This Week in Science

Science  30 Mar 2012:
Vol. 335, Issue 6076, pp. 1542
  1. An Iron Separator

    CREDIT: BLOCH ET AL.

    Petroleum processing initially yields a mixture of saturated and unsaturated hydrocarbons—the feedstocks for fuels, plastics, pharmaceuticals, and a wide range of other commercial products. At present, distillation is the primary means of separating the components of this mixture. A sorbent or membrane-based approach to separation could reap substantial energy savings. Bloch et al. (p. 1606) found that an iron-based metal organic framework material shows promise for very efficient sorption-based separation of ethane and ethylene, propane and propylene, and several other light hydrocarbon mixtures. Neutron diffraction directly revealed the binding motifs at the iron centers that selectively pinned down the olefins while the saturated hydrocarbons passed by.

  2. Chiral Ice

    Water ice, even at the lowest temperatures, is not completely “frozen”—its lattice structure allows for multiple equivalent ground states and it thus retains finite entropy even at absolute zero. Equivalent structures are realized in frustrated magnets called spin ices, where spins interact ferromagnetically, and in the even more exotic artificial spin ices, which are fabricated arrays of nanoscale magnets. Branford et al. (p. 1597) studied the transport behavior of an artificial spin ice with a honeycomb geometry during upward and downward sweeps of an external magnetic field, which revealed a field-asymmetric peak when the magnetic field was applied parallel to the current and the voltage was measured transversely. Micromagnetic simulations suggest that the asymmetric response is a result of the loops of opposite handedness forming at the edges of the sample, resulting in an overall chirality of the transport response.

  3. Electrons Beat Phonons

    The phenomenon of superconductivity, in which a material suddenly (below a certain transition temperature Tc) becomes a perfect conductor with zero electrical resistance, can be roughly explained in terms of Bose-Einstein condensation of pairs of electrons. In conventional superconductors, the formation of these so-called Cooper pairs is mediated by lattice deformations (phonons), but this mechanism is insufficient to explain the high Tc of cuprate superconductors. Other mechanisms, such as magnetic fluctuations, have been proposed which originate with the electrons themselves rather than the lattice. Dal Conte et al. (p. 1600) used time-resolved optical spectroscopy of an optimally doped cuprate to show that the temporal evolution of the reflectivity is consistent with the electronic contribution being dominant and is able to account for the high Tc by itself.

  4. Quantum Mechanical Coupling

    CREDIT: KOLKOWITZ ET AL.

    Observing the induced patterns of iron filings as a magnet is moved nearby, is a mainstay experiment of elementary science kits. Scaling down to the motion of the magnet and the size of the “sensing” particles enters the realm of quantum nanomechanics, where the motion of the vibrating system is quantized. That motion, however, is difficult to observe and manipulate. Kolkowitz et al. (p. 1603, published online 23 February; see the Perspective by Treutlein) coupled the single-mode vibration of a magnetized nanomechanical resonator to the quantum mechanical two-level spin system associated with the nitrogen vacancy center in diamond. The evolution of the spin degree of freedom was directly mapped to the mechanical motion, providing the opportunity to probe minute mechanical motion that would otherwise be undetectable.

  5. A New Lease on Half-Life

    Radiometric dating relies on measuring the abundance of long-lived radionuclides relative to the abundance of their radiogenic decay products—a process determined by the original radionuclide's half-life. For primordial radionuclides that decay slowly, such as 146Sm decaying to 142Nd, this method provides the timing of some of the earliest processes in solar system history. Using accelerator mass spectrometry, Kinoshita et al. (p. 1614) provide a revised estimate for the 146Sm half-life of ∼68.7 million years, which is 30% shorter than the previously accepted value. This shorter half-life suggests that reductions need to be made in the estimated ages for differentiation of Earth's mantle and the solidification of the Moon's magma ocean and for other more recent processes.

  6. Making the CoTC

    In eukaryotes, cotranscriptional cleavage (CoTC) of nascent RNA transcripts at the polyadenylation (polyA) site is involved in transcription termination of RNA polymerase II genes. The Dicer endoribonucleases, on the other hand, are generally associated with gene silencing through the generation of small RNAs from double-stranded RNA. Now, Liu et al. (p. 1621) show that transcriptional read-through of the Arabidopsis FCA gene is regulated by DICER-LIKE 4 (DCL4). DCL4 associated with the FCA gene downstream of the polyA site and repressed transcriptional read-through. The C terminus of DCL4 has a similar domain structure to the yeast endoribonuclease Rnt1, involved in CoTC, which suggests that DCL4 is involved in repressing read-through of endogenous FCA by prompting cotranscriptional cleavage.

  7. Spreading the Excitation

    Inhibitory interneurons balance network excitation, control spike-time precision of principal neurons, and control synchrony within and across brain regions. Vervaeke et al. (p. 1624, published online 8 March) combined electrophysiology, immunohistology, and numerical simulation to investigate the properties of Golgi cells, the main inhibitory interneurons in the input layer of the cerebellar cortex. The dendrites of these neurons were truly passive and acted as linear cables. Gap junctions were distributed nonuniformly on the Golgi cell surface, with a higher density on distal dendrites. Thus, gap junction-mediated lateral excitation preferentially enhanced the distal inputs, enabling distal synapses to drive network activity more effectively.

  8. Building the Brain

    Brain connectivity is often described as a network of discrete independent cables analogous to a switchboard, but how is the physical structure of the brain constructed (see the Perspective by Zilles and Amunts)? Wedeen et al. (p. 1628) used high-resolution diffusion tensor imaging in humans and four species of nonhuman primates to identify and compare the geometric structure of large fiber tracts in the brain. Fiber tracts followed a highly constrained and regular geometry, which may provide an efficient solution for pathfinding during ontogenetic development. Much of development occurs through elaboration and assembly of semiautonomous building blocks. Chen et al. (p. 1634) applied statistical analysis to the form of the human cortex in brain-imaging studies that compared more than 400 di- and mono-zygotic twins. The findings suggest that the structure of the human cortex is defined by genetics.

  9. Sleeping Around the Clock

    CREDIT: ROGULJA AND YOUNG

    Fruit flies do not sleep all night like humans, but they do manage to spend about half of their day in bouts of inactivity that have similarity to sleep in other species. Rogulja and Young (p. 1617) searched for genes in neurons whose products regulated sleep. Depletion of the protein regulator of cyclin A1 (Rca1) caused the flies to be slow to go to sleep and to get less total sleep in a day because their sleep episodes were shorter. The mechanism by which Rca1 influences sleep is not yet clear. It appears not to disrupt circadian rhythms. However, the neurons that express Rca1 in the fly brain are located near the neurons that make up the circadian clock, potentially allowing for coordination of circadian behavior with sleep regulation.

  10. Cost-Benefit Analysis

    Mounting resistance to infection is costly, requires energetic input, and may thus compromise fecundity. Duffy et al. (p. 1636; see the cover) tested the relationships between productivity, predation, and mortality in a combination of observations of a natural lake and an experimental replica of a clonal zooplankton-parasitic yeast population. In the wild, epidemics of the yeast could exceed 60% and cause significant host mortality. In this situation, the clonal zooplankton host faces the physiological dilemma of either increasing resistance to deal with infection or of safeguarding fecundity. Zooplankton that feed quickly can reproduce quickly, but also end up ingesting more yeast spores. However, because fish tend to cull infected hosts, fish predation counters infection. Ultimately, both wild and model systems showed that lakes with high productivity (more nitrogen) and/or few fish supported greater epidemics of yeast and more resistant hosts, whereas less productive lakes, or those with more fish, had smaller epidemics and hosts with higher susceptibility to the yeast.

  11. Dissecting Rapamycin Responses

    Long-term treatment of mice and other organisms with the drug rapamycin extends life span. But, at the same time, the drug disrupts metabolic regulation and the action of the hormone insulin. Lamming et al. (p. 1638; see the Perspective by Hughes and Kennedy) dissected the action of rapamycin in genetically modified mice and found, encouragingly, that these two actions of rapamycin can be separated. Rapamycin inhibits a protein kinase complex known as mTORC1, and this appears to provide most of the life-lengthening effects of the drug. However, rapamycin also acts on a related complex known as mTORC2, and it is the disruption of mTORC2 action that produces the diabetic-like symptoms of decreased glucose tolerance and insensitivity to insulin.

  12. A Better Date

    Uranium-lead (U-Pb) dating, which is one of the most commonly used methods of radiometric dating for old terrestrial materials, operates by comparing the ratio of trace levels of U with its nuclear decay product Pb. This dating method, and the similar Pb-Pb method, assumes that the ratio between the two most common U isotopes (238U and 235U) is constant. By accurately measuring the 238U/235U ratio in a suite of minerals representing a range of tectonic environments, Hiess et al. (p. 1610; see the Perspective by Stirling) demonstrate that this ratio is more variable than was previously thought. The variability does not reflect any systematic bias with time, location, or temperature—suggesting that ideally 238U/235U should be determined for every sample to calculate ages. In the absence of such data, a revised 238U/235U ratio for zircon minerals could significantly modify previous age estimates using U-Pb and Pb-Pb dating techniques.

  13. Choosing the Right Path

    RNA molecules are synthesized in the cell nucleus, yet many have to be moved to the cytoplasm to be processed and/or to effect their function. Different classes of RNA are transported from the nucleus by different transport systems. Messenger RNAs (mRNAs) and uridine-rich small nuclear RNAs (U snRNAs) are transcribed by RNA polymerase II and are capped and bound by the cap-binding machinery in the nucleus but are exported by different protein complexes. The feature that distinguishes the two classes of RNA is their length: U snRNAs are short and mRNAs are long. Using an in vitro system and human tissue culture cells, McCloskey et al. (p. 1643) show that the length of the RNAs is measured by the heterogeneous nuclear ribonicleoprotein (hnRNP) C tetrameric protein complex. The hnRNP C cannot bind to the short U snRNAs, allowing the U snRNA–specific export adaptor protein, PHAX, to bind and mediate export. Longer mRNAs are bound by hnRNP C, which prevents the binding of PHAX, thus identifying these RNAs for export from the nucleus via the mRNA pathway.

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