This Week in Science

Science  22 Jun 2012:
Vol. 336, Issue 6088, pp. 1480
  1. Slide and Find


    Transcription factors rapidly find their specific binding sites on chromosomal DNA. It has been proposed that searching is facilitated by complementing three-dimensional diffusion with one-dimensional diffusion along DNA. Such sliding on DNA has been observed in vitro, but whether and how far transcription factors slide along chromosomes in vivo is unclear. Hammar et al. (p. 1595) used single-molecule imaging to demonstrate that the lac repressor slides into its chromosomal operators in living cells. The average sliding distance was about 45 base pairs, and the repressor frequently slid over its operator before binding.

  2. Master Regulator

    Sympathetic neurons and the adrenal medulla are part of the autonomic nervous system, which is important in the control of a variety of bodily functions and in responses to stress. Interactions between the nervous system and the vascular system have been poorly explored. During development, sympathetic neurons and adrenal medulla cells are derived from the same precursors—neural crest cells—embryonic cohorts that undergo massive migration in the body. Using blood vessel–specific gene manipulation in chicken embryos, Saito et al. (p. 1578) revealed a role for the dorsal aorta in regulating the early migration of neural crest cells and later in development on the segregation of adrenal medulla and sympathetic neurons. The dorsal aorta expresses multiple soluble morphogenetic and growth factors that regulate complex morphogenesis in a spatiotemporal manner. Furthermore, in mice, the morphogenesis of the adrenal medulla was controlled both by the aorta and the adrenal cortex.

  3. Straining Suspended Graphene


    The electronic properties of graphene are best displayed by suspended sheets free from contact with an underlying substrate. Klimov et al. (p. 1557) probed how deformation of suspended graphene sheets could lead to further tuning of its electronic properties with a scanning tunneling microscope; the graphene sheets could also be deformed via an electric field from an underlying electrode. Spectroscopic studies reveal that the induced strain led to charge-carrier localization into spatially confined quantum dots, an effect consistent with the formation of strain-induced pseudomagnetic fields.

  4. A Spike Inside the Dome

    The transition temperature Tc of iron-based superconductors has a dome-shaped dependence on chemical doping, and the superconductivity that develops underneath may obscure a potential quantum critical point (QCP) residing at absolute zero. With the aim of detecting signatures of this quantum criticality, Hashimoto et al. (p 1554; see the Perspective by Sachdev) measured the penetration depth of the pnictide series BaFe2(As1−xPx)2 as a function of x. A sharp peak right around the point where Tc has a maximum (x = 0.30) was observed, implying that the superfluid density diminishes sharply where one would expect it to be the most robust. This unusual finding is interpreted as a sign of a QCP at x = 0.30.

  5. From Wnt Signals to Telomerase Activity

    Telomerase activity is associated with stem cell renewal and cancers, whereas a decrease in telomerase activity is seen during cell differentiation and senescence. Wnt/β-catenin signaling is also a critical regulator of stem cells, and deregulation of the pathway is associated with cancer. Now, Hoffmeyer et al. (p. 1549; see the Perspective by Greider) have found a link between these two pathways. In embryonic stem cells, β-catenin was able to regulate telomerase expression and activity directly. Similar observations were obtained in adult stem cells, a model of intestinal tumors, and human cancer cells.

  6. Exploiting Defects in a Jam

    Phase-change materials that can readily switch between crystalline and amorphous states are increasingly finding use in nonvolatile memory devices (see the Perspective by Hewak and Gholipour). Using high-resolution transmission electron microscopy, Nam et al. (p. 1561) show that for Ge2Sb2Te5, the application of an electric field drives crystal dislocations in one direction, leading to their accumulation and eventual jamming, which causes the phase transition. Loke et al. (p. 1566) found that by applying a constant low voltage to Ge2Sb2Te5, they could accelerate its phase-switching speeds, without harming the long-term stability of the switched state.

  7. No More Fusion Confusion

    Biophysical models explain membrane fusion as a sequence of steps—including membrane contact, formation of a fusion stalk (merger of proximal monolayers), development of contact between distal monolayers that may or may not expand (hemifusion), and, finally, rupture of this diaphragm resulting in the opening of a fusion pore. Biological membrane fusion reactions are often driven by so-called SNARE proteins. By using a reconstituted membrane fusion system, Hernandez et al. (p. 1581, published online 31 May) have now been able to correlate precisely the states of SNARE zippering with intermediate structures along the fusion pathway. The results suggest that a tightly docked state, with a membrane distance so close that no proteins fit in between them, represents a critical fusion intermediate as a consequence of SNARE zippering. This intermediate is incompatible with a SNARE-driven stalk or with a ringlike arrangement of SNAREs depicted in most current models of membrane fusion.

  8. Cavity-Induced Minimum

    Tuning the strength and range of interactions in cold atomic gases is crucial to their role as quantum simulators. Most atom-atom interactions are short-ranged. One way to extend the range is to couple the gas to an optical cavity, which can propagate interactions between atoms, making the interactions effectively long-ranged. This system has been used to observe a transition to a “supersolid” phase characterized by a checkerboard atomic density order. Mottl et al. (p. 1570, published online 17 May) used Bragg spectroscopy to measure the excitation spectrum of an ultracold gas of Rb-87 atoms as the interaction strength was varied. Consistent with theoretical predictions, a minimum was observed in the excitation energy, similar to that observed in roton excitations of the superfluid helium.

  9. Tropical Carbon Loss

    Accurate and precise measures of tropical deforestation and the resulting carbon emissions are needed in order to formulate climate policy. Harris et al. (p. 1573; see the Perspective by Zarin) used satellite observations of deforestation within the tropics of three continents to estimate that gross annual carbon emissions were approximately 0.8 Pg Cyr−2 (Pg = 1015 g) for the years 2000 to 2005, from the loss of 43 million hectares of forest. This result, which is about one-third of some previous estimates, should serve as a baseline for future assessments of changes in the rate of loss of tropical forests.

  10. Plants That Eat Animals

    Apart from some spectacular exceptions, such as pitcher plants and Venus fly traps, most plants are thought to acquire nitrogen passively from microbial decomposition and the activities of nitrogen-fixing bacteria. Metarhizium species are common endophytes—fungi that live within plant tissues without causing disease. This genus is also found ubiquitously in soil, where they parasitize insects. In a series of microcosm experiments, Behie et al. (p. 1576) investigated whether these fungi could couple their endophytic life-styles with their parasitic modes and be a conduit by which plants could obtain nitrogen from animals. Radio-labeled moth larvae were added to the microcosms in which bean and grass plants were grown, and when the larvae were inoculated with fungi, it was only a matter of days before the nitrogen label was detected in the plants.

  11. No Crossing Over


    To ensure the correct division of chromosome during the reduction division of meiosis, homologous chromosomes undergo double-strand breaks that—through crossing over and recombination—link the homologs together (and importantly introduce diversity into the genomes of gametes). But only a minority of these crossovers results in recombination—most are directed into non-crossover pathways. Lorenz et al. (p. 1585), working in the yeast Schizosaccharomyces pombe, and Crismani et al. (p. 1588), working in the higher plant Arabidopsis thaliana, looked for the factors that limit crossovers and promote non-crossover pathways. The homolog of the human Fanconi anemia complementation group M (FANCM) helicase protein was found to be a major meiotic anti-recombinase, which could drive meiotic recombination intermediates into the non-crossover pathway.

  12. Blasting Through

    The fungus that causes rice blast disease, Magnaporthe oryzae, can lead to devastating reductions in rice yields. M. oryzae enters the plant by developing specialized infection structures called appressoria. Appressoria generate enormous internal turgor pressure that somehow creates invasive forces that physically break the plant cuticle. Dagdas et al. (p. 1590) found that a toroidal (doughnut-shaped) filamentous actin network forms at the base of the appressorium at the precise point where the penetration peg, which ruptures the rice leaf cuticle, will emerge. This network is scaffolded by means of four septin guanosine triphosphatases, which form a dynamic ring structure that colocalizes with F-actin. The findings reveal how fungi translate extreme pressure into localized physical force.

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