This Week in Science

Science  09 Jan 2004:
Vol. 303, Issue 5655, pp. 141
  1. Going Out on Longer Carbon Dates

    Dating with radiocarbon (14C) is complicated by environmental processes that move carbon of varying ages between geochemical reservoirs like the ocean and the atmosphere, and thus perturb the carbon record in samples such as tree rings and sediments. Accurate radiocarbon dating requires a well-calibrated age scale. The current official radiocarbon curve, developed by INTCAL, now extends to 26,000 years before the present. Hughen et al. (p. 202; see the Perspective by Bard et al.) present a 14C calibration and activity record from the Cariaco Basin, off the coast of Venezuela, that extends to 50,000 years ago. These data, in conjunction with highly accurate (but sparse) data from corals, records from other sediments, and curves from speleothems (cave formations), should help produce a detailed and accurate 14C age calibration back to ∼50,000 years ago, the useful limit of the technique.

  2. Sized by Climate

    The differentiation of biotic versus abiotic control of macroevolution requires the assembly of morphological data on a large number of species over long time intervals in a well-characterized environment. Schmidt et al. (p. 207) present such data for foraminiferal assemblages of the past 70 million years which show that their dramatic size increase during the Neogene is correlated to latitudinal temperature gradients and surface water stratification. Foraminiferal macroevolution was dominated by climatic and paleoceanographic processes, rather than by expanding variance that would arise through increasing species richness.

  3. A Creepy Pulse

    One section of the San Andreas Fault in central California creeps along rather steadily at about 30 millimeters per year. Nadeau and McEvilly (p. 220) have identified quasi-periodic pulses of characteristic microearthquakes that extend quasi-continuously across tens of kilometers of the creeping section of the fault. The cumulative slip rates of the microevents are typically altered along some region where there has been a medium to large earthquake or at a transitional region from creeping to locked behavior on the fault. Nevertheless, the periodicity persists across the creeping section, which suggests that an unknown process, probably starting below the seismogenic zone (below 10 kilometers depth), is imposing an extensive and coherent time-varying load on the fault.

  4. Salmon Safety

    Annual production of farmed salmon has increased by a factor of 40 during the past two decades. Hites et al. (p. 226; see the news story by Stokstad) investigated levels of toxic contaminants for a large number of samples of both wild and farmed salmon. Farmed salmon contained contaminant burdens higher than in wild salmon, with regional variation in these burdens. Risk analysis showed that, in some cases, contaminant levels in farmed salmon were high enough to detract from the health benefits of eating fish.

  5. Calcium and the Developing Brain

    Calcium signaling communicates many effects of activity in the neural system. Short-term effects may be mediated by protein modifications, whereas long-term effects may be mediated by changes in gene transcription. Aizawa et al. (p. 197; see the Perspective by Jefferis et al.) have identified a calcium-responsive protein called CREST that is a transcriptional transactivator. Embryonic development was unaffected in mice lacking CREST, but these mice displayed severe defects in cortical and hippocampal dendrite development. Cortical neurons from mice lacking CREST showed deficiencies in dendritic growth in response to depolarization. Thus, the effects of electrophysiological activity on gene expression and subsequent neuronal development may be mediated by CREST.

  6. Consciously Trying to Forget

    Sometimes people may try to actively forget and consciously suppress unwanted memories. Repression of memories has been controversial because the mechanisms and the neural networks underlying suppression of unwanted memories are unclear. Anderson et al. (p. 232) used functional magnetic resonance imaging in a cued recall paradigm and discovered a novel form of reciprocal interaction between the prefrontal cortex and the hippocampus. Part of the activated network was similar to that recruited by motor inhibitory mechanisms. Inhibition at different levels of response output, motor responses in a reaction-time task, and the retrieval of specific memories may all share common properties.

  7. Bring Order to Nanostructures

    The usefulness of nanostructures is often limited by how well these structures can be organized. Mao et al. (p. 213) used viruses to create uniform nanowires from nanoparticles for a wide range of magnetic and semiconducting materials. The authors build on an approach in which peptide sequences in the M13 bacteriophage coat protein are screened for their ability to grow a particular type of nanoparticle. The long virus coat protein allowed the nanoparticles to grow in proximity, and a subsequent annealing step created the final nanowire crystallites. Corso et al. (p. 217) describe a surface film of BN grown on the close-packed (111) surface of rhodium whose nanostructure appears to be driven by lattice mismatching. On most metals, no more than a monolayer of BN forms through the thermal decomposition of borazine, (HBNH)3. However, on Rh(111), a bilayer forms as a mesh of 2-nanometer (nm) holes spaced 3 nm apart in a hexagonal lattice. The holes appear to relieve the large lattice mismatch of the substrate and the film and may serve as a robust template for confining molecules, as the authors show for C60.

  8. Mitochondrial Variation in Northern Migrations

    Understanding the phylogeny of human lineages has been helped by studying the variation in mitochondrial DNA. Ruiz-Pesini et al. (p. 223) have looked at the patterns of nucleotide substitution in more than 100 human mitochondrial DNA sequences from a globally diverse sample. The results provide evidence that variation at specific mitochondrial DNA sites associated with energy-deficiency disease may have permitted humans to adapt to harsher northern climates.

  9. Pro Bono Genomics

    The risk of developing osteoporosis is influenced by life-style factors as well as by genetically determined variations in bone mineral density (BMD). Combining conventional mouse genetics with microarray analysis, Klein et al. (p. 229) identified Alox15, encoding the enzyme 12/15-lipoxygenase, as a candidate gene affecting BMD. Mice with a targeted mutation in Alox15 had a higher peak BMD than did controls, and pharmacological inhibitors of 12/15-lipoxygenase increase BMD in two rodent models of osteoporosis.

  10. Upon Further Reflection

    The squid Euprymna scolopes carries its own light source, the luminescent bacterium Vibrio fischeri, within a crypt-filled cavity called the light organ. Stacks of silvery reflective plates are arranged around the lateral and dorsal sides of the cavity and direct the bacterially generated light downward. Unlike most biological reflector materials, which are composed of purine crystals, the major component of these stacks is a protein with an unusual amino acid composition that Crookes et al. (p. 235) have isolated, characterized, and named reflectin.

  11. Enzymes on the Fast Track

    Many studies of enzyme catalysis, both experimental and theoretical, have generated a cornucopia of claims for how rate acceleration is actually realized. In their Review, Garcia-Viloca et al. (p. 186) describe modern transition-state theory and use this formulation to explain how these varied proposals can be accommodated either as components of the exponential factor (pertaining primarily to the free energy of activation) or the pre-exponential factor (encompassing tunneling, recrossing, and nonequilibrium distributions).

  12. Resolving Holliday Junctions

    Homologous recombination between sister DNA molecules plays a vital role in the repair of potentially dangerous double-strand DNA breaks, which left unattended could result in genome instability and tumor formation. Although much is known in eukaryotes about the components that mediate the early steps of the repair process, which result in the formation of a DNA crossover or Holliday junction complex, little is known about the proteins involved in the resolution of the Holliday junction. Liu et al. (p. 243; see the Perspective by Symington and Holloman) extend previous work characterizing a resolvase activity in cell extracts. Paralogs of the recombination and repair protein RAD51, and particularly RAD51C and XRCC3, were important for the disassembly of the Holliday junction complex and the liberation of the repaired DNA strands.

  13. Account for the Ocean's Mix

    Large-scale ocean circulation has a downwelling component of denser water, formed locally at just a few sites in the high latitudes of the Northern and Southern Hemispheres, that sinks to the bottom. A compensating flux of less dense water upwells more diffusely over a wider and less well-determined area of the rest of the ocean. A detailed understanding of how and where the deepest waters are mixed with other water masses has been lacking. Naveira Garabato et al. (p. 210) present observations from the Southern Ocean which show that enhanced turbulent mixing over rough topography is more intense and widespread there than in the lower latitude oceans. This study helps address how water in the deep ocean returns to the surface and sheds light on ways in which nutrients may be delivered to surface waters.

  14. Regulation of Early Imprinting

    Genes from parents are not always expressed equally. In some cases, either the mother's or father's copy is consistently silenced, even when their DNA sequences are identical. How this process, known as imprinting, arises in the germ cells is not fully understood. Fedoriw et al. (p. 238) used transgenic RNA interference in mouse oocytes to knock down a known imprinting regulator protein, CTCF. The region of DNA normally bound by CTCF and hypomethylated in oocytes was methylated in knockdowns. This process led to defects in pre-implantation development and suggests that CTCF not only responds to the germline methylation status of imprinted genes, but determines it as well.

  15. Transposons and Retroelements

    A puzzling feature of yeast Ty1 transposons is that mutations in an RNA debranching enzyme, which cleaves intron RNA lariats at their branch point after the completion of splicing, inhibits both Ty1 transposition and cDNA formation. Yet the Ty1 element contains no known introns. Cheng et al. (p. 240; see the Perspective by Perlman and Boeke) now provide evidence that, even given their lack of introns, Ty1 transcripts contain a 2'-5' branch characteristic of an RNA lariat, indicating an entirely unanticipated step in the life cycle of the transposon. The location of the branch suggests that it may play a role during the formation of Ty1 cDNA by facilitating the transfer of nascent minus-strand Ty1 cDNA from the upstream region of the Ty1 RNA template to the downstream region. The similarity of Ty1 to animal retroviruses further suggests that the branch may be widely conserved among retroelements.