This Week in Science

Science  06 Apr 2007:
Vol. 316, Issue 5821, pp. 13
  1. Glaciers on Mars


    Most of the water ice on the surface of Mars is locked up in the polar caps. The Mars Express orbiter has used its radar to penetrate to the base of the layered deposits on the north pole. Now Plaut et al. (p. 92, published online 15 March) have mapped the south polar-layered deposits. The radar penetrates 3.7 kilometers with little attenuation, which suggests that these deposits are almost pure water ice. The base of the deposits shows a set of buried depressions that may be past impact craters. The deposits themselves total 1.6 × 106 cubic kilometers, equivalent to a global water layer approximately 11 meters thick.

  2. Ocean Conditions Past and Present

    The formation of cold, dense water in the North Atlantic Ocean today helps drive meridional overturning circulation, in which warm water flows north over cold water flowing south, but conditions may have differed during the Last Glacial Maximum (LGM) 21,000 years ago. Lynch-Stieglitz et al. (p. 66) review our understanding of this problem. The pace of deep Atlantic circulation during the LGM was nearly as vigorous as it is now, but patterns of sea surface temperatures and the distribution of water masses were different, indicating that different mechanisms drove circulation then. Furthermore, during the last interglacial, around 125,000 years ago, land and sea surface temperatures at high latitudes were higher than they are today, and sea level was 4 to 6 meters higher. Did deep ocean conditions contribute to melting of the Greenland and Antarctic ice sheets? Duplessy et al. (p. 89) analyzed cores from the Atlantic and Southern oceans and show that North Atlantic Deep Water was warmer during the last interglacial than it is today. Using two models, they infer that extra heat would have been transferred to Circumpolar Deep Water in the Southern Hemisphere, which would have melted more of the Antarctic Ice Sheet.

  3. Dissecting Oxide Dislocations

    Imbalances in stoichiometry in layered oxides, such as at grain boundaries, can affect their electrical and mechanical properties. Less is known about the structure of dislocations, which are defects in the crystalline ordering, and the role they may play. Shibata et al. (p. 82) use high-resolution electron microscopy to study dislocations in aluminum oxide. Two nonstoichiometric partial defects form close to each other, and at high temperatures, the motion of the partial defects occurs with those on adjacent planes.

  4. Acid Buried in Base


    Chemists often tailor reaction conditions by manipulating the temperature or acidity of the medium. In contrast, enzymes cannot grossly alter their surroundings, and rely instead on internal cavities that tune the molecular environment of an individual docked substrate. Pluth et al. (p. 85) mimic this strategy using a synthetic cage-like cluster that self-assembles from ligands and metal ions in solution. The electrostatic environment inside the cluster stabilizes cations, and so favors protonation of guest molecules. The cage can function as an acidic enclave in a basic solution and be used to perform acid-catalyzed orthoformate hydrolysis in a surrounding basic medium.

  5. In Sync Several Times

    An organism or cell can synchronize its oscillatory behavior with that of its neighbors, as in the blinking of fireflies or the beating of cardiac cells. Shim et al. (p. 95) studied the behavior of two coupled nanomechanical beams, a conceptually simple system that nonetheless shows rich dynamic behavior. The beams are driven at a wide range of frequencies. Frequency-locking or entrainment occurred in a number of regions in which the two beams synchronize to a single resonance. These resonators may be of use in signal processing and communication.

  6. Electrons Feel the Drag

    When current flows in one layer of a bilayer system, electron-electron interactions can drag current in the other layer. Measurements of this Coulomb drag effect are important for understanding coupled electronic and correlated electron systems. Price et al. (p. 99; see the Perspective by Lerner) report on the observation of giant fluctuations, four orders of magnitude greater than that expected, of the Coulomb drag resistance, which result in an alternating positive and negative frictional force on electrons. The authors propose a model in which electrons in the two layers interact in the ballistic regime, characterized by large momentum transfers, where the local electron properties become important.

  7. Engineering Isoprenoid Builders

    Isoprenoids, a diverse family of natural products, are built from five-carbon building blocks using four coupling reactions. Enzymes that catalyze chain elongation and cyclopropanation have been identified, however, enzymes that catalyze branching and cyclobutanation have not. Thulasiram et al. (p. 73; see the Perspective by Christianson) show that all four reactions can be catalyzed by engineered enzymes that are chimeras of a chain elongation enzyme and a cyclopropanation enzyme. The products have the same stereochemistry as the natural products, suggesting that enzymes catalyzing the four reactions evolved from a common ancestor.

  8. The More You Know, the More You Learn

    The ability to remember complex new information often depends on prior knowledge of the topic. This is because we have already formed a relevant mental schema as a framework. Tse et al. (p. 76; see the Perspective by Squire) used rats to study the effects of prior learning of schemas on the ability to acquire new episodic associations. These associations were acquired faster when the animals were first trained on a consistent set of associations than when they occurred in the context of a novel set of associations. The acquisition of novel associations was dependent on the hippocampus. However, within 48 hours the associations were independent of the hippocampus, which is substantially faster than typical memory consolidation. Thus, animals—like people—can bring activated mental schemas to bear during learning.

  9. Motor Mechanics


    Kinesin-1 is a two-headed molecular motor that takes 8-nm steps along microtubules. At each step, one molecule of adenosine triphosphate (ATP) is hydrolyzed and between steps kinesin pauses until another molecule of ATP binds. Now Alonso et al. (p. 120; see the Perspective by Hackney) show that kinesin-1 interacts with free tubulin heterodimers in solution and that this system too is gated by ATP. The observed behavior would not be predicted by current models for the motor mechanism that include a role for the microtubule lattice in the gating mechanism.

  10. Understanding Inositol Pyrophosphates

    Inositol pyrophosphates are relatively poorly understood, highly phosphorylated members of the inositol polyphosphate family. Two studies describe related advances in signaling involving inositol pyrophosphates. Mulugu et al. (p. 106) purified an inositol pyrophosphate synthase from yeast that has two catalytic domains. The enzyme, called Vip1, appears to act as a switch, with its catalytic activity determined by the local pH. Lee et al. (p. 109) purified a molecule that regulates the yeast Pho80-Pho85-Pho81 complex, a protein complex containing a cyclin, a cyclin-dependent kinase (CDK), and a CDK inhibitor. The active molecule turned out to be myo-D-inositol heptakisphosphate (IP7), which is synthesized through the kinase activity of Vip1.

  11. Sizing Up Man's Best Friend

    In contrast to most mammalian species, Canis familiaris (the domestic dog) shows extreme diversity in body size. Sutter et al. (p. 112, cover) show that a single allele of the gene encoding insulin-like growth factor-1 (IGF-1) is shared by all small dog breeds but is nearly absent from giant dog breeds, implying that sequence variation in the IGF-1 gene plays a causal role in dog size. Discovery of the IGF-1 gene was facilitated by its localization within a genomic signature, or haplotype block, that probably arose as a result of centuries of dog breeding by humans.

  12. Spliceosome Assembly

    Human Prp31 is a protein in the spliceosome that is essential for pre-mRNA splicing. It is assembled onto the spliceosome after 15.5K protein binds to an RNA component, U4 small nuclear (sn)RNA. Liu et al. (p. 115) present structural and biochemical data of hPrp31-15.5K-U4 snRNA complexes that give insight into this hierarchical assembly. hPrp31 presents both RNA and protein binding surfaces, making it a true ribnucleoprotein (RNP) binding protein. Binding occurs through the nucleolar protein (Nop) domain, which may act as a molecular ruler that measures the length of an RNA stem to achieve RNP binding specificity.

  13. A Sound Source of Power

    Smaller devices tend to have lower power requirements but also less space for storing power in batteries or by other means. An alternative is to harvest mechanical energy that could pass through a device from an external source, such as sound waves. Wang et al. (p. 102) have extended their observation of deflection-induced charge separation in polar ZnO nanostructures to produce a device that can generate electrical power from ultrasound-driven vibrations. The vibrations cause a platinum-coated zigzag electrode to move up and down, which causes a series of piezoelectric semiconducting nanowires to bend back and forth. Because different wires are actuated at different times, a continuous electrical current is generated.