This Week in Science

Science  20 Jun 2008:
Vol. 320, Issue 5883, pp. 1559
  1. Toward Attosecond Technology


    The interaction between intense light pulses and rare (noble) gas atoms results in the ionization of the atoms and the subsequent emission of shorter light pulses of much shorter wavelength, extending into the ultraviolet. Control over this process is necessary for the development of attosecond technology. Optimizing the interaction, Goulielmakis et al. (p. 1614) demonstrate the single-cycle ionization of neon atoms by an intense single-cycle infrared driving pulse and the concomitant generation of isolated, sub-100-attosecond pulses of extreme ultraviolet light. Such short pulses enter the atomic unit of time scales where electrons in atoms and electron-electron effects can start to be probed.

  2. Enigmatic Chondrules

    The conditions that formed the most abundant solids in the early solar system, known as chondrules, have been enigmatic. Chondrules record high-energy processes in the solar nebula and represent an important starting condition for understanding the subsequent evolution of the solar system, including the distribution of material and the formation of larger planetesimals. Alexander et al. (p. 1617; cover) show that the sodium concentration of chondrules is too high to be explained by current models, which invoke evaporation at modest to low particle densities. Instead, chondrules seem to have formed in locally dense regions of the nebula. While this suggestion accounts for the general chemistry of the chondrules, explaining the existence of such locally dense regions remains a challenge.

  3. Motivation, Morals, and Behavior

    The design of social programs often relies upon Adam Smith's “invisible hand” whereby individuals are assumed to choose their actions primarily on the basis of self-interest. Society may often therefore structure economic incentives that are likely to induce individuals to act in a fashion that benefits the social group as a whole. On the other hand, empirical observations and experimental investigations have demonstrated in individuals the existence of prosocial behaviors, such as altruism. Bowles (p. 1605) reviews these recent studies and describes how the failure to take these behaviors into account when designing incentives can lead to entirely unexpected and counterproductive outcomes.

  4. Constant Constants?

    Fundamental constants such as the speed of light, the gravitational constant, and the electron mass are thought to be both basic to a physical picture of the universe and unchanging over time and space. As measurements get more and more precise, the constancy of the constants can be checked. One approach involves ever more precise atomic clocks, but these are restricted to terrestrial laboratory measurements. What about the possibility of changes in physical constants at great distance or just after the Big Bang? Murphy et al. (p. 1611) use absorption of light by molecular clouds between our solar system and a distant quasar to establish a bound on the possible variation of the ratio of the proton mass to the electron mass.

  5. A Greener Greenland

    Melting of the Greenland ice sheet is expected to cause much of the rise in sea level that will occur as global climate continues to warm, although it still is not clear how much of the ice sheet will disappear as air temperatures rise. One way to infer how vulnerable the ice sheet is to temperature variations is to determine how it was affected by climate change in the past. De Vernal and Hillaire-Marcel (p. 1622; see the Perspective by Steig and Wolfe) constructed a pollen record for southern Greenland for the past million years, from a marine sediment core just off the coast. Large changes in ice extent and vegetation accompanied changes in temperature, illustrating the susceptibility of the ice sheet to climate change, and providing a guide as to what to expect with future warming.

  6. Changes in Altitude


    The rate at which the ice sheets in Greenland and the Antarctic change mass is determined by the difference between the rate of ice loss at the margins and ice gain in the center, both of which are largely uncertain. Mass gain in the interior of the Antarctic Ice Sheet has been particularly difficult to determine, because snow accumulation varies over multiple time scales, and the thickness of the firn (the porous, upper layer of the ice sheet formed by fallen snow and not yet fully transformed into ice) is difficult to document for large regions. Helsen et al. (p. 1626, published online 29 May; see the Perspective by Cuffey) show that accumulation variability dictates changes in firn layer thickness, and thus mainly determines ice sheet elevation changes as observed by satellite radar altimetry.

  7. Guts Galore

    Spectacular numbers of microbes live in mammalian guts and lend their hosts important nutritional functions. Ley et al. (p. 1647, published online 22 May) have made a comparative metagenomic study of the fecal flora of human beings and 60 other mammal species, living in zoos and in the wild, to see how taxonomic position and diet affect the composition of the internal flora and to learn how these relationships have co-evolved. Although there is a general trend to herbivores harboring the most diverse communities and carnivores the least, overall the relationship between a flora and its host is specific: Baboons in the St. Louis Zoo have much the same gut flora as wild baboons in Namibia.

  8. Phylogenetic Error Correction

    Molecular sequence alignment methodology is a key tool for analyzing evolutionary relationships. However, Löytynoja and Goldman (p. 1632) show that current methods are making systematic errors that bias results of evolutionary inferences, e.g., in comparative genomics. These errors are not corrected by sampling more sequence data—indeed, the error grows in response to denser sampling. These systematic errors can be avoided by performing multiple alignments for studies in phylogenetics and sequence evolution.

  9. Clutching the Flagellum

    The bacterial flagellum is one of a very few rotating motors in biology and has been extensively studied with respect to its assembly and function. A variety of transcription factors control flagellar gene expression and chemotaxis systems control the direction of flagellar rotation and cell behavior. Blair et al. (p. 1636; see the Perspective by Berry and Armitage) identify a novel functional component of the motor, a clutch that allows the cell to arrest flagellar rotation. This clutch protein appears to play a role in motility control and is co-regulated with biofilm formation, when bacteria will stop moving.

  10. Glial Cell Tuning


    Astrocytes are the major class of nonneuronal cells in the brain and account for close to half of the cells in the mammalian cerebral cortex, yet their function is virtually unknown. Evidence is now accumulating for an active role of astrocytes in brain function. Schummers et al. (p. 1638; see the Perspective Wolf and Kirchhoff) investigated astroglial calcium responses in the visual cortex of the ferret by two-photon laser-scanning microscopy. Visual cue-induced calcium signals were monitored in neurons and astrocytes simultaneously at high temporal and cellular resolution. The receptive field properties (response kinetics, orientation, and localization) of the astroglial network consistently followed neuronal activity. Simultaneous recordings of hemodynamic signals and calcium responses in neurons and astroglia revealed that astrocytes have a key role in coupling neuronal activity to vascular signals critical for noninvasive brain imaging.

  11. Shorter Messages in Proliferating Cells

    Sequences in the 3′ untranslated regions (UTRs) of messenger RNAs (mRNAs) control mRNA stability, translation, and subcellular localization. Sandberg et al. (p. 1643) analyzed changes in the expression of mRNA isoforms differing in their 3′UTRs in response to activation of T lymphocytes. Activated cells tend to express higher proportions of shorter 3′UTR isoforms, which lack regulatory sequences such as microRNA target sites that commonly inhibit protein expression. This shift toward expression of shorter 3′UTR isoforms occurs in activated immune cells in both human and mouse and correlates with cellular proliferation across many cell types and tissues.

  12. Intracellular Subversion Strategy

    A large number of putative bacterial proteins contain eukaryotic ankyrin repeat homology domains (ank). These same bacteria also possess potential type IV secretion systems, which can inject bacterial effector proteins into their eukaryotic host cell cytosol. It is thus possible that these ank-containing genes could encode such effector proteins. Now Pan et al. (p. 1651) show that ank-containing genes in both the facultative intracellular pathogen Legionella pneumophila and the obligate intracellular pathogen Coxiella burnetii encode proteins that are indeed translocated into host cells during infection by a process that requires the bacterial Dot/Icm type IV secretion system. One of the L. pneumophila ank-containing proteins, AnkX, prevented microtubule-dependent vesicle transport and helped internalize L. pneumophila to evade phagosome-lysosome fusion.

  13. Probing a Volcanic Lava Lake's Depths

    Because of the high temperatures of magmas (near 1000°C), it is generally thought that stable isotopes do not fractionate noticeably between the melt and newly formed crystals. Teng et al. (p. 1620; see the Perspective by Weyer) tested this notion in a well-controlled setting, the lava lake of Kilauea Iki. This lake formed during an eruption in 1959 and has been slowly cooling. Drilling was used to collect a series of samples through a zone of partial melt in the lake's core and into crystals that accumulated at its base. Iron isotopes were found to fractionate between the melt and crystals. The fractionation, not seen for Mg or Li isotopes, may reflect iron's variable redox state, and can be used to trace magmatic processes.

  14. Fly Genome Evolution

    The evolutionary forces acting on copy-number variation, and particularly whole-gene duplications, are largely unknown. Using whole-genome tiling arrays and a hidden Markov model, Emerson et al. (p. 1629, published online 5 June) detect polymorphism for gene duplications and deletions in 15 natural lines of Drosophila melanogaster. Generally speaking, copy-number polymorphisms were deleterious. Deletions, when compared to duplications, were generally much less frequent, much smaller, and especially rare in coding regions. Among duplications, mutations overlapping coding regions were more deleterious than full-gene duplications or duplications in intergenic regions. Moreover, duplications occurring on the X-chromosome were more deleterious than those falling on autosomes, indicating that evolution on the X-chromosome proceeds differently from the autosomes.

  15. Regulating the Regulators

    Much of the control of cell division comes down to turning on or off the activity of the cyclin-dependent kinases. Seki et al. (p. 1655) help clarify how the transition from the G2 phase of the cell cycle into mitosis is controlled in mammalian somatic cells. The depletion of Bora, a protein encoded by a gene actively transcribed at G2 phase of the cell cycle, prevented activation of another key kinase in cell-cycle control, Polo-like kinase 1 (Plk1). Bora directly interacted with Plk1, enhancing phosphorylation of Plk1 by yet another kinase that plays a role in cell cycle regulation, Aurora A.

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