This Week in Science

Science  01 Apr 2005:
Vol. 308, Issue 5718, pp. 13
  1. Marine Biology and Climate


    The CO2 content of the anthropogenically unperturbed atmosphere has varied consistently between a minimum of around 180 parts per million (ppm) at glacial maxima and a maximum of around 280 ppm during interglacials. Why peak glacial intervals and warm periods have such apparently well-constrained atmospheric CO2 concentrations is still unexplained. Climate models uniformly fail to account for the observed difference through physical mechanisms alone, so marine biological processes often have been invoked as a probable cause. Kohfeld et al. (p. 74) combined multiple records of biological activity from more than 150 marine sediment records extending back into the penultimate glacial period. They show that ocean biology could have been responsible for no more than half of that difference during that time, which implies that physical processes must somehow be responsible for the rest.

  2. Probing Luttinger Liquids

    Strongly interacting electron systems in low dimensions provide a theoretically tractable system for studying such complex electronic interactions. However, experimental realization and probing of such systems has lagged behind the theoretical efforts. Auslaender et al. (p. 88) now play catch-up with their study of the fractionalization of electrons in a well-controlled one-dimensional system. Using a pair of coupled semiconducting wires with variable electron density, they directly measured the spin and charge excitations of the system and followed the dispersive behavior of the electronic transport as a function of the strength of the Coulomb interaction. They found clear evidence for the theoretically predicted separation of the spin and charge degrees of freedom.

  3. Sneaking a Peak at Creep

    Creep is the slow deformation of a material that occurs when it is held under constant load, such as the gradual stretching of a piano or violin string. During creep in a metal or metallic alloy, voids form and grow with time, in addition to changes in the texture and orientation of the crystal grains. Pyzalla et al. (p. 92) have developed a technique to monitor all of these changes at once, and they use it to track creep in a brass alloy. They confirm that the transition from homogeneous creep to localized deformation occurs late in the creep process and suggest ways of how other simultaneous diffraction and tomography measurements may be used to track crack growth or load partitioning within a composite.

  4. Translational Repression at 5.5 Angstroms


    Gene expression must be tightly regulated at both the level of transcription and translation. X-ray crystallography studies have helped to elucidate the mechanism by which threonyl-tRNA synthetase regulates its own expression. Jenner et al. (p. 120) now examine the complex that includes the ribosome, structured messenger RNA (mRNA) carrying a regulatory domain, and initiator transfer RNA, with structural information shown at 5.5 angstrom resolution. The path of the mRNA on the 70S ribosome in the presence of initiator tRNA and the localization of the regulatory element of mRNA on the ribosome suggest the molecular mechanisms by which translational repressors can work.

  5. Rafts and Regulation

    Regulation of the transcription factor NF-κB is central to the activation of T cells in the immune system. Activation of the T cell receptor leads to accumulation of a group of signaling proteins within lipid rafts in the plasma membrane. Somehow this process results in activation of the IKK (IκB kinase) complex, which leads to activation of NF-κB. Lee et al. (p. 114; see the Perspective by van Oers and Chen) provide a mechanism that helps link these processes. The 3-phosphoinositide-dependent kinase 1 (PDK-1) interacts with and activates another protein kinase, PKCθ, which in turn interacts with components of the IKK complex and is required for their recruitment to the lipid rafts. PDK1 also interacts with a scaffold protein CARD11, which in turn recruits the Bcl10 and MALT1 proteins. The latter proteins mediate ubiquitination of a component of the IKK complex—the signal that eventually activates NF-κB.

  6. A Trusting Trustee

    For most of us, games are merely a source of enjoyment, but mathematicians and economists have analyzed their theoretical and experimental aspects, and now neuroscientists are taking their turn. The trust game involves exchanges of money between two players, where the amounts transferred reflect an inclination to trust (or mistrust) the generosity of the other player. King-Casas et al. (p. 78; see the cover and the news story by Miller) examined the neural correlates of these inclinations during the course of repeated interactions within pairs of subjects separated by thousands of miles. The cut-and-thrust character of the game establishes the investor's reputation in the mind of the trustee, and the trustee's intention to increase the repayment equates to the well-known reinforcement learning signal that predicts reward.

  7. Long-Distance Synchrony

    How do distant brain areas communicate with each other? It is thought that neurons increase their impact on target groups through precise oscillatory synchronization. Long-range coherence modulation might represent a general mechanism for regulating the flow of information within the nervous system. To test this idea in human volunteers, Schoffelen et al. (p. 111) combined magneto-encephalography and electromyographic recordings during the performance of a basic reaction time task, where the subjects implicitly learned the increasing or decreasing probability of a signal. The coherence of gamma-band (40 to 70 Hertz) oscillations between the motor cortex and the spinal cord did indeed make motor outputs more effective.

  8. Jurassic Termite Trouble

    A few mammals, such as the anteater and some rodents, have evolved special limbs for digging and teeth for a diet feasting on just a few types of abundant social insects. These mammals appeared in the Paleocene, about 30 to 40 million years ago. Luo and Wible (p. 103) now describe a fossil of a similarly specialized mammal that appeared about 150 million years ago and appears to represent a new, but now extinct, line of basal mammals. The fossil, Fruitafossor windscheffelia, has large forelimbs, specialized for digging, and hollow teeth, probably used for feeding on termites.

  9. Listen, Learn, Freeze


    Rats and mice learn to freeze when they hear a tone previously encountered at the same time as an electric shock. This type of learning takes place in the amygdala. Rumpel et al. (p. 83, published online 3 March 2005) examined the cellular basis of the learning by tagging the glutamate receptors that are recruited into synapses during learning with a subunit that can be detected electrophysiologically. They found that fear conditioning drives these receptors into synapses in about 35% of the cells in the lateral amygdala. Inhibition of this recruitment inhibits the formation of the tone-shock memory. If only 10 to 20% of the synapses are inactivated, learning is blocked. Thus, synaptic modification is required for behavioral learning, which is unexpectedly sensitive to the loss of a small fraction of modified synapses.

  10. Animal Horizons

    The Doushantuo Formation, China, preserved perhaps the earliest examples of animals, including many exquisite embryos. Its deposition after a global glacial period has raised questions about the relation between climate stability and major evolution cycles. Condon et al. (p. 95, published online 24 February 2005; see the Perspective by Kaufman) now provide uranium-lead ages from zircons for the Doushantuo Formation that bracket its deposition between 635 and 550 million years ago. This formation, approximately 100 meters thick, was deposited gradually during a very long interval. Most of the fossils in the upper part of this formation may be correlative with other metazoans worldwide.

  11. Dust in the Wind and Sea

    Dust contains iron, an essential nutrient for marine phytoplankton and a primary control on marine productivity. Climate, in turn, modulates the sources, amount, and sites of deposition of dust. Jickells et al. (p. 67) review what is understood about this complicated Earth system, concentrating on the linkages between the various components. Gaps in our current understanding of the cycle are large enough that it would seem premature to embark on ambitious geoengineering schemes that would attempt to reduce atmospheric CO2 concentrations through iron fertilization of selected areas of the surface ocean.

  12. Not the Same Difference

    Much of the recombination in the human genome occurs in “hotspots.” The genomic mechanisms and evolutionary forces that direct recombination to specific locations, however, remain unknown. Winckler et al. (p. 107, published online 10 February 2005; see the Perspective by Jorde) studied fine-scale recombination rates over a large span of orthologous DNA in both humans and chimpanzees. Although the species share 99% identity at the level of DNA sequence, in no case did the location of the hotspots coincide, and the rates of recombination across three 500-kilobase regions were significantly different. Thus, recombination hotspots evolve rapidly, and the rate of their evolution is different from that of DNA sequence.

  13. Growing Fuels for Developing Countries

    Biomass fuels are vital resources for many people in developing nations, where they can provide more than 90% of household energy needs. However, the use of biofuels also emits air pollutants that currently cause more than 1.6 million annual deaths globally from respiratory diseases, and it creates a major fraction of regional greenhouse gas emissions. Bailis et al. (p. 98) present a detailed database of household energy use for countries in sub-Saharan Africa, and a set of energy forecasts for various demographic trajectories and energy scenarios. They estimate the current and future impacts of these scenarios on human health and greenhouse gas emissions, and develop an integrated picture of the health costs and benefits of transitions from wood to charcoal or to fossil fuels.

  14. More Silencing, Please

    A number of classes of proteins are involved in gene silencing mediated by small RNAs (siRNAs and miRNAs)—dicers chop double-stranded (ds)RNAs into small RNAs, argonautes are involved in target recognition (and in some cases, target RNA cleavage), and RNA-dependent RNA polymerases (RDRs) can convert single-stranded RNA into dsRNA. Herr et al. (p. 118, published online 3 February 2005) add two newly discovered factors to this list in plants, which together show high homology to the family of RNA polymerases and which have accordingly been named subunits of RNA Pol IV. The two subunits, together with RDR2, are involved in the silencing of transgenes and an endogenous retroelement in the flowers of Arabidopsis, and are needed for production of both heterochromatin-associated and nonheterochromatin-associated siRNAs.