This Week in Science

Science  15 Mar 2002:
Vol. 295, Issue 5562, pp. 1969
  1. In Brevia

    Analysis of the fungi that inhabit the roots of a grass by Vandenkoornhuyse et al. (p. 2051) reveal an extraordinary diversity of species, from every major fungal group, as well as yet unidentified species.

  2. Radiation from Asia

    The diversity and range of mammals increased greatly after the Paleocene/Eocene boundary (about 55 million years ago), and new groups appeared on continents throughout the Northern Hemisphere. On the basis of primarily phylogenetic analyses, Asia has been suggested as a likely center of origin. Bowen et al. (p. 2062; see the Perspective by Beard) now provide paleontologic evidence pointing to an Asian origin for a major mammal family, the Hyaenodontidae, which includes a number of large extinct carnivores, and implying an Asian origin for other major mammal groups, including the primates.

  3. High-Pressure Photopolymerization

    1,3-Butadiene is highly unsaturated and very reactive. It readily dimerizes at ambient conditions, and polymerization produces a mix of cis and trans isomers. Citroni et al. (p. 2058) show that in a diamond-anvil cell under high-pressure conditions (above 0.7 gigapascals), the constrained molecular environment limits the reaction outcomes of trans-butadiene. Under dark conditions, only the cycloaddition product, vinylcyclohexene forms, and when irradiated with 488-nanometer light, pure trans-polybutadiene forms. The authors interpret the polymerization results in terms of an extended lifetime of the S1 excited state.

  4. Catalytic Metals and Their Supporting Casts

    Recently, a transmission electron microscope has been developed that allows high-resolution analysis of catalytic metal nanoparticles dispersed on oxide supports in the presence of reactive gases and at high temperatures. After some further modification, Hansen et al. (p. 2053) now achieve 0.14-nanometer resolution, which allows them to study nanoparticles of metals with lower atomic numbers, such as iron, nickel, and copper, with atomic resolution. The authors show that nanoparticle dynamics and their interactions with the supporting material influence the performance of catalytic systems such as the industrial methanol synthesis catalyst.

  5. Life Below the Sea Floor

    Live bacterial cells appear to inhabit sediments as far as 1 kilometer beneath the sea surface. These communities may account for as much as one-third of Earth's total biomass, but the biological activity of these cells is extremely low, and the bulk of any metabolic activity always takes place within the uppermost centimeters of the sediment. D'Hondt et al. (p. 2067) review the data on these sediments gathered from around the world during the past 15 years of the Ocean Drilling Program and found striking differences between two distinct realms of sediments. Although methanogenesis occurs in both, the methane is destroyed oxidatively when coupled to sulfate reduction. Hence, the open-ocean sediments are rich in sulfate and relatively methane-depleted, whereas the more strongly methanogenic, ocean-margin sediments are relatively sulfate-depleted.

  6. Rates for Attaining Fit States

    Hexavalent chromium, such as the chromate anion, can be toxic in high doses and resides in many contaminated groundwater systems (as made famous by the movie Erin Brockovich). One source has been leakage or waste from large baths or ponds of chromium used to electroplate metals. Reduction to Cr3+ reduces its mobility and hazard. Ellis et al. (p. 2060; see the Perspective by Blowes) survey the distribution of stable isotopes of chromium in groundwater and the Earth and show that the isotopic ratio in ground water can be used to assess rapidly the rate of ongoing reduction.

  7. Life-Styles and Variability

    Why do different animal species vary so greatly in population variability? Sæther et al. (p. 2070; see the Perspective by Coulson et al.) identify two distinct demographic processes operating in bird populations and report a strong covariation between demographic processes and population variability. Recruitment-driven populations (in which there is a high reproductive rate) were characterized by weak density regulation and a large environmental stochasticity that resulted in high population variability. In survival-restricted populations (in which the reproductive rate is typically low and survival long), strong density regulation at high densities, and small estimates of the environmental variance were found, leading to small fluctuations in population size.

  8. Caging the Histone Code

    Chromatin plays an important role in the regulation of gene expression. Covalent modification of the tails of the histone proteins, which make up the nucleosome around which DNA is wrapped, can alter the response of a gene to the transcription machinery. For example, methylation of Lys9 in the tail of histone H3 results in epigenetic repression of gene expression. The chromodomain-containing protein HP1 binds to methylated Lys9, but it has not been clear why only a subset of chromodomain-containing proteins interact with chromatin. Jacobs and Khorasanizadeh (p. 2080; see the cover) have determined the structure of the HP1 chromodomain bound to a methylated H3 tail. The methylammonium group is recognized by a hydrophobic cage in HP1, and the H3 tail completes the β-sandwich architecture of the chromodomain.

  9. Protein Degradation Maintains Plant Defenses

    The innate immune response by which plants defend against certain pathogens includes interactions between a signal from the pathogen and a specific resistance (R) gene in the plant. Although there are a variety of R genes, and a veritable multitude of pathogens, the plant tends to respond with a limited repertoire of physiological responses. Azevedo et al. (p. 2073) and Austin et al. (p. 2077) analyze the signaling cascade initiated by activation of disease resistance (see the Perspective by Nishimura and Somerville). Several R proteins trigger responses through the RAR1 protein, which in turn interacts with the SGT1 proteins. These results implicate SGT1 in ubiquitin-targeted protein degradation. The two SGT1 variants in Arabidopsis seem to have divergent functions. Thus, at least part of the signaling cascade triggered by response to a pathogen may involve protein degradation.

  10. Cutting and Pasting RNA

    Although macromolecular complexes that decode messenger RNA (mRNA) and synthesize pre-mRNA have been resolved, one of the most enzymatically challenging reactions, the splicing of pre-mRNAs, has been difficult to assess structurally. Fortunately, the group II self-splicing introns follow a catalytic pathway similar to that of pre-mRNA splicing, and these introns display sequence similarities to the RNA components of the macromolecular spliceosome complex. Zhang and Doudna (p. 2084) describe the crystal structure of the catalytic core of the group II intron and find that the critical nucleophilic adenosine is part of an unpaired, two-nucleotide bulge, extruded from the RNA double-helical scaffold.

  11. Little Fleas Have Lesser Fleas

    Bordetella spp. cause significant human respiratory disease, and to accommodate the changing environments they encounter, they cycle through a series of phenotypic changes, including changing the nature of the surface molecules that also act as receptors for the tails of bacteriophages that prey on these bacteria. Liu et al. (p. 2091; see the Perspective by Hatfull) have discovered a family of phage that have a marked tropism for a phase in the Bordetella life cycle when several virulence and colonization factors are produced; here, pertactin is the primary virus receptor. However, they also found phage that have tropism for a motile phase of the bacterium's life cycle, as well as various intermediate forms. Thus, the phage can generate variants that can infect the bacterium at different stages in its life cycle by means of a reverse transcriptase-driven mechanism that causes mutations in a tail assembly protein.

  12. Natural Killer Cells Overcome Rejection

    Reconstitution of the hematopoietic system by bone marrow (BM) transplantation operates on a knife edge. Engraftment can be improved by allowing some degree of mismatch between the tissue antigens of the donor and recipient. In diseases such as leukemia, engraftment can also provide a powerful means for helping to destroy host-derived cells. However, the greater the mismatch, the more vigorously residual T cells in the donor BM will react against the recipient tissues, leading to graft-versus-host disease (GVHD). Two reports address how natural killer (NK) cells help prevent rejection (see the Perspective by Kärre). Ruggeri et al. (p. 2097) observed that for NK cells, a greater level of NK cell reactivity against host antigens correlated with a higher incidence of successful transplantation. In mice, preconditioning with purified host-reactive NK cells removed the need for usual preablation of recipient bone marrow by irradiation, and these animals did not develop T cell-mediated GVHD seen in control mice. Host-reactive NK cells may ablate antigen-presenting cells in the host and, in so doing, prevent them from inducing antihost T cells. Activation of NK cells can also be prevented by ligands that bind inhibitory receptors on the NK cell surface. Wang et al. (p. 2094) observed that in mice lacking the phosphatase SH2-containing inositol phosphatase (SHIP), the repertoire of inhibitory NK receptors was skewed in favor of select receptors that could recognize foreign as well as self ligands. When transplanted with bone marrow, these mice failed to reject bone marrow from a mismatched donor. Incidence of GVHD was also absent in these mice, suggesting a possible role for host-derived NK cells in GVHD.

  13. At the Hop

    Most electronic excitation processes of molecules at surfaces that result in molecular movement either lead to desorption or uncontrolled surface diffusion. Komeda et al. (p. 2055) show that the scanning tunneling microscope (STM) can be used to cause migration of CO molecules on the (110) surface of palladium, which exhibits a row-and-trough morphology. Tunneling electrons from the STM tip cause CO molecules to hop laterally along the rows. However, on the Cu(110) surface, no such hopping is seen, despite a lower barrier for this process. Modeling shows that differences in anharmonic coupling between the C-O stretching mode and the lateral hopping mode account for the difference in hopping probability.

  14. Blood and Iron Fractionation

    Recent improvements in mass spectrometry have opened up the study of several heavy stable isotopes, including those of iron. Most of the application has been in bacteria or mineral deposits. Walczyk and von Blanckenburg (p. 2065) investigated the iron isotope signatures in humans, where iron is involved in many biological processes and is an important dietary supplement. They find that there is a large fractionation between diet and blood samples (the largest depletion of iron found in any sample, biological or geologic), and that women show a larger depletion than men that likely reflects greater dietary absorption of iron through the intestines by women.

  15. Akt-ing Against a Loss

    Mutations in the tumor suppressor protein PTEN occur in a variety of cancers. Its major enzymatic activity is to dephosphorylate phosphoinosides, including phosphoinositide-3,4,5-trisphosphate (PIP3). In the absence of PTEN, cellular levels of PIP3 increase, which result in overgrowth phenotypes and lethality in Drosophila larvae. However, PIP3 binds to numerous signaling molecules, and so it is not clear if there is a specific interaction that loss of PTEN function primarily affects. Stocker et al. (p. 2088) show that flies can live in the absence of PTEN if the interaction of PIP3 with the serine-threonine kinase Akt is decreased through a mutation in its PH domain. At least in the fly, Akt appears to be the principal target of PIP3.

  16. Superconductivity in a Spin-Ladder Cuprate

    Schön et al. (Reports, 28 September 2001, p. 2430) reported that, through field-effect doping of molecular beam epitaxy (MBE)-grown thin films, they had induced superconductivity in the nominally insulating spin-ladder compound [CaCu2O3]4. In a comment, Ingle et al. argue that the work referenced by Schön et al. provides “no unequivocal evidence of a spin-ladder arrangement of the Cu and O atoms in the film studied,” and conclude that “without such evidence, any claim for superconductivity due to doping of a spin ladder is premature.” Schön et al. respond that additional work has confirmed a very close fit between the ladder-plane unit cell parameters in their thin films and those in a variety of two-leg ladder cuprates reported elsewhere in the literature. They also review other lines of evidence that lead them to conclude that they “have observed clear evidence of a spin-ladder arrangement.” The full text of these comments can be seen at