This Week in Science

Science  25 May 2007:
Vol. 316, Issue 5828, pp. 1093
  1. Coherent Single-Electron Source


    Just as the availability of single photons has enabled the development of secure communications via quantum cryptography, it is expected that an electronic analog would allow similar quantum coherent manipulations in an electronic circuit. Fève et al. (p. 1169; see the Perspective by Giblin) have developed a coherent single-electron source in which the electrons are emitted from a quantum dot into a quantum-coherent two-dimensional conductor. The trigger is the application of a potential step to the quantum dot. The source can also be used as an alternating-current standard for possible applications in metrology.

  2. Dark Heavy Dwarfs

    When a massive galaxy collides with another, it can jettison gas that subsequently clumps up to form a crowd of surrounding dwarf galaxies. Theoretical models predict that such tidally produced dwarfs should not contain dark matter, so their gravity should tally with the stars and gas within them. Bournaud et al. (p. 1166, published online 10 May; see the Perspective by Elmegreen) have used gas motions to measure the masses of a series of dwarf galaxies lying in a ring around a massive galaxy that has recently experienced a collision. They find evidence for dark matter within the dwarfs—double that expected from the stars alone. If this extra material is in the form of unseen cold molecular gas originating in the massive galaxy, then it must be more common than now thought and could account for a part of the missing baryons in the universe.

  3. Drying the American Southwest

    The large human population and widespread industrial and agricultural economies of the southwestern United States and northern Mexico depend on the cheap and ready availability of water that may be affected by changes in precipitation, evaporation, groundwater storage, and river flow as climate warms. Seager et al. (p. 1181, published online 5 April) show that a broad array of climate models agree that this region will dry substantially during the next century and that conditions as dry as those of the Dust Bowl will become the normal ones in the region. These changes would be caused by atmospheric circulation patterns that create a poleward expansion of the subtropical dry zones. This aridification should be unlike any climate state that exists in the instrumental record.

  4. Direct Aromatic Coupling

    Linking two aromatic rings is a key reaction in organic synthesis that generally requires one coupling partner to be modified with a halide and the other appended with an electropositive group, such as boron or tin. Stuart and Fagnou (p. 1172; see the Perspective by Ellman) describe a palladium catalyst that promotes the direct coupling of benzene to indoles without the need for any additional activating groups. The reaction proceeds by C-H bond activation and proves remarkably selective for the cross-coupled product; no benzene or indole dimers are observed. High regio-selectivity for bonding to the 3-carbon position of a wide range of substituted indoles was seen.

  5. Early Iron Deficit

    The timing and processes of the formation of the solar system can be mapped out by measuring long-lived isotopes in meteorites. One such system is 60Fe-60Ni, with a half-life of 1.5 million years; 60Fe only forms in stars. Bizzarro et al. (p. 1178; see the news story by Kerr) find that differentiated meteorites show a very minor deficit of 60Fe compared to Earth, Mars, and chondrites. The authors suggest that the oldest solar system material formed in the absence of 60Fe and that 60Fe was injected into the proto-planetary disk about 1 million years after the solar system formed. This material may have come from a neighboring star, such as a particularly iron-rich supernova.

  6. Engineering Herbicide Resistance


    Engineering of crop plants to be resistant to herbicides can allow agricultural strategies that are kinder to the land, with reduced need to till the earth. However, herbicides that are currently in widespread use are starting to lose effectiveness because of increases in resistant weed populations. Behrens et al. (p. 1185; see the news story by Service) describe how crop plants can be engineered to be resistant to dicamba, a well-known environmentally friendly herbicide. By transferring a bacterial gene to the crop plants, the authors engineered plants resistant to the herbicide. However, for the time being at least, weeds are not resistant and die back.

  7. DNA Damage-Response Teams

    DNA damage is often a key event in triggering malignancy (see the Perspective by Petrini). Much of the cellular response to DNA damage is mediated by two protein kinases, ATM (ataxia telangiectasia mutated) and ATR (ATM and Rad3-related). Matsuoka et al. (p. 1160) report a proteomic screen that implicates more than 700 proteins in the cellular response to DNA damage caused by ionizing radiation. Antibodies that recognized the phosphorylated forms of peptides containing consensus phosphorylation sites recognized by ATM or ATR were used to search for previously unrecognized substrates. These results provide a resource for identification of previously unrecognized proteins that function in control of DNA damage in mammalian cells. Three reports, Wang et al. (p. 1194), Sobhian et al. (p. 1198), and Kim et al. (p. 1202), describe a complex of proteins that interact with the breast cancer-associated tumor suppressor gene product BRCA1 and implicate covalent modification of proteins by ubiquitination in regulating the functions of BRCA1 and its partners in the cellular response to DNA damage. A complex of BRCA1 with the protein Bard1 is known to have ubiquitin ligase activity. In the present work, BRCA1 formed a complex at sites of DNA damage with RAP80, a protein with a ubiquitin-interacting motif domain, and RAP80 contributed to localization of BRCA1 to sites of DNA damage. A third protein, Abraxas, appears to mediate interaction of BRCA1 with RAP80. BRCA1 complexes also contained BRCC36, a deubiquitinating enzyme. The DNA damage checkpoint that halts division of cells with damaged DNA was defective in cells lacking RAP80. Thus, the BRCA1-Abraxas-RAP80 complex appears to target BRCA1 to sites of DNA damage.

  8. Marijuana and Developmental Damage


    The effects of marijuana are mediated by cannabinoid receptors on neurons in the brain, and a causal relationship between marijuana use during pregnancy and permanent cognitive deficits in the offspring has been identified. Berghuis et al. (p. 1212) now define the molecular hierarchy that controls marijuana actions within single neurons and show that activation of cannabinoid receptors by their natural ligands controls the establishment of functional connections between neurons in the brain. These findings define the cellular context through which prenatal marijuana use perturbs brain development.

  9. Real-Life Transcription Factor Dynamics

    Transcription factors bind to specific sites on chromosomal DNA to regulate gene expression. How transcription factors find their target DNA is generally thought to occur through a combination of diffusion through the cytoplasm as well as diffusion along DNA segments. Elf et al. (p. 1191) use single- molecule techniques to study the specific and nonspecific binding of a model transcription factor, the lac repressor, in living Escherichia coli. Lac repressor searching for an operator spends 90% of its time diffusing along DNA. If the repressor does not find an operator within 5 milliseconds, it falls off the DNA and diffuses through the cytoplasm to bind to another DNA segment. Such single-molecule approaches will move us toward a quantitative understanding of biochemical processes as they occur in living cells.

  10. From Membrane Curvature to Fusion

    Soluble NSF attachment protein receptor (SNARE)-dependent tethering and zippering of vesicles to target membranes promotes vesicle fusion with target membranes. In the synapse, the relevant SNAREs are synaptobrevin2, syntaxin1, and SNAP25, but synaptic vesicle fusion also requires other proteins. Martens et al. (p. 1205, published online 3 May) show that SNARE-dependent tethering and zippering is only part of the bilayer fusion mechanism—synaptotagmin is also needed. In cell-free experiments, synaptotagmin-1 induced positive membrane curvature in a calcium-dependent manner, which made the membrane more fusogenic and helped bring membranes into contact with each other.

  11. Motor Motion Captured

    Myosin V is a two-headed molecular motor that alternates the positions of its leading and trailing heads to move unidirectionally along actin filaments in a “hand-over-hand” mechanism. Shiroguchi and Kinosita (p. 1208) have now directly visualized this walking motion. Each head is attached to a long and stiff neck. The adenosine triphosphate-dependent power stroke causes the neck of the leading head, which is bound to the actin track, to lean forward. The trailing head is lifted from the actin track, and a free swivel connection at the neck-neck junction allows the lifted neck to undergo extensive Brownian rotation in a diffusive search for the next binding site. The forward movement of the leading neck moves the pivot point forward so that the unbound head lands at a forward site.

  12. All Mixed Up

    Various isotope ratios that can be measured in primitive meteorites reflect processes that took place in the nebula from which the solar system formed. Anomalies that have been seen in many isotope systems could be explained by patchiness in the protosolar nebula, which raises questions about how the Earth differentiated to grow its metal-rich core that locked up heavy isotopes in the interior. Carlson et al. (p. 1175) have analyzed Ba, Nd, and Sm isotope compositions in a variety of chondrite meteorites and find deficiencies corresponding to slow-neutron and proton-capture nuclear processes. The abundances of Sm and Nd in chondrites are lower compared with terrestrial rocks, consistent with previous suggestions, but not for Ba, contrary to earlier work. The Sm and Nd differences are consistent with early planetary differentiation of the Earth.

  13. Pathogen Biochemistry

    Ribonucleotide reductases (RNRs) undertake the chemically challenging task of cleaving a carbon-oxygen bond in ribonucleotide substrates to provide precursors for DNA synthesis. RNRs from different organisms use different strategies to generate a cysteine radical in the catalytic subunit R1. Jiang et al. (p. 1188) show that the RNR in the human pathogen Chlamydia trachomatis uses a high-valent manganese and iron cofactor to generate the cysteine radical, a mechanism that is unprecedented in enzyme redox chemistry. The strategy may allow the pathogen to remain active in the presence of oxidants that are part of the host's immune response.