This Week in Science

Science  13 Oct 2006:
Vol. 314, Issue 5797, pp. 220
  1. Ancient Metazoan Embryos


    The Doushantuo Formation of China contains many embryos of early (Late Precambrian) metazoans. Hagadorn et al. (p. 291) have used x-ray imaging to reveal the internal structures of these embryos, which consist of a few to nearly 1000 cells. Some of the embryos show evidence of asymmetric cell division similar to many higher metazoans. None of the 162 embryos have any epithelial development, a hallmark of sponges. Instead, they are likely representative of stem-group metazoans.

  2. Gently Arranging the Sheets

    Self-assembly allows complex objects to be fabricated merely by mixing components with the right sort of surface interactions, but for small particles, a template or patterned surface is usually needed to ensure the proper ordering. Tang et al. (p. 274 show that they can assemble free-standing sheets of cadmium telluride particles in solvent without the need for surface walls or a two-solvent interface. Instead, the assembly is driven by a combination of small driving forces, such as dipole moments and hydrophobic attractions.

  3. Timing Different Outcomes

    Catalysts can favor one set of products over another by reducing the energy required to reach particular conformations along one of the reaction trajectories. Sussman et al. (p. 278; see the Perspective by Rabitz) show that a similar effect can be induced by an intense infrared (IR) laser field in the photochemical dissociation of IBr. The reaction was initiated with a visible pulse. By applying a time delay, the IR field could be moved along the reaction trajectory, where it modified the energy landscape through Stark shifting to favor either of two pathways. The Stark pulse need not be resonant with an absorption band of the reacting molecules, so the technique should be applicable across a very wide range of substrates.

  4. Controlling Spins in Diamond Defects

    Various implementations of controlling and manipulating individual quantum states are currently being explored for possible applications in quantum information processing. Childress et al. (p. 281, published online 14 September) describe nuclear magnetic resonance experiments on individual defect centers in diamond, which acts as a single electron solid-state qubit. Using the observed spin-echo signals and extensive modeling of the hyperfine interactions, the authors reconstruct the local environment of the center and show that its electron spin coherence properties are determined by surrounding carbon-13 nuclear spins. They argue that their ability to address and manipulate electron and nuclear spins may be a possible route to quantum information processing.

  5. The Real Deal


    Truly tumor-specific molecules expressed on the cell surface are ideal targets for monoclonal antibodies in cancer therapy. Schietinger et al. (p. 304) report how a cell surface wild-type protein was transformed into a tumor-specific cancer target molecule. A mutation in a chaperone gene (Cosmc) abolished the activity of a glycosyltransferase and created novel antigenic epitope on a wild-type transmembrane protein. The combination of a monosaccharide with a wild-type protein sequence generated a syngeneic, high-affinity, highly specific, and therapeutically potent target. Mutations in the chaperone gene have also been found in patients with the autoimmune disease Tn syndrome.

  6. Golden Origins

    The origin of many hydrothermal gold deposits (those associated with magmas) has remained enigmatic: Does the gold come from the magma, or was it concentrated from the surrounding rocks, and how fast was it deposited? Simmons and Brown (p. 288; see the Perspective by Heinrich) have now obtained a direct sample of a brine emanating from a magma associated with one of the world's largest hydrothermal gold ores in Papua New Guinea. The sample, obtained via deep drilling, contains 15 parts per billion of dissolved gold. Given the flux of brine from the magma, the magma could produce the entire deposit within 50,000 years.

  7. Cancer, One Gene at a Time

    Knowing which genes are recurrently mutated in cancer cells helps illuminate the molecular pathways that underlie tumorigenesis. In a pilot study, Sjöblom et al. (p. 268; published online 7 September; see the cover and the 8 September news story by Kaiser) have sequenced 13,000 protein-coding genes in human breast and colorectal cancers and developed methods for distinguishing harmless sequence changes from causal mutations. Almost 200 candidate cancer genes (CAN genes) were mutated at significant frequency, many of which had not been previously implicated in tumorigenesis. Notably, there was little overlap of CAN genes mutated in breast and colorectal cancers, nor was there substantial overlap in different tumor specimens derived from the same tissue.

  8. Dealing with DNA Damage

    For an organism to remain healthy, cells with damaged DNA must either pause in the cell cycle for a repair job or succumb to elimination by apoptosis. Huang et al. (p. 294; see the Perspective by Bartek and Lukas) provide a mechanism through which cells with genomic damage may switch between these alternative fates. DNA damage activates a checkpoint signaling mechanism that arrests the cell cycle in part by inhibiting activity of cyclin-dependent kinase 2 (CDK2). The authors now find that CDK2 may also couple to the machinery controlling cell death. Normally, the transcription factor FOXO1 is phosphorylated by CDK2. In cells with extensive DNA damage, reduced phosphorylation of FOXO1 allows its translocation to the nucleus, where it enhances expression of apoptosis-inducing genes.

  9. Failing Relationship


    Aphid insect pests on plants possess a selection of symbionts in specific organs called bacteriomes. The composition of the symbiont flora varies between species and strains of aphid, and the bacteria show an intriguing range of degrees of genome reduction. Pérez-Brocal et al. (p. 312; see the Perspective by Andersson) have been investigating the genome of Buchnera aphidicola BCc, which inhabits the aphid Cinara cedri, along with the cohabiting secondary symbiont Serratia symbiotica. They found a highly reduced genome, of about 420 kilobases and 362 protein-coding genes, that is only two-thirds the size of Buchnera strains in other aphids. Compared with its symbiont partners, it has lost many functions. Instead, the cohabiting Serratia seems to have taken over the “lost” metabolic functions, and indeed, seems to be supporting the tiny genome as well as the host, which may mean that Buchnera aphidicola BCc has embarked on a path to extinction. In Brevia, Nakabachi et al. (p. 267) describe the limited genome of another bacterial endosymbiont, Carsonella ruddii.

  10. Branching Out

    At puberty, the female mammary gland is transformed from a simple epithelial tubule into an elaborate ductal tree. Underlying this global change in tissue architecture are changes in the behavior of individual cells. Certain cells within the tubules are instructed to branch, whereas their neighbors, only a few cell diameters away, are not. Using a micropatterning approach to engineer mouse mammary epithelial tubules in culture, Nelson et al. (p. 298) show that the position of branching depends on the initial tubule geometry and is determined by a local minimum in the concentration gradient of autocrine inhibitory morphogens, including transforming growth factor-β.

  11. Rethinking Viral Resistance

    In their investigations of herpes simplex encephalitis, Casrouge et al. (p. 308, published online 14 September) found that a single gene defect can impair resistance to the causative herpes simplex virus, but not others. The rare autosomal recessive mutation was present in a conserved gene encoding the endoplasmic reticulum membrane protein UNC-93B, which is thought to be involved in the cellular antiviral immune response. Cells from these subjects displayed defective response of some interferon genes, which would explain the children's severe defect in controlling viral replication. In contrast, the seemingly robust immunity of the subjects to other pathogens suggests, rather unexpectedly, that a single gene defect can lead to a pathogen-specific form of immunodeficiency.

  12. For Peat's Sake

    Although good records exist of how the atmospheric concentrations of methane (CH4) and carbon dioxide (CO2) varied over the last deglacialtion and the Holocene, why they changed as they did still is a mystery. One terrestrial biosystem that has been invoked as a potentially important driver, particularly of methane variability, is peatlands. In order to help better relate peatland development to the atmospheric inventories of CH4 and CO2 during the past 17,000 years, MacDonald et al. (p. 285) assembled and analyzed more than 1500 radiocarbon dates of peatland initiation across Northern Eurasia and North America. They found a pattern of expansion similar to that of the rapid CH4 increase of the Bølling-Allerød warm period and to its rise and sustained peak during the early Holocene. The expansion of peatlands also coincides with the decrease in CO2 at the beginning of the Holocene.

  13. Not-So-Simple Switches

    Riboswitches are structured RNAs that bind metabolites and, dependent on this binding, control gene expression. In most cases the metabolite-binding acts as a simple “on-off” switch. Now Sudarsan et al. (p. 300) report tandem riboswitch architectures that facilitate more sophisticated control. The 5′ untranslated region of Bacillus clausii metE RNA includes two riboswitches that respond independently to two different metabolites to function as a two-input Boolean NOR logic gate.

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