This Week in Science

Science  01 Feb 2002:
Vol. 295, Issue 5556, pp. 761
  1. In Brevia

    Parthenogenesis, the process by which an egg can develop into an embryo in the absence of sperm, has been used by Cibelli et al. (p. 819) to derive embryonic stem cell lines (from the primate Macaca fascicularis) that can differentiate into a range of cell types, including cardiomyocyte-like cells, smooth muscle cells, adipocytes, ciliated epithelium, and neurons.

  2. Value-Added Peapods

    Recent work has shown that hybrid structures consisting of single-walled carbon nanotubes filled with C60, known as “peapod” structures, can be prepared. A scanning tunneling microscopy study by Hornbaker et al. (p. 828; see the cover) of these peapod structures indicates that the electronic density of states of the nanotube are altered by the addition of the C60 molecules. The periodic distribution of the C60 within the nanotube and the ability to move the fullerenes may offer the possibility to tune the orbital hybridization and to form structures with desirable electronic properties not available to the individual constituents.

  3. Collective Action

    Theory suggests that when electrons in metals are confined in a one-dimensional wire, they interact collectively, instead of independently as they do in bulk metals. The verification of these predictions has proven difficult experimentally. Auslaender et al. (p. 825; see the Perspective by Zülicke) have observed the electronic excitations in a pair of parallel wires, one with a gap along its length, as they studied the transport of electrons in the complete wire. The electrons can only get into the wire by tunneling from the broken wire, but only if the electronic states match up. The dispersion relation of the elementary excitations that they mapped by varying the energy (with a voltage bias) and momenta (with a magnetic field) of the injected electrons is in agreement with collective behavior.

  4. Metallocenes Sans Carbon

    Ferrocene is a “sandwich” compound in which an iron atom >is bound to two aromatic cyclopentadienyl (C5H5) rings. Like benzene, ferrocene and many other metallocenes show thermal stability and reactivity reminiscent of benzene and have found several uses, such as polymerization catalysts. The ability of this structure to stabilize aromatic species has allowed the synthesis of aromatic phosphorus compounds in which one C5H5 ring is replaced by P5 (P being isoelectronic with CH). Urnezius et al. (p. 832) now report the “one-pot” synthesis of [(η5-P5)2Ti]2− from highly reduced Ti complexes and white phosphorus. Despite its electron-deficient character, salts of this compound show remarkable stability toward air and heat.

    CREDIT: URNEZIUS ET AL.
  5. Gut-Level Genomics

    The FoxA family of transcription factors, which includes the pha-4 gene of Caenorhabditis elegans, is critical for the development of the digestive organs in a wide range of species. Using a genomics-scale approach, Gaudet and Mango (p. 821) compared microarrays from C. elegans embryos lacking a pharynx with microarrays from embryos having excess pharyngeal tissue. This analysis identified 240 genes that were preferentially expressed in the pharynx, and PHA-4 directly regulated almost all of the pharynx-specific genes that were examined in detail. Furthermore, the relative affinity of PHA-4 for different DNA sites linked to these genes correlates with the time the genes are activated in vivo.

  6. Glancing Goals

    Saccades, small rapid eye movements when jumping from one fixation point to another, are mostly controlled by the frontal cortex. Seidemann et al. (p. 862) combined electrical microstimulation and optical imaging to study the mechanisms underlying the planning and execution of saccades in the frontal eye field and cortical area 8Ar in its direct vicinity. Stimulation of these regions sufficient to evoke saccades produces a concomitant wave of depolarization succeeded by a large and prolonged hyperpolarization. The direction and amplitude of the evoked eye movements depend on the time point of saccade initiation with respect to this sequence. If a saccade is initiated during depolarization, it will be contralateral (eye movement to the opposite side of the body), but if it is initiated during hyperpolarization, it will be ipsilateral (to the same side). Saccade amplitude is correlated to the relative magnitude of the depolarization or hyperpolarization state.

  7. The State of Atmospheric Flux

    Satellite data have been used to understand primary processes that affect the Earth's radiation and to observe long-term flux trends. Aerosols can affect Earth's albedo (reflectivity) directly, by reflecting or absorbing radiation, or indirectly through their effects on cloud formation. Bréon et al. (p. 834) have compared satellite measurements of cloud droplet radii to the amount of small aerosols in the atmosphere in order to construct a global view of how aerosols affect cloud droplet size. They find distinct differences in the properties of clouds over land and over oceans, and suggest that anthropogenic emissions may be responsible for much of this variability on a global scale. Significant interannual variations in Earth's radiative flux linked to large-scale atmospheric circulation changes, such as the El Niño-Southern Oscillation (ENSO), are particularly evident in the tropics. Recent satellite measurements have detected even longer term trends (see the Perspective by Hartmann). Analysis by Chen et al. (p. 838) of data for the past 15 years on the fluxes of emitted thermal and reflected solar radiation shows that the observed decadal positive trend in outgoing longwave radiation is the result of stronger Hadley-Walker circulation. This general strengthening of tropical circulation, which occurred primarily during the first half of the 1990s, could be the result of either natural climate variability or anthropogenic forcing. Wielicki et al. (p. 841) present a compilation of tropical top-of-atmosphere radiative-flux data from two decades of satellite measurements which shows that the energy budget is much more dynamic and variable than was previously believed. Radiative flux variations of 1 watt per square meter (W/m2) can influence the output of climate models. They report rapid changes in longwave radiation, often related to ENSO or large volcanic eruptions, of as much as 8 W/m2, seasonal variations of 5 W/m2 in shortwave fluxes, and decadal drifts of 2 to 4 W/m2.

    CREDIT: WIELICKI ET AL.
  8. Inching Along

    Two different mechanisms have been proposed for the movement of kinesin along microtubules in 8-nanometer steps, a “hand-over-hand” mechanism, where the two kinesin heads alternately move past each other, and an “inchworm” mechanism, where the two heads do not swap places. Hua et al (p. 844; see the news story by Couzin) immobilized kinesin molecules through the carboxyl-terminal end of the neck domain and measured orientations of microtubules moved by single enzyme molecules. The kinesin-mediated linkage between the microtubule and the surface was sufficiently torsionally stiff that hand-over-hand stepping would produce 180° rotations of the microtubule relative to the immobilized neck. No 180° microtubule rotations were observed, and the results instead support an inchworm mechanism.

    CREDIT: HUA ET AL.
  9. Chaperoning Neuronal Degeneration

    In a fly model of Parkinson's disease, Auluck et al. (p. 865; see the Perspective by Helfand) have demonstrated a role for the molecular chaperone Hsp70 in protecting dopaminergic neurons in the presence of pathological levels of α-synuclein. When the authors went on to examine Lewy bodies in postmortem samples from human patients, these characteristic pathological lesions also contained high concentrations of molecular chaperones. These results suggest that modulating the activity of chaperone proteins may help in treating this debilitating disease.

  10. A Green Light for Transgenes

    Silencing of retroviral sequences during embryonic development is believed to be a developmental safeguard against uncontrolled expansion of these parasites. However, this shut-off of gene expression has represented a major obstacle for scientists trying to create transgenic animals. Lois et al. (p. 868) have shown that transgenic mice and rats can be generated at high frequencies by infection of single-cell embryos with recombinant lentiviral vectors. The transgene (in this case, green fluorescent protein) was expressed at high levels and could be made tissue-specific by coupling it to an appropriate promoter. The transgene was transmitted through the germ line to the next generation. Although it is not proposed that this approach replace other methods, it may be especially useful for quickly and inexpensively developing many transgenic lines and for other species in which pronuclear injection or other methods have not worked.

  11. Bacterial Amyloids

    Amyloid fibers are associated with a variety of disease states, including prion diseases and systemic amyloidosis. Chapman et al. (p. 851) now show that extracellular fibers expressed by the bacterium Escherichia coli are a bacterial form of amyloid. Curli fibers possess several amyloid-specific characteristics—for example, they aggregate to form fibers that bind to the dye Congo red. However, in the intact bacterium, the expression of curli requires the concerted action of several gene products. Understanding how curli are assembled may help in our understanding of pathological amyloids in human disease.

  12. Flexibility Is the Key

    Bacterial RNA polymerase contains five subunits (α2ββ'ω) in addition to its DNA-binding subunit (δ). In many bacterial genes, the δ subunit directly binds to two promoter elements that are centered at about -10 and -35 relative to the transcription start site. However, δ generally cannot bind to these elements on its own. Kuznedelov et al. (p. 855) now examine the mechanism by which δ binding is effected. After δ binds to the core polymerase, the “flexible flap” domain of the RNA polymerase β subunit interacts directly with δ to induce a conformational change that allows δ to then simultaneously bind to the -10 and -35 promoter elements for subsequent transcription initiation. Binding of different δ factors to the core polymerase may allow for differential specificity in promoter recognition.

  13. Golgi Partitioning

    Single-copy organelles, like the Golgi, need to be partitioned faithfully to daughter cells during cell division. There has been much debate as to the mechanism by which the Golgi complex partitions. Golgi enzymes may recycle quantitatively to the endoplasmic reticulum (ER) at the beginning of mitosis, so that a brand new Golgi forms de novo after mitosis is complete, or the Golgi may vesiculate and leave a template of clustered membranes and matrix components that is required for Golgi reassembly after mitosis. Seemann et al. (p. 848) looked at Golgi matrix components during mitosis when Golgi membrane structures had been recycled to the ER after treatments prior to entry into mitosis. Matrix components were faithfully partitioned to daughter cells. Thus, it could be that both modes of transmission are required—distributive partitioning of membrane, but specific partitioning of matrix depending on the status of the Golgi upon entry into mitosis.

  14. O2-Sensing Model Further Modified

    When oxygen becomes limiting (hypoxia), mammalian cells respond by increasing transcription of genes that enhance oxygen delivery or that facilitate metabolic adjustment to reduced oxygen availability. The hypoxia-inducible transcription factor HIF-α plays a pivotal role in this adaptive response, and recent work has revealed that HIF-α activity is controlled in part by oxygen-dependent proline hydroxylation, which reduces protein stability. Lando et al. (p. 858; see the Perspective by Bruick and McKnight) identify asparagine hydroxylation as a second oxygen-dependent protein modification regulating HIF-α activity and show that the critical asparagine target resides in the transactivation domain of HIF-α. Hydroxylation of this asparagine, which occurs under normal oxygen conditions, appears to suppress the transcriptional activity of HIF-α by preventing its interaction with the coactivator protein p300.

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