This Week in Science

Science  14 Sep 2007:
Vol. 317, Issue 5844, pp. 1465
  1. Switching Magnetism on the Spot


    In magnetic hard drives, information is typically written by application of a magnetic field, and readout is performed with a separate electrical probe. The bit densities that can be achieved this way are limited by stray magnetic fields that can affect nearby bits and possibly destroy information that was already stored. The use of spin-polarized current to locally control and read out the magnetization is expected to overcome such problems, but the underlying mechanisms involved in spin-polarized magnetization switching remain unclear. Krause et al. (p. 1537) show that spin-polarized current from a scanning tunneling microscope tip can be used to both manipulate and read out the magnetization in small islands of iron atoms. The magnetization switching in the island is dominated by a spin-torque effect exerted by the spin-polarized current, whereas the Oersted field (magnetic field arising from current flow) is small.

  2. A No-Win Solution for Checkers

    Computer scientists have traditionally used games such as chess as test cases for research in artificial intelligence. Less challenging games that have a small search space can be completely solved with computers by examining every possible set of moves from a given starting position. Chess has an immense search space that would require the fastest computers eons to solve, but other games provide tough but feasible challenges. Schaeffer et al. (p. 1518, published online 19 July; see the 20 July news story by Cho) report their solution of the game of checkers. If black moves first, and the opponents execute perfect play, the game ends in a draw. The analysis began in 1989 and required dozens of computers for a complete solution.

  3. Strings of Stars

    Gravity caused the first stars that formed in the early universe to collapse in overly dense regions. These regions were seeded by clumps of dark matter, particles that neither glow nor interact with light except gravitationally. Most modeling of the first stars has used “cold” dark matter, but it is possible that the dark matter was “warm” if it was made of more energetic fundamental particles. In computer simulations that include warm dark matter, Gao and Theuns (p. 1527, see the cover and the Perspective by Bromm) show that the faster motions of the warm dark matter erased very small density structures, and quite stable elongated gas clouds formed instead that fragmented to produce strings of stars. Thus, the pattern of the first stars may tell us about the dark matter content of the universe.

  4. Graphene Billiards


    With its distinctive band structure and mechanical stability, graphene (isolated sheets of graphite) has been predicted to exhibit a number of exotic transport properties. However, the transport of carriers around the Dirac point (where the electronic bands meet in momentum space) that gives rise to many of the predicted properties has remained controversial. Miao et al. (p. 1530) systematically studied transport properties around this region in device structures of various sizes in which the carrier densities could be varied. Carriers in the graphene have a large coherence length that causes its transport to depend on geometry. In effect, the wave functions of electrons and holes can interfere as they are scattered from the edges of the graphene sheet, which acts like a quantum coherent billiard.

  5. Pass the Sulfur, Please

    Carbon and sulfur isotopic signatures provide the main evidence for the identification of the earliest life on Earth. Details in the signatures of the several sulfur isotopes can now be used to track metabolism. It was previously suggested that a large fractionation in the 34S versus 32S isotopes implies that sulfate-reducing bacteria were present in rocks dated to about 3.5 billion years ago. Philippot et al. (p. 1534; see the Perspective by Thamdrup), making use of 33S data, show these rocks record the presence of organisms that metabolized and disproportionated elemental sulfur. Several such organisms are present near the base of the phylogenetic tree.

  6. Human Interactions

    Humans have continuously interacted with natural systems. Liu et al. (p. 1513) review the intricate nature of the organizational, spatial, and temporal couplings of human and natural systems. Case studies on different continents suggest that couplings have evolved from direct to more indirect interactions, from adjacent to more distant linkages, from local to global scales, and from simple to complex patterns and processes. An appreciation of such interactions should help in the development of effective policies for ecological and socioeconomic sustainability. Humans not only interact with nature but with one another in groups. Lim et al. (p. 1540) have adapted concepts of phase separation familiar in chemistry and physics to study patterns in global populations that can help predict and perhaps prevent conflicts. They posit that violence arises at boundaries between regions that are not sufficiently well defined. A model based on spatial distributions of ethnic groups gave good predictions about regions of violence in the former Yugoslavia and in India.

  7. Tailor-Made Toll-Like Receptor

    Laboratory-based immunology has revealed much about the role of innate immune receptors from insects to mammals, but to what extent do such receptors protect humans from infections? Zhang et al. (p. 1522) report a primary human immunodeficiency that points to a dedicated role for a Toll-like receptor (TLR) in protection from infection with a single specific virus, without any apparent influence on other pathogens. Herpes simplex virus (HSV) causes encephalitis in children carrying a mutant allele of TLR3, which normally regulates the antiviral interferon response to virus nucleic acid in the central nervous system and in dendritic cells of the immune system. Maintenance of TLR3 in the innate armory of humans may have been driven by viral infection. These results suggest that other similarly narrow host-pathogen interactions may have also co-evolved.

  8. Functional Evolution of Proteins

    The direct identification of protein evolution mechanisms requires comparing proteins through evolutionary time. The sequence of the 450-million-year-old ancestor of vertebrate mineralocorticoid (MR) and glucocorticoid (GR) was previously determined by phylogenetic analysis, and the ancestor was shown to have MR-like hormone specificity. Ortlund et al. (p. 1544, published online 16 August; see the 17 August news story by Service) used structural, functional, and phylogenetic analysis to determine how specific mutations resulted in a change from MR-like to GR hormone specificity. They find evidence for epistatic interactions where a substitution changed the conformation at another site. Substitutions that had no immediate functional effect, but affected stability to allow subsequent functional switching mutations, played an important role in GR evolution.

  9. An Airy Meal


    The fixation of atmospheric nitrogen into ammonia that is essential to human nutrition and global ecosystems is performed by free-living bacteria and by symbionts in plant root nodules. Lechene et al. (p. 1563; see the Perspective by Kuypers), using multi-isotope imaging mass spectrometry with the stable isotope of 15N, measured nitrogen fixation by symbiotic bacteria. They traced the utilization of fixed nitrogen, in this case by animal rather than by plant host cells.

  10. Doubling Up Antibody Specificity

    The light and heavy chains that make up antibodies both carry variable regions at their ends that combine to form the highly diverse antigen-binding sites of the antibody molecule. Immunological dogma states that a single B cell generates antibodies of one defined specificity (each molecule carries identical, symmetric heavy-light chain combinations), but one particular class of antibody known as immunoglobulin G4 (IgG4) has been suspected of breaking this rule. Van der Neut Kolfschoten et al. (p. 1554; see the Perspective by Burton and Wilson) now provide direct evidence that IgG4 can swap a heavy-light combination on one fragment antigen-binding arm for another, which creates antibodies with dual specificity. Furthermore, in a model of disease that relies on cross-linking by antibodies, the loss of single specificity (and the loss of the ability to cross-link) was effective at reducing disease.

  11. Regulatory Motifs in Making Muscle

    During metazoan development, multiple genes are co-expressed so that interacting gene products can be produced in the same place and time. Brown et al. (p. 1557) examined how genes that function together are coordinately expressed by dissecting cis-regulatory elements in 19 co-regulated Ciona genes that encode components of a muscle multiprotein complex. Assays defined the cis-regulatory elements through mutational analyses, and mutant-construct gene expression in muscle cells was quantified to estimate the activity of each regulatory motif. A comparison between the divergent species C. intestinalis and C. savignyi revealed that motif arrangements differ widely among co-regulated genes within a species but orthologous motifs are evolutionarily conserved.

  12. Membranes Under Attack

    Proteins containing membrane attack complex/perforin (MACPF) domains play an important role in mammalian immune defense by disrupting membranes of invading microorganisms or infected host cells. Two studies and now provide insight into the mechanism of MACPF function. Rosado et al. (p. 1548, published online 23 August 2007) determined the crystal structure of a bacterial MACPF protein at 2.0 angstrom resolution, and Hadders et al. (p. 1552) determined the structure of the MACPF domain of the human complement component C8α at 2.5 angstrom resolution. Surprisingly, both MACPF domains were structurally similar to pore-forming, cholesterol-dependent cytolysins (CDCs) from Gram-positive bacteria. The mechanism of pore formation by CDCs is known, and the structural results suggest that lytic MACPF proteins may use a similar mechanism to disrupt cell membranes.

  13. Diversity Needed Within and Among Species

    Two key elements of biodiversity are the genetic diversity within species and the diversity of species in ecological communities and ecosystems. These elements are usually studied separately, with little attention to possible interactions between the two levels of diversity. Lankau and Strauss (p. 1561) provide empirical evidence for feedback loops that allow the maintenance of diversity at both levels. Genetic variation in an allelochemical trait of Brassica nigra is necessary for the coexistence of this species and other competitor species. At the same time, genetic variation in B. nigra was only maintained when multiple competitor species were present in the community. Some B. nigra genotypes were strong competitors against other species, but poor competitors against their own species. An important message of these results is that the conservation of species diversity in natural habitats might also require conservation of the mechanisms that maintain genetic diversity within species.