This Week in Science

Science  06 Jun 2003:
Vol. 300, Issue 5625, pp. 1472
  1. Separate and Entangled

    Superconducting quantum bits (qubits) are currently being developed as promising candidates for implementations in quantum computers. Although the quality of single qubits has been improving with coherence times sufficiently long to allow manipulation of the states of the qubit, the ability to couple, or entangle, qubits together will be required for quantum computing to be realized. Taking initial steps along this route, Berkley et al. (p. 1548) entangled two macroscopic quantum systems, current-biased Josephson junctions, that were separated by 0.7 millimeters but coupled via capacitors. Spectroscopic studies show that the electronic energy levels correspond to those expected for entanglement of the two qubits.

  2. Tracking Carbon Sinks

    The sequestering of atmospheric CO2 by ecosystems is the focus of two studies. European terrestrial ecosystems may comprise a net carbon sink analogous to that seen for North America. Net uptake has been inferred from inverse atmospheric models that are based on CO2 fluxes. Janssens et al. (p. 1538) compare estimates derived from both atmospheric reconstructions and ecosystem inventory approaches to quantify the carbon balance of Europe's four dominant ecosystem types: forests, grasslands, croplands, and peatlands. They find that European terrestrial ecosystems sequester only 7 to 13% of European fossil emissions and that this is only about one-third that of the previous estimates. Although the uncertainties within both estimates remain large, the differences are likely the result of how non-CO2 compounds such as methane are treated. The effect of climate on plant growth in general has been assessed by Nemani et al. (p. 1560), who constructed a global record for the period from 1982 to 1999. They compared the data with relevant information on precipitation and temperature to infer where major changes have occurred. Most of the increase in plant growth that they documented occurred in the tropics, particularly in South America, where decreases in cloud cover increased primary production.

  3. Optical Analogs of Two-Dimensional NMR

    A key development in nuclear magnetic resonance (NMR) was the introduction of multidimensional spectroscopy, in which one NMR spectrum evolves under the influence of subsequent magnetization pulses of defined phase and duration. In optical spectroscopy, analogous experiments can be performed with phase matching, in which short pulses create a coherent excited state that can then be perturbed with additional pulses that come in along different directions. Tian et al. (p. 1553; see the Perspective by Jonas) now show that shaped ultrafast optical pulses in a collinear geometry can be used to select particular nonlinear polarizations that can enhance or suppress particular coherences. In a model system (a rubidium vapor), the effect of such “phase cycling” on cross peaks provided information on how different excited states observed in fluorescence were coupled or interacted anharmonically.

  4. How Does Your Comet Glow?

    About 8 years ago, x-ray satellites showed that comets emit x-rays, but the underlying mechanism that produces the emission has proven elusive. Beiersdorfer et al. (p. 1558) have obtained high-resolution spectra of ions trapped in gases using the spare x-ray microcalorimeter spectrometer from the ASTRO-E mission. The laboratory spectra can be explained by double-capture of electrons in a charge-exchange process. These lab measurements were used in an emission model that was able to fit the spectrum of comet Linear C/1999 S4 taken by the Chandra X-ray Observatory. Charge-exchange between solar wind ions and neutral gases in the coma is sufficient to explain the x-ray emissions.

  5. From Hotspots to Melting Pots

    During the glacial periods of the Pleistocene, the ranges of many organisms contracted into isolated pockets or “refugia.” One popular hypothesis has been that diversity within a species will decline away from refugia because of successive founder events during postglacial colonization. Petit et al. (p. 1563) performed a meta-analysis of data from a large number of European plant species and found that the overall pattern of diversity is in conflict with this hypothesis. Although most species had the most genetically divergent populations in the southern refugial areas (the “hotspots”), the most diverse populations were typically located at more distant northerly latitudes. The authors argue that this pattern most likely is caused by admixture of divergent lineages coming out of different refugia (“melting pots”).

  6. Seeking Out Single-Stranded DNA

    The checkpoint signals that cells generate in response to DNA damage are mediated by the ATR protein kinase, which functions in a complex with the ATRIP protein. Zou and Elledge (p. 1542; see the Perspective by Carr) found that the ATR-ATRIP complex was not recruited to sites of DNA damage in cells that lacked replication protein A (RPA). Although RPA functions in processes such as replication and recombination, it also appears to initiate the checkpoint signal by binding to single-stranded DNA and then interacting with the ATR-ATRIP complex, which phosphorylates substrate proteins at the damage site.

  7. Wet Mantle Below the Mediterranean

    Water can be carried into the mantle by subduction, but it is unclear how much of this water remains in the subducted slab when it enters the mantle transition zone [depths of 410 to 660 kilometers (km)]. Van der Meijde et al.(p. 1556) examined the frequency dependence of seismic waves at the 410-km velocity discontinuity and found evidence for greater-than-expected water content in the transition zone below the Mediterranean. Several subduction zones may transport water into the region. The phase transition of olivine to wadsleyite at the 410-km discontinuity would enhance the water content because the latter mineral retains more water in its structure.

  8. Cutting Down the Middle

    During cell division, two daughter cells are physically separated when an actin ring forms in between the two new daughter nuclei and constricts. Pardo and Nurse (p. 1569) describe a novel and unexpected role for the equatorial microtubular structures in the control of the actin ring localization during cytokinesis in fission yeast. In the absence of these structures, the actin ring migrated dramatically to one end of the cell while the two nuclei frequently collapsed together in the middle of the cell. The equatorial microtubular structures are thus likely to be essential for maintaining accurate positioning of the division plane should cytokinesis be delayed.

  9. A Disrupter of Insulin Signaling

    The protein kinase Akt is a key component in insulin signaling. Du et al. (p. 1574) searched for proteins that directly interacted with Akt and found a protein they call TRB3, a mammalian homolog of the Drosophila tribbles protein, that functions as an inhibitor of the kinase. The amount of TRB3 RNA in livers from diabetic mice was increased compared to that in the wild type. Infection of mice or of rat FAO hepatoma cells with adenovirus expressing TRB3 caused hyperglycemia and reduced responses to insulin, respectively. The results establish TRB3 as component in metabolic control by insulin and as a potential therapeutic target for treatment of type II diabetes.

  10. Spikes, Learning, and Hippocampal Neurons

    What are the patterns of neuronal activity by which memories are established in the brain? An important step in this direction would be the identification of neuronal firing that occurs specifically during encoding and consolidation of memory. Wirth et al. (p. 1578) recorded from individual neurons in the monkey hippocampus and found that the activity of cells changed in parallel with the animals' learning of a location-scene association task. In a significant proportion of the cells, changes in activity were sustained after the animal had learned an association, whereas in others the changes were more temporary. These findings show that new associative learning is signaled by changes in a hippocampal neuron's stimulus-selective response properties.

  11. No More DNA, We're Full

    Tetrahymena, a single-celled eukaryote, organizes its genome into a germline micronucleus and a somatic macronucleus. The DNA in the macronucleus is substantially rearranged, with up to ∼15% being deleted. This deletion is programmed by the micronucleus and is thought to involve the RNA interference (RNAi) machinery. Yao et al. (p. 1581; see the Perspective by Selker) show that direct injection of double-stranded RNA, the substrate for RNAi, results in the specific and efficient deletion of homologous genomic regions. Foreign DNA sequences introduced into the micronucleus are efficiently deleted from daughter somatic macronuclei. This process prevents their expression in vegetative cells and suggests that this RNA-directed DNA deletion system operates to disable invading DNA.

  12. Polishing the Number of Rice Genes

    The rice genome is useful not only for understanding rice, but other cereal crops as well. The Rice Chromosome 10 Sequencing Consortium (p. 1566; see the Perspective by Bevan) now offers a closer-to-finished sequence of chromosome 10, the smallest of the 12 rice chromosomes. Compared with previous draft sequences of the rice genome, this more polished sequence analysis finds, among other various statistics, that the median predicted size of genes is roughly 40% greater and the total predicted number of genes is roughly doubled in comparison to previous predictions.

  13. Stressing Out the Neighbors

    Several experimental studies have shown that nanocrystalline metals deform by a different mechanism in comparison to coarser grained materials and can resist plastic deformation at high stress. They also tend to exhibit a brittleness that leads to a “dimpling” of fracture surfaces on the larger size scale of several grains. Hasnaoui et al. (p. 1550) used large-scale simulations to show that these small grains tend to move in a cooperative fashion. However, if a special grain boundary is present that does not typically move under stress, such as that at a twin boundary, it can pin its neighboring grains and prevent plastic deformation from occurring.

  14. The Ins and Outs on Pathogen Screening

    All multicellular organisms have common families of mechanisms that operate during early stages of responses to pathogen attack. Studies in insects and mammals have revealed the role of the Toll family and of the pattern recognition molecules Nod 1 and 2 in activating similar, but not identical, signal-transduction pathways. Lipopolysaccharide is not the sole pattern recognized by the innate immune system, Girardin et al. (p. 1584) show that intracellular Nod1 specifically detects a tripeptide bearing an exposed diaminopimelate amino acid derived from the peptidoglycan of Gram-negative bacteria. In contrast, Nod2, which is present on monocytes, macrophages, and dendritic cells, senses a peptidoglycan dipeptide motif common to all bacteria (unresponsive mutants of Nod2 have been implicated in Crohn's disease). Thus, Nod1 allows discrimination between intracellular Gram-positive and Gram-negative microorganisms and helps to shape subsequent adaptive immune responses.

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