This Week in Science

Science  26 Mar 2004:
Vol. 303, Issue 5666, pp. 1941
  1. Imaging Oxygen Absences


    Recent advances in high-resolution transmission electron microscopy have made it possible to distinguish between light atoms that have almost the same electron contrast, but measurements to date have only been qualitative in nature. With the aid of an aberration-corrected microscope, Jia and Urban (p. 2001) determined that in BaTiO3, only 68% of the oxygen sites are filled when they imaged along a Σ 3{111} twin boundary. This absence of oxygen leads to a modified bonding of the Ti and O atoms that reduces the grain boundary energy. Such measurement of the oxygen content are critical to understanding how the presence of vacancies influences the electronic properties of this and other related oxide materials.

  2. Single Photons from a Single Atom

    The development of quantum communication networks will require a source of single photons that can be generated on demand. McKeever et al. (p. 1992) report on a scheme in which a single cesium atom is trapped in an optical cavity. Coupling between the excited atom and the optical mode of the cavity allows single photons to be generated deterministically while strongly suppressing two-photon events.

  3. Stopping Malaria in Mosquitoes

    Although mosquitoes are an obligatory host for malaria parasites, the parasites still must face the insect's immune responses. Little has been known about how such responses might affect the outcome of these stages of the parasite's life cycle. Osta et al. (p. 2030; see the Perspective by Hemingway and Craig) used RNA interference to down-regulate gene expression in mosquitoes, and examined the effects on the development of the sexual stages of a malaria parasite. Inhibition of a gene encoding a leucine-rich repeat protein enhanced parasite development, while inhibition of two genes encoding C-type lectins, led to increased oocyst melanization that blocked parasite development. Thus, mosquito factors can have both protective and antagonistic roles in malaria transmission.

  4. Maintaining Stem Cells and the Germ Line

    The Drosophila germ line provides an excellent model system for studying stem cell biology. Members of the highly conserved Nanos family of translational repressors are required for the migration and survival of primordial germ cells. Using genetic mosaic analysis and regulated Nanos expression, Wang and Lin (p. 2016) now show that Nanos is required for stem cell self-renewal and maintenance in the germ line. The fate of stem cells and their precursors is maintained by preventing the precocious activation of oogenesis. In addition, translational regulation appears to play a crucial role in the establishment and maintenance of stem cells.

  5. Uncovering Oceanic Sulfate Cycles

    Sulfate, a primary oxidant in ocean sediments, has a long oceanic residence (∼10 million years). Thus, its biogeochemical cycle should be relatively stable, and studies of isotopic variations of sulfur in sedimentary pyrite indicate that sulfate concentrations remained essentially constant during most of the past 550 million years. Turchyn and Schrag (p. 2004; see the Perspective by Derry and Murray) now present a 10-million-year high-resolution record of the oxygen isotope of marine barite (BaSO4) that reveals the more dynamic nature of the marine sulfur cycle. Ocean sulfate concentrations were at least 10% lower during the late Miocene than at present, implying that significant changes of the sulfur cycle could have occurred at other times in Earth's history when biogeochemical processes on continental shelves and slopes were disrupted.

  6. Order in the Pseudogap

    The electronic behavior of the high-temperature superconducting cuprates depends on the doping level of carriers in the material. Underdoped samples enter a so-called pseudogap state, which is not superconducting but exhibits gapped electronic excitations, before becoming a superconductor. Because of the high temperature, understanding what happens in this pseudogap state has been difficult to probe microscopically, but thought to be vital in unraveling the microscopic mechanism of superconductivity in the cuprates. Vershinin et al. (p. 1995; see the Perspective by Norman) now present a high-resolution study of the pseudogap state and reveal spatial ordering in the electronic density of states. The results should place strict limitations on various ordering scenarios being put forward to describe the complex phase diagram of these materials.

  7. Complex Anatomy


    In the not too distant future, a list of a cell's entire protein contents will be available, and there are already several ways of mapping which proteins interact in various metabolic and regulatory pathways. Aloy et al. (p. 2026) make use of these approaches in order to tackle the challenge of putting together a list of stable and dynamic protein complexes in yeast. They use tandem affinity purification to isolate complexes and to identify their components. Those proteins with known three-dimensional structures or with sequence similarity to proteins of known structure are then used as anchors to predict probable interactions; fitting of these clumps into experimental electron microscopic maps, in the best cases, makes it possible to diagram how the different proteins within a complex are arranged, as well as how individual proteins participate in different complexes.

  8. Seeing Disks Around Stars

    Circumstellar disks provide important information about stellar and interstellar evolution. β Pictoris (β Pic), an M-type star in the Milky Way, has a massive circumstellar disk, and Kalas et al. (p. 1990; see the Perspective by Mouillet) have now discovered a circumstellar disk around AU Microscopii (AU Mic), a young M-type star located near β Pic. The similarity of the two stars indicates that they formed at the same time from the same source region, but the disk around AU Mic is three times less massive and 100 times less luminous than β Pic. The size and density of the disk around AU Mic suggests that the dust is microscopic and pristine, having undergone fewer collisions, less aggregation, and less ejection from the system.

  9. Formin in Motion


    Formins are proteins involved in the nucleation of actin and associate with the fast-growing end of actin filaments. Higashida et al. (p. 2007) now observe the movement of formins in living cells as actin filaments grow. The movement does not require microtubules or their motors, nor the actin-based motor protein myosin. Instead, the forming protein appears to “surf” along with the growing actin ends. The movement was promoted by a constitutively active form of the small GTP-binding protein, Rho, which plays a role in a variety of actin-remodeling processes.

  10. Patterning the Frontal Lobe

    Regions of the frontal lobe are essential for higher cognitive function. In patients with bilateral frontoparietal polymicrogyria (BFPP), affected individuals show frontal lobe dysfunction (including mental retardation, gait difficulty, language delay, and seizures) and characteristic frontal lobe anatomy (bilaterally symmetrical, and abnormally thin with multiple small, irregular folds). Piao et al. (p. 2033; see Perspective by Holt and Jahn) identify the genetic basis of BFPP as being mutations in a putative G protein-coupled receptor, GPR56. GPR56 appears primarily to affect neuronal progenitors and thus regional patterning of human cerebral cortex.

  11. Synaptic Vesicle Queue Jumping

    During repetitive nerve stimulation, it has been assumed that synaptic vesicles that are mobilized march up to the presynaptic membrane like little soldiers in formation, and that the first ones out are those lying closest to the membrane. Rizzoli and Betz (p. 2037; see Perspective by Rakic) show in frog motor terminals that the vesicles in the “readily releasable pool” (the first ones out) are scattered almost randomly within the nerve terminal. During their movement to the membrane, they pass hundreds of other less mobile vesicles. These privileged vesicles must thus possess distinctive properties, perhaps access to cytoskeletal tracks and motors, that less mobile vesicles lack.

  12. Limiting Diversity in HIV Infection

    The AIDS pandemic is largely the result of heterosexual infection by human immunodeficiency virus-1 (HIV-1), yet this mode of transmission is relatively inefficient. Even though HIV-infected individuals possess diverse quasi-species of the virus, only a narrow population of viruses are transmitted to their partners. Examining recent infections in HIV-1-discordant heterosexual pairs in Zambia, Derdeyn et al. (p. 2019; see the news story by Cohen) found that the infecting viruses were, in fact, derived from single strains in each case, that have distinct envelope glycoprotein characteristics. These infecting strains were also considerably more susceptible to neutralization by antibodies to HIV-1. Thus, the features of the virus being selected against during chronic infection also contribute to more efficient transmission, or replication, within new hosts.

  13. Well Under Control

    Several examples of using feedback algorithms to optimize a quantum mechanical process, such as selecting for different products in a photochemical reaction, are now known, and many successful simulations of quantum control processes have been reported. Despite the differences between systems studied, the large number of variables involved in control, and possible sources of noise, quantum control is remarkably successful. Rabitz et al. (p. 1998) now present a general theoretical argument for why this is so. They assume nothing more about the Hamiltonian of the system other than that it is controllable, i.e., that some values of parameter space lead to the desired final state. What they then show is that all extrema of the system are either maxima or mimina—there appears to be no local minima that trap the control algorithm in a marginally productive outcome.

  14. Signaling Control of Life-Span

    Life-span control in the nematode Caenorhabditis elegans depends on the protein Sir2 and its interaction with Daf-16, a member of the FOXO family of transcription factors. Daf-16 participates in an insulin-like signaling pathway, and inhibition of signaling through this pathway extends life-span. Brunet et al. (p. 2011) studied the mammalian homolog of Sir2 known as SIRT1. SIRT1 is a nicotinamide adenine dinucleotide-dependent histone deacetylase that directly interacts with the mammalian FOXO family transcription factor FOXO3 and contributes to its deactylation. This interaction may enhance or decrease FOXO-dependent transcription, depending on the target gene analyzed. Furthermore, expression of SIRT appears to enhance the effects of FOXO3 in promoting resistance to oxidative stress, but to inhibit FOXO-mediated signaling that promotes cell death. This enhanced stress tolerance and avoidance of cell death could provide a mechanism to increase longevity.

  15. Plant MicroRNAs Fail to Make the Cut

    The functions of the multitudinous microRNAs (miRNAs) found in most eukaryotic organisms are slowly beginning to emerge, with recent research in human tissue-culture cells and Drosophila adding to our knowledge of the roles of the founder members of the group, let7 and lin4, in the nematode worm Caenorhabditis elegans. Chen (p. 2022) now shows that the miRNA172 family of miRNAs regulates flower development in Arabidopsis through the floral homeotic gene APETALA2. Furthermore, miRNA172 works by repressing the translation of the APETALA2 messenger RNA, contrary to recent expectations that plant miRNAs function by cleaving their target RNAs. Thus, this plant miRNA works like a typical animal miRNA.

  16. Dopamine, Excitation, and Reward

    The reward prediction theory of dopamine neuron function suggests a selective role for midbrain dopamine neurons in reward. However, there has been no consensus on whether dopamine neurons change their firing in response to aversive stimuli. Most studies indicate that there is a small population of midbrain dopamine neurons that either increase or reduce their firing in response to aversive stimuli. To settle this matter, Ungless et al. (p. 2040) recorded from neurons in the ventral tegmental area of anesthetized rats and recorded the responses of the rats to tail-pinch stimuli and used subsequent morphological methods to identify the recorded neurons. After observing the responses of dopamine neurons identified by electrophysiological criteria, the authors morphologically identified dopamine neurons as opposed to nondopamine neurons. The small number of neurons that increased their firing during an aversive stimulus were in fact nondopaminergic.