This Week in Science

Science  22 Oct 2004:
Vol. 306, Issue 5696, pp. 571
  1. Graphene Transistor


    Carbon nanotubes are often described as rolled-up sheets of graphene (a single layer with graphite). However, stable isolated graphene sheets have so far not been obtained. Novoselov et al. (p. 666) have now exfoliated single sheets of graphene, one to several atomic layers thick, from a high-quality graphite parent crystal. They fabricated devices with these sheets of carbon, and demonstrate a metallic-channel field-effect transistor. The high mobility observed, in excess of 10,000 square centimeters per volt per second at room temperature, paves the way for further development of these films in device electronics.

  2. How Electrons Get Wet

    Water, unlike ammonia, does not support free electrons in a stable solution, but for more than 40 years, chemists have puzzled over the transient charges that ultraviolet irradiation of water can produce. How are these electrons confined, and how do they affect the water molecules around them? Three reports describe studies that tackle these questions by using clusters of 50 water molecules or fewer so that the interactions with an excess electron can be studied in close detail (see the Perspective by Jordan). Hammer et al. (p. 675, published online 16 September 2004) acquired vibrational spectra of clusters of four to six water molecules. The electron associated primarily with a particular water molecule that has both H atoms pointed away from the rest of the cluster. In the tetramer and pentamer, the electron is bound sufficiently weakly that one quantum of OH stretching energy leads to ionization of that electron within 200 fem- toseconds (fs). Bragg et al. (p. 669, published online 16 September 2004) monitored the relaxation of a photoexcited excess electron in larger clusters (25 to 50 waters). They observed increasing transition rates from the excited p-state to the ground state as the cluster size increases. The time constants (on the order of 100 fs) are consistent with electronic relaxation preceding solvent reorganization, and the data extrapolate well to a comparable mechanism in bulk water. Paik et al. (p. 672, published online 16 September 2004) tracked how the clusters relaxed from the p-state. Solvent reorganization in the ground state occurred on a 300-fs time scale regardless of cluster size (15 to 35 waters); this time scale persists in the bulk. The rates of hydrogen bond breakage in the clusters scaled inversely with cluster size but remained much faster (3 to 13 picoseconds) than statistical predictions would suggest.

  3. Losing Variability in the Noise

    One concern with paleoclimate reconstructions that rely on collections of climate proxy records, such as data from tree rings and corals, is that such reconstructions might underestimate the magnitude of the temperature fluctuations that occurred. In order to test how the methods used might affect the records that they generate, von Storch et al. (p. 679, published online 30 September 2004; see the Perspective by Osborn and Briffa) used virtual data generated by a climate model in a synthetic reconstruction of Northern Hemisphere climate for the past 1000 years. Reconstructions produced by regression-based methods on noisy data suffer from marked losses of decadal- to centennial-scale variability.

  4. Devil in the Details

    Using microarrays tiled across the entire Drosophila genome, Stolc et al. (p. 655) developed an expression map and examined how expression is linked to development and evolution. Gene expression “neighborhoods” were found to be conserved across species, which appeared to also shape genomic rearrangements. Developmental gene expression was analyzed exon-by-exon, and a substantial fraction of noncoding RNAs also seemed to be developmentally regulated.

  5. Jack of All Trades

    For any cell of any organism, shearing both strands of DNA is extremely dangerous. One means of repairing such potentially lethal damage is known as nonhomologous end joining (NHEJ), where unrelated double-stranded DNA ends are trimmed and cleaned up and then joined together. Della et al. (p. 683) isolated the bacterial proteins responsible for NHEJ in order to dissect the process. Rather than housing different functions in different proteins as in eukaryotes, Mycobacterium tuberculosis uses one protein, Mt-Lig, to carry out the trimming, gap filling, and ligation steps of NHEJ. Together with the M. tuberculosis homolog of Ku, a protein critical for bringing the broken DNA ends together, Mt-Lig repaired double-stranded breaks both in vitro, and when expressed in the eukaryote yeast, formed a self-sufficient repair machine.

  6. Photons One-at-a-Time


    Light pulses or beams come in two flavors: Nonclassical, with a known number of photons but no information regarding the phase, and classical, in which there is a definite phase but no precise information on the number of photons, which fluctuates. How many photons does it take to get a classical field? To answer this question, Zavatta et al. (p. 660) describe a technique in which photons can be added one at a time and can observe the evolution from a nonclassical to classical light field. The results may also provide insight into the processes of spontaneous and stimulated emission of radiation. Quantum information processing requires the development of techniques in which quanta can be stored in a register and retrieved some time later. Recent work has shown that it is possible to use a cloud of rubidium atoms to store pulses of nonclassical light, and that light can then be retrieved from the atomic ensemble with a suitable read pulse. However, the number of photons in these pulses was not determined. Matsukevich and Kuzmich (p. 663; see the news story by Seife) extend those earlier studies to the storage and retrieval of single photons within the atom cloud. This approach may provide routes toward distributed quantum networking.

  7. Down Syndrome Critical Region?

    Down syndrome (DS) is thought to result from the expression of an additional copy of a gene, or set of genes, present within a critical region (CR) of chromosome 21, the chromosome present in triplicate in this condition (trisomy 21). Olson et al. (p. 687; see the Perspective by Nelson and Gibbs) used a genetically manipulated mouse model for DS, in which the orthologs of the small number of human genes within the DSCR could be duplicated or deleted. Offspring generated by crossing these animals possessed one, two, or three copies of the DSCR and were compared with an established mouse model for DS with trisomy for a substantially larger region of the chromosome. Using primarily craniofacial and growth parameters, the mice with trisomy for DSCR genes did not possess DS-like features. Thus, DS may not result from simple overexpression of a small handful of genes, but involve instead a complex genetic and developmental interplay.

  8. Regulating Cellular Survival

    The protein phosphatase 2A (PP2A) regulates a number of different transcription factors, some of whose gene targets control cell survival. Kong et al. (p. 695) report that α4, a regulatory subunit of PP2A that can enhance enzymatic activity, is an essential inhibitor of transcriptionally induced cell death, or apoptosis. Multiple cell types derived from mice engineered to lack α4 succumbed to rapid cell death. In the absence of α4, expression of proapoptotic gene targets of the transcription factor p53 increased. The data provide support for the idea that, in animal cells, apoptosis is a default cell fate that must be constantly repressed.

  9. Oscillating Cellular Coding System


    The transcription factor nuclear factor kappa B (NF-κB) is used by different cell types in response to a wide range of stimuli. Once activated in the cytoplasm, it moves into the nucleus to induce the expression of specific target genes before being rapidly inactivated. Nelson et al. (p. 704) combined real-time imaging of single living cells and a computational model to show that maintenance of gene expression in response to a particular stimulus depends on persistent oscillations of the NF-κB complex between these two cell compartments. The role of this oscillatory behavior appears to be to maintain a concentration of activated transcription factor in the nucleus that will ensure the appropriate cellular response.

  10. Wax on the Way Out

    Waxes—lipids of the plant cuticle—coat the surface of all land plants and provide a critical interface between the plant and their environment, but how plants secrete cuticular lipids has not been known. Pighin et al. (p. 702; see the Perspective by Schulz and Frommer) characterized a mutant of Arabidopsis that accumulates wax components inside the epidermis in inclusions that are morphologically similar to inclusions seen in human adrenoleukodystrophy caused by defects in so-called ABC lipid transporters. When the defective gene was cloned from Arabidopsis, it proved to be an ABC transporter of the ABCG subfamily.

  11. Spotting Metals in Single Cells

    Synchrotron high-energy X-rays can be used for detecting and deciphering the interaction of bacteria with environmental metal ions, allowing the detection and location of metals within a single bacterial cell, and the assessment of their redox state. Kemner et al. (p. 686) detected nanominerals in hydrated cells and could observe and quantify chemical changes depending upon the physiological status of the cells—free or attached to a substrate or after heavy metal-induced killing. The methodology described should be valuable in environmental analysis and life detection.

  12. High Throughput Gene Regulator Analysis

    It is an ongoing challenge to extract genetic insights as rapidly as possible, making the most efficient use of resources. Liao et al. (p. 690) have used a computational approach and a haplotype map of 16 inbred mouse strains to identify genes and regulatory elements without the delays imposed by generating crosses. This approach was used first to confirm previously identified loci associated with major histocompatibility complex (MHC) phenotypes and the ability to regulate the response to aromatic hydrocarbon. Gene expression profiles were then used to identify a novel regulatory element associated with variation of the levels of expression of an MHC class II gene among 10 inbred strains.

  13. Regulator of Calcium Signaling

    Many calcium-dependent signals are mediated by the calcium sensor protein calmodulin. Rakhilin et al. (p. 698) describe a calmodulin-binding protein that appears to function at the interface of two major signaling pathways activated by G protein-coupled receptors—one mediated by the cyclic adenosine monophosphate-dependent protein kinase (PKA) and the other by phospholipase C and intracellular calcium signals. Alterations in signaling by such receptors are associated with a number of major diseases of the nervous system. This protein, called RCS for regulator of calcium signaling, bound to and inhibited calmodulin. RCS binding to calmodulin was enhanced when RCS was phosphorylated by PKA. Phosphorylated RCS inhibited activity of the calcium-calmodulin-dependent protein phosphatase (PP2B). Phosphorylated RCS is thus poised both to enhance signaling through PKA (because substrates of PKA are often dephosphorylated by PP2B) and, at the same time, to inhibit the action of calmodulin promoting a range of calcium-dependent signals.