This Week in Science

Science  10 Feb 2006:
Vol. 311, Issue 5762, pp. 737
  1. Hitching a Ride on the Chromosome


    The Kaposi's sarcoma-associated herpesvirus (KSV) does not integrate into its host but is maintained as a stable episome. In order to be distributed to daughter cells, the virus associates with human chromosomes. Barbera et al. (p. 856) show that the viral latency-associated nuclear antigen (LANA) binds directly to specific chromosome components, the core histones H2A and H2B. LANA could not bind in systems that lack these two histones. The crystal structure of the complex revealed that a hairpin formed when LANA interacts with a particular acidic region formed by H2A and H2B within the nucleosome.

  2. Cosmic Magnetism

    Primordial magnetic fields arose in the hot young universe as a by-product of the gravitational collapse of cosmic structures. Ichiki et al. (p. 827, published online 5 January; see the Perspective by Durrer) show that primordial magnetic fields are strong enough to explain the fields seen in galaxy clusters and galaxies today. For a range of cosmic scales, they calculate how seed magnetic fields are produced by currents caused by the differing motions of charged protons and electrons as photons scattered off them during cosmic epochs before the first atoms formed.

  3. Assembling a CO Triangle


    The Fischer-Tropsch process uses catalysts and high temperature and pressure conditions to synthesize hydrocarbons from CO and H2. However, efforts to link CO units more selectively under milder conditions have been largely unsuccessful, in part because of the high strength of the CO triple bond. Summerscales et al. (p. 829; see the Perspective by Wayland and Fu) have used a uranium complex to assemble three CO units in a triangular ring joined through the carbons and suspended between two U centers, each of which donates an electron to produce a (CO)32− dianion. Structural data and density functional theory suggest that uranium f orbitals are especially suited to stabilizing the structure.

  4. Swiveling in a Net

    Liquid water is held together by a net of intermolecular hydrogen (H) bonds that constantly break and reassemble. Rotation of water molecules would seem to require small diffusive steps as donated H-bonds are gradually transferred between acceptors. Numerical simulations by Laage and Hynes (p. 832, published online 26 January 2006) support a more delocalized mechanism in which rotation is controlled by coordination changes at the H-bond accepting partners in the solvation shell. Thus, rotation is generally restricted, but when bulk coordination is simultaneously added to the current acceptor and removed from a nearby potential acceptor, the donor molecule rapidly swivels from one to the other.

  5. Shooting Methane Blanks

    Numerous rapid increases in the concentration of atmospheric methane occurred during the last glacial period and deglaciation, associated with abrupt climate warming events. The “clathrate gun” hypothesis argues that the source was methane clathrates below the sea floor that were rapidly destabilized by ocean warming. Sowers (p. 838) tested that hypothesis with measurements of the isotopic composition of hydrogen in methane trapped in bubbles of the GISP2 Greenland ice core for several episodes of rapid warming during the last glacial period and the last deglaciation. He finds no evidence that methane clathrates, which have a unique hydrogen isotopic signature, contributed significantly to the methane concentration peaks.

  6. In a Wider Warm Spell

    A number of unusually warm or cold intervals can be seen in most proxy records of temperature of the last millennium, so how can we assess the relative magnitude of the current warm period? Osborn and Briffa (p. 841) compared the geographic extent of late 20th-century warming in the Northern Hemisphere to the distribution of both warm and cold intervals for the last 1200 years by adopting specific thresholds to define warm and cold periods in order to avoid questions about of the absolute magnitude of warm and cold events, and they considered only a subset of the data chosen specifically for its value as a temperature proxy. They find that the continuing warmth of the late 20th century is the most widespread and longest temperature anomaly of any kind since the 9th century A.D.

  7. Modulating the Scaffold

    Signaling complexes are often preassembled into complexes. So-called scaffold proteins help to maintain these complexes and can contribute to specificity in various signaling systems. Bhattacharyya et al. (p. 822, published online 19 January; see the Perspective by Breitkreutz and Tyers) show that the role of such scaffolds can go beyond support and spatial localization. In yeast, mating pheromone causes activation of a series of kinases that all interact with the scaffold protein Ste5, and signal transduction through this pathway activates the mitogen-activated protein kinase Fus3. When Fus3 binds to Ste5, this interaction causes an allosteric partial activation of Fus3's kinase activity. Fus3 then appears to provide negative feedback in the system by phosphorylating the Ste5 scaffold.

  8. Basic Body Design


    Why have certain features of animal body plans, such as bilateral symmetry, been conserved since the early Cambrian period, whereas at the species level, there has been a continuous accumulation of changes? Davidson and Erwin (p. 796) propose that the genetic regulatory networks associated with development contain three components that differ in their evolutionary conservation. Evolutionarily inflexible subcircuits (“kernels”) perform essential upstream functions in building given body parts, while other small subcircuits (“plug-ins”) have been repeatedly co-opted to diverse developmental purposes, leaving highly flexible, individual cis-regulatory linkages to regulate detailed phenotypic variation.

  9. Self-Promoting Signals

    Release of proapoptotic factors from the mitochondria leads to cell death, and signaling events appear to occur “upstream” or “downstream” of the mitochondria. This neat organization is challenged by Lakhani et al. (p. 847; see the Perspective by Adrain and Martin) in an analysis of knockout mice lacking caspase 3 and caspase 7, both thought to be “downstream.” Caspases 3 and 7 are activated when clipped by other caspases after they have been stimulated by molecules released from the mitochondria. In the knockout animals, not only was the “downstream” event, apoptosis, inhibited, but “upstream” events, such as loss of the integrity of the mitochondrial membrane and release apoptotic factors, were also delayed. These unanticipated results may indicate that caspase 3 and caspase 7 act to promote mitochondrial signals that lead to their own activation and raise a “chicken or egg” conundrum regarding the initiation of the mitochondrial death signals.

  10. Role for Translation in Maintaining Totipotency

    Germ cells are totipotent—they can give rise to all different cell types. Ciosk et al. (p. 851) now show that the translational regulators MEX-3 and GLD-1 maintain totipotency in the germ line of the nematode Caenorhabditis elegans. When these two factors were eliminated, ectopic cells were found in the gonad due to the differentiation of germ cells into somatic cell types such as muscle, neurons, and intestinal cells. This transdifferentiation was associated with a loss of germ cell features such as P granules and germ cell proteins. These “worm teratomas” may be useful as a genetically tractable model system for understanding teratoma biology.

  11. Word on the Street

    To understand what forces control the emergence of extraordinarily successful songs, movies, or plays, Salganik et al. (p. 854; see the Perspective by Hedström) have assessed the influence of social information, that is, information about what other people are watching and listening to, on market performance. By querying students online about their assessments of a defined set of songs, the authors show that access to social information increases the tendency for certain songs to do well, and that the quality of the song is only partly reflected in its market performance.

  12. Depressed Mouse Needs Long-Term Treatment

    What are the neurobiological mechanisms through which psychosocial experience may alter the activity of the mesolimbic dopamine system? Berton et al. (p. 864); see the news story by Holden) demonstrate that long-lasting behavioral and molecular changes develop in mice after suffering a series of aggressive encounters. The persistent social aversion seen in these mice can be completely normalized by chronic (but not acute) treatment with clinically effective antidepressants. The growth factor brain-derived neurotrophic factor (BDNF) is required within dopaminergic reward regions for these behavioral alterations to unfold.

  13. Going Their Very Separated Ways

    In turbulent flows, neighboring particles are often sent on very different pathways, which is why this type of flow is exploited in industrial mixing processes. One long-standing question is whether the intermixing depends on time only, or if it also depends on the initial separation distance between any two particles. Bourgoin et al. (p. 835) show that for violently turbulent flows, the rate of particle separation is also dependent on their initial separation distance, in agreement with long-standing predictions of Batchelor.

  14. Marked Out for Activation

    In eukaryotes, genomic DNA is packaged by histones into nucleosomes within chromatin. Chromatin plays a critical role in the regulation of gene expression, through the covalent modification of the histone N-terminal tails. One of the most prevalent histone modifications is the acetylation of lysine 16 in the tail of histone H4 (H4 K16Ac), which is generally considered to be an activating mark. Shogren-Knaak et al. (p. 844; see the news story by Marx) assayed the function of this modification in vitro by reconstituting nucleosome arrays on which the only modification is H4 K16Ac. As expected, the mark prevented compaction and cross-fiber interactions of 30-nanometer? like chromatin fibers. In vivo, the mark was enriched in decondensed chromatin. H4 K16Ac also interfered with the ability of a chromatin remodeling enzyme to shuffle a single nucleosome on DNA. Thus, H4 K16Ac influences both the structure and function of chromatin.

  15. Neurotransmitters and Higher Cognitive Functions

    The activity of the human prefrontal cortex is modulated by ascending neurotransmitter systems. However, the differential modulation of cognition by distinct neurochemical systems in man has yet to be elucidated. Chamberlain et al. (p. 861) demonstrate in healthy human volunteers a double-dissociation for involvement of noradrenaline and serotonin in two cognitive functions that are critically implicated in everyday behavioral regulation and in the manifestation and treatment of psychiatric illnesses.