This Week in Science

Science  17 May 2002:
Vol. 296, Issue 5571, pp. 1197
  1. In Brevia

    Using the power of the Very Large Telescope and a new technique, Della Valle and Gilmozzi. (p. 1275) have found four of the most distant novae, which can be used as “standard candles” to measure the shape and expansion of the universe.

  2. Triassic-Jurassic Impact?

    The cause of the extinction of many species at the Triassic-Jurassic boundary is unknown. Olsen et al. (p. 1305; see the news story by Kerr) found an enrichment of iridium at the boundary that may be associated with an extraterrestrial bolide impact. An impact may have wiped out many species and left room for the dinosaurs to grow and dominate in the Jurassic.

  3. Six Contentions on Separation

    Social structures are organized such that, for example, one can trace a path through local networks from one individual to another in relatively few steps (the so-called six degrees of separation). Watts et al. (p. 1302) have modeled this finding based on six insights into the structure of social networks formed by groups of individuals. This model may prove useful in developing efficient network searches, whether for people or for pages on the World Wide Web.

  4. Modeling a Heterogeneous Mantle

    Seismic velocities increase abruptly at two depths in the mantle, 410 and 660 kilometers (km). These discontinuities are usually attributed to structural transformations of olivine to ringwoodite at 410 km and ringwoodite to perovskite plus magnesiowustite at 660 km. However, not all seismic data are consistent with this interpretation, and two reports detail further studies of mantle structure. Lebedev et al. (p. 1300) show that the seismic velocities determined beneath southeast Asia and Australia are definitely correlated with the olivine transformations. In particular, the temperature derivative of the change in pressure of each transformation can be extracted from the seismic data. These results should allow Earth scientists to distinguish anomalous mantle dynamics that may be regional in scale from these global structural transformations. Trampert and van Heijst (p. 1297) have completed a detailed survey of the variations in sound velocities of shear waves that propagate in different directions (called seismic anisotropy) in the transition zone. They find lateral variations in the anisotropy that they relate to heterogeneous structures, such as alignment of grains or alignment of melt pockets.

  5. Shooting Atom Clouds

    Solitons, waves that propagate without attenuation or broadening, are usually associated with a nonlinear effect that provides the necessary feedback mechanism that maintains the wave disturbance in its original shape. Khaykovich et al. (p. 1290) now show that bright solitons can be formed in clouds of atoms in a Bose-Einstein condensate by tuning the interaction between atoms from repulsive to attractive prior to launching the atom bunch into a wave guide. Such a matter-wave packet that does not spread with time should prove useful for atom interferometry and studying atomic collisions.

  6. How σ Factor Finds DNA

    A major control point of gene expression is the initiation of messenger RNA synthesis. In bacteria, promoter recognition and melting requires a complex that comprises RNA polymerase and the σ factor. Now Murakami et al. (p. 1280 and 1285) have determined the crystal structures of Thermus aquaticus core RNA polymerase bound to σ and of the holoenzyme complex with a promoter DNA fragment at resolutions of 4.0 and 6.5 angstroms, respectively. The structures give insight into how the holoenzyme recognizes promoters and melts the DNA to give a transcription competent open complex. All of the sequence-specific contacts with the promoter are mediated by the σ subunit, and σ also plays a role in directing the melted DNA template strand into the RNA polymerase active site. The structures provide a basis to design additional experiments that will further improve our understanding of transcription initiation.

  7. DNA Damage and Aging

    A popular theory postulates that aging is triggered by cellular accumulation of oxidative damage. The causal role of oxidative DNA damage in aging is strongly supported in a study by de Boer et al. (p. 1276; see the Perspective by Hasty and Vijg) of mice carrying a mutant version of the DNA helicase gene XPD. This gene causes the rare human disorder trichothiodystrophy (TTD). The XPD mutant mice, which are severely impaired in transcription-coupled repair and mildly impaired in nucleotide-excision repair, exhibited many symptoms of premature aging, including osteoporosis, infertility, early graying, and reduced life-span. Mice doubly mutant for XPD and a second gene required for nucleotide-excision repair, XPA, showed greatly accelerated aging that correlated with an increased cellular sensitivity to oxidative DNA damage. The authors propose that the aging phenotype in the TTD mice is caused by unrepaired DNA damage that compromises transcription, which in turn leads to functional inactivation of critical genes and cell death.

  8. Charged Nanocrystals Light Up in Solution

    Semiconductor nanocrystals can accept a charge carrier (an electron or hole). Compound semiconductor nanocrystals such as CdS, when charged in solution at electrodes, tend to be chemically unstable. Ding et al. (p. 1293) show that organically capped silicon nanocrystals are stable after electron and hole injection in organic solvents and that oppositely charged nanocrystals can recombine in solution and emit light. Greater light emission was obtained by adding coreactants for the recombination reaction, such as oxalate or persulfate anions.

  9. Narrowing in on Jaw Development

    The evolution of vertebrate jaws allowed their diversification into a wide range of feeding niches. Shigetani et al. (p. 1316) examined the expression patterns of genes controlling the developing head of the lamprey, a jawless vertebrate. These genes are expressed in regions producing neural crest with a distinct fate from that seen in the developing heads of higher vertebrates such as chicks and mice. Although the correlation between fate and expression correlation has shifted, the relation between genes in the lamprey is the same as in higher vertebrates. A cascade of signals that is present through a broad area of the neural crest and epithelium of the lamprey has been restricted to the mandibular region in vertebrates.

  10. How to Stick Together

    Cell-cell adhesion relies upon the direct interaction of integral membrane proteins. One of the primary components is a family of molecules known as cadherins, which contain a string of five highly similar domains disposed on the outside of the cell membrane. Boggon et al. (p. 1308) describe the crystal structure of this extracellular portion of C-cadherin and suggest how interactions between C-cadherin molecules on opposing cells and on the same cell serve to create a meshwork that ties the cells together.

  11. Vitamin D Versus Fat

    Mechanistic insight into the epidemiologic relation between colon cancer, vitamin D, and a high-fat diet is now offered by Makishima et al. (p. 1313), who report that a receptor for vitamin D in the intestine also binds a bile acid called lithocholic acid (LCA). Increased amounts LCA are associated with a high-fat diet, and because it is poorly reabsorbed, it passes into the colon. Binding to the vitamin D receptor stimulates the expression of CYP3A, an enzyme that can catabolize LCA. These results may explain how the enteric system protects itself from the toxic effects of LCA and how vitamin D may guard against colon cancer.

  12. RNA Interference Versus Viruses

    Posttranscriptional gene silencing (PTGS) in plants, a form of RNA silencing wherein RNA produced by a gene is destroyed, protects them from invading nucleic acids, mainly viruses and transposons. The biological role of the highly related phenomenon of RNA interference (RNAi) in animals has been less clear. Li et al. (p. 1319) now show that RNAi can be induced by viral infection of insect cells and plays a similar protective role. The virus contains a protein that is able to suppress RNAi, indicating that the virus is in an evolutionary “arms race” with its host.

  13. About Faces in the Brain

    Babies can discriminate among foreign speech sounds at 4 to 6 months of age, but this ability disappears at about 10 to 12 months and is limited to their native languages. Pascalis et al. (p. 1321) report a similar finding in visual processing, in that 6-month-old babies can discriminate individual monkey and human faces. However, 9-month-olds, tested with the same procedure, only discriminate human faces.

  14. Stop-Start Action in NK Cell Activation

    The natural killer (NK) cell receptor LY49H was recently identified as being responsible for rendering certain strains of mice resistant to mouse cytomegalovirus (MCMV). Infection with the virus should alter host proteins that interact with LY49H and lead to NK cell activation. Unexpectedly, Arase et al. (p. 1323; see the Perspective by Vivier and Biron) found that the ligand for LY49H is a type I-anchored glycoprotein encoded by the virus itself. In mice that are resistant to MCMV, the protein m157 activated LY49H+ NK cells, but m157 bound an inhibitory LY49 NK receptor in a susceptible strain of mice. NK cell inhibition through direct interaction between a viral protein and host receptor may have evolved as a viral evasion strategy, which was followed by the evolution of an activatory protein.

  15. Spin Polarization of Injected Electrons

    LaBella et al. (Reports, 25 May 2001, p. 1518) reported that “a large spin-polarized (∼92%) current can be injected into GaAs [gallium arsenide] at high temperatures (100 K),” using a “100% spin-polarized” scanning tunneling microscope (STM) tip as the electron source. Egelhoff et al. comment that, in relating the measured polarization in the light emitted from the GaAs to the spin polarization of the injected electrons, LaBella et al. neglected to corrrect for refraction as the light left the sample; suggest that the group did not measure the spin-relaxation lifetime value of the sample used, which bears on the relationship between the electron spin polarization at recombination and the injected-current polarization; and argue that it is unlikely that the electrons emitted from the STM were indeed 100% spin polarized. Thus, Egelhoff et al. conclude, “an electron spin polarization at recombination of 25.2% can be inferred from these experiments,” not the 92% cited in the original report. LaBella et al., in their response, agree that their analysis did not adequately account for refraction and acknowledge that they did not measure the spin-relaxation lifetime for the sample studied, although they used a published value for GaAs of a similar hole density and temperature.

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