are small ∼22-nucleotide RNAs that regulate gene expression either by causing the cleavage and presumed destruction of target RNAs or by repressing their translation. miRNAs have been found in the genomes of all plants and animals so far analyzed (the exceptions being some fungi). Pfeffer et al. (p. 734) now show the presence of miRNAs in a virus, the “fourth domain” of life. Epstein-Barr virus (EBV), a large DNA virus and one of the most common human viruses, encodes five miRNAs. Bioinformatics analyses, supported in one case by previous research, suggest the EBV miRNAs target both viral and human genes that have the potential to regulate the course of viral infection.