This Week in Science

Science  27 Jul 2001:
Vol. 293, Issue 5530, pp. 569
  1. Nitrogen-Fixing Symbiont

    The α-proteobacterium Sinorhizobium meliloti not only has affinity with important plant (Agrobacterium) and animal (Brucella) pathogens but is also a symbiont of leguminous plants. This symbiosis is responsible for a substantial proportion of the approximately 100 million tons of nitrogen fixed in terrestrial ecosystems. Galibert et al. (p. 668) sequenced the 6.7-megabase genome of S. meliloti, which consists of three replicons (one chromosome and two megaplasmids). Some surprising features include partitioning of several classes of insertion elements among the replicons, the presence of 26 nucleotide cyclases, a type IV secretion system and two types of pili genes, as well as the apparent absence of type III secretion, the sigma factor rpoS, and the nitrogenase nifQ.

  2. Shocking Behavior in Condensates

    Although defects are often unwanted, their controlled introduction can provide useful signatures or probes of a medium. The ability to introduce localized defects into Bose-Einstein condensates (BECs) would be an extremely useful tool to probe the properties of such macroscopic quantum systems and superfluids. Combining their slow-light technique with electromagnetic-induced tranparency, Dutton et al. (p. 663) report the formation of localized defects in a BEC and the response of the BEC to the defect. Small-scale, large-amplitude sound wave collapse in the BEC results in the breakdown of the superfluidity by the formation of topological defects such as solitons and the nucleation of vortices. The results present a superfluid analog to classical shock waves.

  3. Building β-Helical Polymers

    Although less common than α-helices and β-sheets, the β-helix motif, in which parallel β-strands coil up to form a large helix, can occur, such as in the fibrous form of transthyretin, which is associated with a number of diseases, and are also present in insect antifreeze proteins. Cornelissen et al. (p. 676) have synthesized a polymeric isocyanopeptide that also organizes into β-helices but with variation on the motif. The backbone of the polymer forms a central helix, about which the adjoining peptides form sheetlike arrangements. This structure shows stability in both organic solvents and in water, and it may be possible to use these materials as templates for crystal growth, anchors for metal catalysts, or for organizing nonlinear optical chromophores.

  4. Crystal Packing Directs C70 Polymerization

    When subjected to pressure, the fullerene C60 can polymerize through cycloaddition to form three-dimensional networks. Similar reactions for the next higher known fullerene, C70, have been elusive, and pressurization normally forms only C70 dimers. Soldatov et al. (p. 680), following a lead from theoretical modeling, show that the packing of the C70 crystal makes all of the difference. Unlike cubic packed crystals, hexagonally packed crystals contain a twofold screw axis that allows zig-zag polymer chains to form under pressure. The resulting structures were verified by x-ray diffraction and spectroscopy.

  5. When the Crust Took Shape

    Although the use of the uranium-lead system for dating may be more familiar, the radioactive decay of lutetium to hafnium (176Lu to 176Hf) has also been a critical tool for dating old crust. Scherer et al. (p. 683; see the Perspective by Kramers) have determined a more precise value for the decay constant of 176Lu to 176Hf through age comparisons. Their value agrees with recent decay counting experiments and is smaller than previous age comparison values. Their result pushes back the appearance of Earth's first crust from 4.1 billion to 4.3 billion years ago and indicates that differentiation occurred quickly after the formation of the planet.

  6. Volcanic Piston

    In June 2000, the volcano on Miyake Island, Japan, developed a relatively large caldera at its summit amidst a small flurry of minor eruptions. Caldera formation was synchronous with some very-long-period seismic signals that indicated the outflow of magma at depth. The signals indicated that the magma was flowing away from Miyake Island to the northwest toward Kozu Island. To explain this unusual combination of observations, Kumagai et al. (p. 687; see the Perspective by Scarpa) have developed a model of a “piston” of solid material within the conduit that pushes down into the shallow magma chamber and forces magma outflow at depth. Each downstroke of the piston increases the size of the caldera.

  7. Prides and Prejudice

    Recent models have shown the circumstances in which reproduction can be apportioned among female group members in social animals. Many empirical studies have focused on species in which reproduction is highly skewed. Packer et al. (p. 690; see news story by Pennisi) have examined the degree of reproductive skew within African lion prides using data collected during the past few decades, which tell a different story. They compared the observed variance in reproduction within prides with the variance produced by simulations that under or overestimated variance in reproduction. In most cases, reproductive output was partitioned fairly evenly among the females in a pride. These results provide a rare example of egalitarianism in an animal society.

  8. A Model for Control of Chagas Disease

    Chagas disease is a widespread, chronic, and ultimately fatal disease of the rural poor in Latin America that is caused by the parasite, Trypanosoma cruzi. This protozoan is transmitted by blood-sucking triatomine bugs. It has long been suggested that improving the quality of housing could control this disease. Cohen and Gürtler (p. 694) have now mathematically modeled the complex set of variables that contribute to disease transmission within a household. They offer simple recommendations for exclusion of pets and structural improvement of sleeping areas that would virtually eliminate Chagas disease in human beings.

  9. Getting to Grips with SNARES

    The SNARE proteins are involved in the promotion of faithful fusion events during membrane traffic. Tochio et al. (p. 698) examined the structure of one of the SNARE proteins, Ykt6p, from yeast using nuclear magnetic resonance spectroscopy. The amino-terminal structure did not resemble that of another well-characterized SNARE protein, syntaxin, but rather has characteristics in common with the actin regulatory protein profilin. Ykt6p may adopt different conformations that would affect its incorporation into productive fusion complexes.

  10. Death Without Entanglements

    Many human neurodegenerative diseases are characterized by abnormalities in the conformation and phosphorylation of the microtubule-binding protein tau, and it has been speculated that these abnormalities play a causal role in neuronal killing. Wittmann et al. (p. 711; see the 15 June news story by Ferber) have created a genetic model of these so-called “tauopathies” by expressing human wild-type and mutant tau in the nervous system of the fruit fly Drosophila melanogaster. The transgenic flies developed many features of the human tauopathies, including adult-onset progressive neurodegeneration and early death. Curiously, however, the dying neurons showed no signs of neurofibrillary tangles—the large filamentous aggregates of tau—that are a prominent feature of tauopathies in humans and rodent models. Because of its genetic accessibility, the fly model is expected to provide new insights into the cellular mechanisms that underlie tau-mediated neurotoxicity.

  11. Receptor for an Inflammatory Lipid

    An orphan G protein-coupled receptor called G2A has important immunoregulatory roles because its deletion in mice results in an autoimmune disorder similar to systemic lupus erythematosus. Kabarowski et al. (p. 702; see the Perspective by Carson and Lo) have now determined that a lipid present in cell membranes and serum called lysophosphatidylcholine (LPC) is the activating ligand of this receptor. LPC has previously been implicated in chronic inflammatory conditions such as systemic autoimmune diseases and atherosclerosis. The G2A-LPC interaction altered the migratory behavior of T cells.

  12. Destined for Destruction

    When cells are starved, they tend to cannibalize excess proteins. Kuroda et al. (p. 705; see the Perspective by Gottesman and Maurizi) examined this process in bacteria and discovered that inorganic polyphosphate appears to be a key factor in promoting the specific degradation of unassembled (excess) ribosomal subunits, presumably in order to recycle their constituent amino acids. They propose that the inorganic polyphosphate binds noncovalently to the ribosomal proteins and targets them for degradation by the adenosine triphosphate-dependent protease, Lon.

  13. How One Phosphate Changes Actin

    Monomeric actin binds adenosine triphosphate (ATP) and associates with itself to form actin polymers, the core component of the thin filaments that make up the cytoskeleton. Polymerization can take place in the absence of ATP hydrolysis, which occurs at a slow rate and appears to increase the propensity of monomers to dissociate from the filament (see the Perspective by De La Cruz and Pollard). How this works has been unclear because the available structures of actin (necessarily in complex with other proteins to prevent polymerization) have not revealed any differences between the ATP and adenosine diphosphate (ADP) states. Otterbein et al. (p. 708) present a structure of uncomplexed actin with ADP bound and thus describe the conformational changes triggered by nucleotide hydrolysis and phosphate release.

  14. High-Valued Dielectrics Unraveled

    The perovskite oxides are considered to be ideal replacement insulators to address the demand for the ever-shrinking size of microelectronic circuitry because of their large dielectric constant—often more than three orders of magnitude greater than that of silicon dioxide. One such material, CaCu3Ti4O12 (CCTO), which has a room-temperature dielectric constant of ∼105, is particularly attractive for applications because it also exhibits good thermal stability over a wide temperature range (100 to 600 K). However, an explanation for the unusual temperature dependence below 100 K, where the dielectric constant shows a 1000-fold reduction but without the accompanying structural phase transition found in other high dielectric constant materials, has remained elusive. Homes et al. (p. 673) now present optical conductivity data over a wide frequency and temperature range which indicate that microscopic mechanism for the behavior of this material is associated with the transition from covalent to a more ionic contribution of the bonding within TiO6 octahedra in the unit cell.

  15. The Y Chromosome and the Replacement Hypothesis

    Ke et al. (Reports, 11 May 2001, p. 1151), in an analysis of three Y chromosome markers characteristic of African origins in 12,127 males from 163 Asian populations, found no support for even a minimal contribution to the gene pool from previous Asian hominids—a result consistent with the “Out-of-Africa” or replacement hypothesis for the origin of modern humans. Hawks comments that the Ke et al. study relied too heavily on a single line of evidence, and that a “balanced view” that encompasses both other genetic data and archeological and fossil evidence argues against a simple replacement hypothesis “for East Asians, or indeed for any ancient human population.” Jin and Su respond that Y chromosome markers, although not the only genetic evidence, represent “the best choice so far,” and that historical and fossil evidence, valuable as a snapshot of events, “can hardly illuminate whether human beings living at that moment actually contributed to the extant populations.” The full text of these comments can be seen at