This Week in Science

Science  16 Apr 2004:
Vol. 304, Issue 5669, pp. 356
  1. Twins Help Twice


    Pure metals such as copper have low electrical resistivity but also tend to be soft. These metals can be made stronger by alloying them with a second component, or through the introduction of dislocations and grain boundaries, but all of these methods significantly increase electrical resistivity. Metals can also be strengthened through the introduction of large numbers of twin defects, which occurs when two crystalline grains grow with a large boundary angle between them. Lu et al. (p. 422) now show that the resistivity increase is much smaller in copper synthesized with a large number of twinned crystals. Furthermore, when the twinned crystal grains are small, the increase in strength is significant.

  2. Only One Calmodulin Required

    Calmodulin interactions with Ca2+ channels unify two vital but diverse functions: Ca2+ regulation of channels themselves, and local Ca2+ triggering of nuclear transcription factors. Crucial to these functions are the number of calmodulins regulating each channel, and the number of calmodulins privy to the local Ca2+ signal of each channel. Mori et al. (p. 432; see the Perspective by Levitan) fused calmodulin to the cytoplasmic tail of the channel protein. A single calmodulin could reproduce the Ca2+-dependent inactivation characteristic of the native channel. The local concentration of calmodulin at the mouth of the channel was estimated to be several orders of magnitude greater than that in the cytoplasm. This value corresponds to a swarm of about 25 calmodulins poised at the channel mouth, presumably ready to carry Ca2+ signals to various targets such as nuclear transcription factors.

  3. Optimizing Metabolite Production on Chips

    Optimization of enzyme pathways is important in order to generate maximum amounts of commercially valuable metabolites. Jung and Stephanopoulos (p. 428) have generated messenger RNA-enzyme fusion molecules using in vitro translation and immobilized them on a solid support by hybridization with homologous capture DNA. Using this method, they determined the relative concentrations of enzymes that would maximize the yield of trehalose in its five-step synthesis pathway.

  4. Home and Away


    One of the enduring enigmas in biology is how songbirds find their way from their tropical winter feeding grounds to their temperate summer breeding grounds every year. Lab-based research has showed that birds can perceive a variety of orientation cues when presented in isolation, but how these cues interact to guide wild birds on a daily basis during their migration has remained frustratingly obscure. Cochran et al. (p. 405; see the news story by Stokstad p. 373 A) tracked individual thrushes migrating through the midwestern United States and found that the birds use a simple orientation mechanism. They use the magnetic field—a cue that is permanently available even under totally overcast skies—but calibrate its shifting direction with the sunset, a cue that always indicates “west.”

  5. Iron in the Oceans

    In several locations of the world ocean, phytoplankton are far less abundant than the available macronutrients could support. This paradox is of considerable importance for climate, because ocean productivity exerts a major control on the concentration of atmospheric CO2. A number of experiments have shown that adding iron, an essential micronutrient, to ocean surface water in these regions increases productivity and the export of carbon to deeper waters. However, questions still remain about the magnitude of the effects, and how other factors, such as low concentrations of silicic acid, might also limit productivity. These issues have been the focus of the Southern Ocean Iron Experiment (SOFeX) (see the Perspective by Boyd). Coale et al. (p. 408; see the cover) present an overview of this field campaign, in which iron was added to patches of surface waters in the Southern Ocean in two regions, one low in silicic acid north of the Antarctic Polar Front, and one high in silicic acid to the south. Iron fertilization increased productivity, and silicic acid, a critical nutrient for diatoms, exerted a secondary control over phytoplankton growth and community structure. They also found that particulate material formed in the surface layer released nitrogen more rapidly than carbon as it sank and was remineralized. In an effort to determine whether this iron fertilization enhanced carbon sequestration, Buesseler et al. (p. 414) found that the iron addition to the high-silicic acid waters of the southern patch produced small but measurable increases in carbon export. Their measurements of 234Th, a radioactive nuclide with a high particle affinity and a short half-life, show that sinking particles transported carbon below the mixed layer, thus avoiding the rapid return of CO2 to the atmosphere. The relatively modest increase in carbon export does not appear to be large enough to make iron fertilization a viable method for sequestering anthropogenic CO2, however. Bishop et al. (p. 417) measured particulate organic carbon concentrations and the particle flux from the surface to deeper waters in the low-silicic acid northern patch with autonomous profiling devices they call “Lagrangrian Carbon Explorers.” The productivity of the fertilized patch was greatly enhanced compared to areas not amended with iron, and greater-than-expected carbon export to below the mixed layer occurred.

  6. Iron in the Skies

    Night-luminous clouds that appear high above the stratosphere during the polar summers have been described for more than a century. However, little is known with certainty about these polar mesospheric clouds (PMCs), which form at altitudes between 82 and 87 kilometers and that are believed to be highly sensitive to water vapor pressure and temperature. Their frequency of occurrence and brightness have varied markedly during the past century, although their altitudes have remained quite constant. Plane et al. (p. 426; see the Perspective by Hunten) now present lidar observations of iron scavenging by PMCs. Iron atoms are delivered into the mesosphere by the ablation of meteoroids. At the altitude of PMCs, a local minimum in iron abundance shows that these clouds scavenge essentially all of the iron present, probably by iron uptake on the surfaces of the ice particles that make up the clouds. These observations provide a rare example where heterogeneous removal is directly observed in the atmosphere.

  7. Sensing Calcium During Membrane Fusion

    The minimal machinery required for calcium-stimulated fusion, such as occurs during regulated secretion or neurotransmitter release, has been the subject of much controversy. Using a reconstituted system, Tucker et al. (p. 435) now show a robust requirement for the cytoplasmic tail of the synaptic vesicle protein synaptotagmin to act as calcium sensor during SNARE-mediated fusion in reconstituted liposomes.

  8. Same Time, Same Place


    When we hear a sentence, we assess whether it makes sense semantically and if the content of the sentence agrees with what we know about the world. Although it would seem plausible that the second assessment could be made only after the first had been settled, Hagoort et al. (p. 438) provide two kinds of brain imaging evidence for an extensively parallel processing of these truths in time and in space. Measurements of evoked brain potentials (with electroencephalography) and neuronal activity (with functional magnetic resonance imaging) indicate that the left inferior prefrontal cortex subserves processing of both meanings.

  9. Cryptosporidium Genome Comes Out of Hiding

    The analysis of microbial genomes is an important approach in the generation of novel therapeutics. Abrahamsen et al. (p. 441) have generated a complete genome sequence of the intestinal parasite, Cryptosporidium parvum. Unlike Plasmodium and Toxoplasma, C. parvum lacks mitochondrial and apicoplast genomes. In addition, many metabolic pathways—such as the Krebs cycle, pentose phosphate shunt, and de novo synthesis of amino acids and nucleotides—are missing. Analysis of the genome suggests reasons for the failure of some therapeutic approaches and identifies potential drug targets that appear to be of plant or bacterial origin.

  10. Mitochondrial Toxicity in Alzheimer's Disease

    Aggregates of amyloid beta (Aβ) protein are a characteristic of Alzheimer's disease (AD), and these aggregates are toxic to neurons. Mitochondrial dysfunction is a hallmark of this neuronal toxicity. Now Lustbader et al. (p. 448) show that Aβ interacts directly with Aβ-binding alcohol dehydrogenase (ABAD) in the mitochondria of AD patients and in transgenic mice. A crystal structure shows that binding of Aβ prevents nicotinamide adenine dinucleotide binding to ABAD, which results in a loss of activity of ABAD and promotes leakage of reactive oxygen species, mitochondrial dysfunction, and cell death. Thus, the interaction of Aβ with ABAD in mitochondria may play a role in the pathogenesis of AD.

  11. Gazing into the Vortex

    Patterned magnetic nanostructures offer a wide range of potential applications, from data storage to spintronics. Understanding the response and behavior of the magnetization dynamics of such structures will be important to identify and improve upon their functionality. Choe et al. (p. 420) describe results of a time-resolved x-ray imaging technique that allows them to characterize the dynamics with both high temporal and spatial resolution and to uncover a wealth of microscopic information about the magnetic dynamics on the nanoscale.

  12. Picking Apart Complex Traits

    Analysis of complex traits will require resources that can partition the genetic effects of autosomes, sex chromosomes, and mitochondria. Singer et al. (p. 445) have generated chromosome substitution strains (CSSs) in which each chromosome is individually substituted from a donor (A/J) strain onto a recipient C57BL/6J mouse. The study reveals a large reservoir of genetic variation that affected different behavioral and physiological traits and implicated specific chromosomes as harboring quantitative trait loci (QTL). The use of CSSs can lead to the detection of more QTLs with fewer mice and with greater sensitivity and reliability than traditional approaches based on segregating populations.

  13. The Actor and the Critic in the Brain

    Adaptive behavior is the ability to orient toward specific goals in the environment and to control actions flexibly in pursuit of those goals. During instrumental conditioning, an organism learns the relation between its own responses and positive or negative outcomes. Models of reinforcement learning provide a two-process account of instrumental conditioning. One component is a temporal difference learning prediction error signal, used as a critic to update successive predictions of future reward. The second component is an actor, which modifies stimulus-response-reward associations so that actions associated with greater long-term reward are chosen more frequently on subsequent trials. Using functional magnetic resonance imaging and behavioral studies, O'Doherty et al. (p. 452) compared the influence of rewarding versus neutral stimuli during classical and instrumental conditioning. As predicted, in the instrumental conditioning task, subjects chose the rewarding stimulus over the neutral one and, in the classical conditioning task, responded more quickly to stimuli that predicted reward.