This Week in Science

Science  26 Nov 2004:
Vol. 306, Issue 5701, pp. 1433
  1. Dendrimer Templates


    Organic dendrimers consist of a central core structure, surrounded by successive branches or arms, that sprout outward much like the branches on a tree. Landskron and Ozin (p. 1529) have functionalized the ends of dendrimers with siloxy groups and templated them with organic surfactants. The dendrimers organize to form a hierarchical structure with well-defined microporous channel walls and ordered mesoporous channels.

  2. Romancing the Shaken Stone

    The surface of asteroid 433 Eros is heavily cratered, covered with loose regolith and ubiquitous boulders. The regolith shows evidence for sliding down slopes and ponding in small valleys, and has evidently obscured small craters (diameters less than 100 meters), even though the asteroid has minimal gravity. Richardson et al. (p. 1526; see the Perspective by Asphaug) show that the regolith movements are caused by seismic reverberations after impact events. Their model of this process finds that the number of observed and buried craters on Eros is consistent with the modeled impactor population in the main asteroid belt where Eros resides.

  3. Imaging Hydrogen in Diamond

    The thermal, mechanical, and electronic properties of diamond make it a desirable material to use in high-power electronics. However, the preparation techniques for synthetic and thin-film diamond that produce material of sufficient quality unavoidably introduce hydrogen into the structure. Reichart et al. (p. 1537) introduce a technique based on proton-proton scattering that allows the hydrogen in the diamond to be imaged. A knowledge of where the hydrogen resides and in what amounts should help in optimizing deposition and synthetic processes.

  4. Synthetic Motors That Reverse

    Biological motors can display reversible motion, such as the F1F0-adenosine triphosphatase motor. A chemically synthesized rotary motor that displays reversible unidirectional motion is reported by Hernández et al. (p. 1532), in which a smaller ring moves between positions defined along a larger ring. The stepwise addition of reagents destabilizes noncovalent bonding at one site on the larger ring, which allows the small ring to move but only after deprotection and reprotection steps allow it to reach a more favorable recognition site. The small ring can be returned back to its starting position with a similar sequence of reagents. The authors note that unlike random motion between the sites, chemical energy must be expended for the motion to be deterministic. CREDIT: HERNANDEZ ET AL.

  5. Eruption Precursors: This Wave or That

    Seismic anisotropy, in which a shear wave can be split into fast and slow moving modes by oriented minerals or structures such as aligned cracks, may be useful for determining the state of stress beneath a volcano. Gerst and Savage (p. 1543) found that the anisotropy beneath Ruapehu volcano, New Zealand, changed because of the pressurization and depressurization of the magma system when magma was erupted and new magma filled the evacuated conduits.

  6. Positive Epistasis in HIV-1 Evolution

    What is the evolutionary benefit of recombination and sexual reproduction? One class of theories suggests that recombination has been favored by selection because of its influence on epistatic interactions, whereby a gene at one locus influences the expression of a gene at another. Retroviruses such as human immunodeficiency virus-type 1 (HIV-1) offer the opportunity to test such theories because they exhibit rates of recombination sufficiently large to provide, statistically significant sample sizes. Bonhoeffer et al. (p. 1547; see the Perspective by Michalakis and Roze) analyzed a data set of nearly 10,000 HIV-1 sequences with precise fitness estimates, based on an assay that measures the total production of progeny virus after a single full round of replication. They find evidence for positive epistasis, which calls into question theories that are based on negative epistasis. In addition, it appears that recombination slows down, rather than accelerates, the evolution of drug resistance in HIV-1.

  7. A Bacterial Nose for NO

    Nitric oxide is an important signaling molecule in mammals, where it acts in part when sensed by a heme protein, soluble guanylate cyclase. Nioche et al. (p. 1550, published online 7 October 2004) searched for ancestral proteins with related NO-binding heme domains in the bacterium Clostridium botulinum. NO is toxic to C. botulinum, and the bacterium actively moves away from nitrite-preserved meat. The authors identified a bacterial protein with an extreme (femtomolar) binding affinity for NO, and elucidated the crystal structure of a related molecule from Thermoanaerobacter tengcongensis. NO-binding domains thus provide prokaryotes with a highly sensitive sensor for NO.

  8. Evolution Through Compensation

    Comparisons between the previously sequenced genomes of the fruit fly, Drosophila melanogaster, and its relative, D. pseudoobscura, have allowed Kulathinal et al. (p. 1553, published online 21 October 2004) to explore the landscape of protein evolution. Amino acid replacements that are harmful in D. melanogaster were often observed as the wild type in D. pseudoobscura. Similar results were seen with the more distantly related mosquito, Anopheles gambiae. Thus, compensating mutations must occur and become fixed very frequently in populations.

  9. Integrating Gene Interaction Data

    Genes can interact in many more ways than through direct protein-protein associations. Lee et al. (p. 1555) have developed a unified scoring scheme that enables integration of different kinds of data weighted according to the data quality. An integrated network of Saccharomyces cerevisiae genes was built from co-expression, phylogenetic, gene-fusion, as well as physical and genetic interaction data sets. The addition of different kinds of data resulted in greater certainty that the linkages made were correct and made it easier to predict gene function.

  10. Jnking Atherosclerosis


    Atherosclerosis is the most common cardiovascular disease in Europe and North America. The c-jun-NH2-terminal kinase (Jnk) family is implicated in atherogenesis. Ricci et al. (p. 1558) addressed the function of JNK in atherogenesis, using atherosclerosis-prone apolipoprotein E (ApoE)-deficient mice simultaneously lacking either Jnk1 or Jnk2. Jnk2 deletion strikingly reduced plaque formation in ApoE deficient mice. However, deletion of Jnk1 revealed only a slight effect on atheroma formation. Pharmacological inhibition of overall Jnk activity substantially suppressed atherosclerosis in ApoE-deficient mice. Specific inhibition of JNK2 activity may thus represent a therapeutic approach to ameliorate atherosclerosis.

  11. Bone Marrow Contribution to Gastric Cancers?

    Although the cellular origin of epithelial cancers, such as gastric cancer induced by Helicobacter pylori infection, remains to be established, a prevailing assumption is that they derive from resident epithelial stem cells. In contrast to this theory, Houghton et al. (p. 1568; see the news story by Marx) find that gastric cancers caused by experimental Helicobacter infection in mice were of bone marrow, rather than epithelial cell, origin. Bone marrow-derived cells from donor mice were tracked in chronically infected recipients and predominated in the gastric mucosa where they displayed features of neoplastic progression, eventually forming epithelial cancers. If an equivalent contribution of bone marrow-derived cells to epithelial cancers could be established in humans, this finding would significantly revise our understanding of the origin and progression of malignancy.

  12. Compact DNA and Gene Regulation

    The DNA of all eukaryotes is compacted into chromatin, the primary unit of which is the nucleosome. Although the structure of the nucleosome core bound to DNA is known to atomic resolution, the higher order, compacted structures of chromatin, and the role of this compaction in regulating gene expression, are less clear (see the Perspective by Mohd-Sarip and Verrijzer). Dorigo et al. (p. 1571) analyzed the first level of higher order chromatin organization, the 30-nanometer fiber, using in vitro reconstituted nucleosome arrays cross-linked for stability. Unlike the classical solenoid model for the 30-nanometer fiber, which forms a “one-start helix,” the fibers assume a “two-start helix” of nucleosomes. The Polycomb Group (PcG) genes are critical for metazoan development and maintenance of developmental patterning. It has been suggested that PcG proteins repress genes by nucleating a condensed chromatin structure. Francis et al. (p. 1574) now confirm the compaction of a nucleosomal array by the addition of PcG proteins to chromatin.

  13. Bison Prehistory Revealed

    In an international collaboration of more than 15 museums, Shapiro et al. (p. 1561; see the news story by Pennisi) used mitochondrial DNA sequences from more than 350 late Pleistocene and Holocene bison bones to record evolutionary processes in real-time throughout the late Pleistocene. The genetic diversity of Beringian bison populations underwent a catastrophic decline immediately before the Last Glacial Maximum, well ahead of the arrival of humans in the New World. Old World steppe bison are all descended from a re-invasion from the New World around 90,000 to 120,000 years ago, and New World bison are descended from a small population of bison isolated to the south of glacial ice barriers.

  14. Cicadas Feed the Forests

    Cicadas spend most of their lives as subterranean larvae, emerging synchronously as adults to sing and mate for a few short weeks. This life cycle is taken to extreme in the North American Magicicada spp, which emerge every 17 years, most recently in the summer of 2004. Yang (p. 1565; see the Perspective by Ostfeld and Keesing) reports that that the decomposition of millions of dead cicada bodies during the emergence year represents a significant resource pulse for forests. Additions of cicada carcasses to experimental forest plots led to significant increases in bacteria, fungi, and soil nitrogen in forest soils, and may explain why trees show significant growth increments in the months following a cicada outbreak. Forest dynamics on a broad landscape scale may thus be influenced by the life history of a single herbivore species.

  15. Characterization of Nanoscale Objects

    Rayleigh scattering with a white-light supercontinuum source has now been used to characterize nanoscale objects. Sfeir et al. (p. 1540, published online 28 October 2004) show that the output signal in their experiments does not necessarily require particular sample properties, such as the ability to luminesce, and thus represents a general characterization technique. For example, carbon nanotubes are usually synthesized as complex mixtures, but the supercontinuum white-light scattering technique allows all of the samples to be probed in parallel. An individual signature of the nanotubes can then be probed further with other complementary and individual nanotube characterization tools.