This Week in Science

Science  16 Aug 2002:
Vol. 297, Issue 5584, pp. 1089
  1. In Brevia

    A method developed by Yan et al. (p. 1143) for determining the variation in expression levels of genes in normal humans shows that inherited variations are fairly common.

  2. Pictures of Quasiparticle Interference

    Interference patterns of electron waves, such as those surrounding defects, have often been observed with scanning tunneling microscopy (STM). Hoffman et al. (p. 1148; see the 26 July news story by Cho) have used STM to look more closely at the “checkerboard” pattern formed in the high-temperature superconducting cuprate, Bi2Sr2CaCu2O8+δ. By Fourier transforming the energy and spatial data, they can capture the dispersion relations of the superconducting quasiparticles in real space. The results, which suggest that the patterns arise from the interference of quasiparticle states located at different parts of the Fermi surface, may provide an explanation for the various incommensurate phenomena observed in these materials.

  3. Getting a Kick Out of Light

    In the usual photoionization process, the absorption and emission of photons between the light field and the photoexcited electron packet is an arbitrary process that broadens the energy distribution of the electrons. However, there is no net exchange of energy between the two, and the drift velocity of the electron packet is unchanged. Kienberger et al. (p. 1144; see the Perspective by Lewenstein) used ultrafast x-ray pulses only several hundred attoseconds (1 attosecond is 10−18 second) in duration to generate a packet of electrons. They could then control where the packet interacted relative to the phase of much more intense (but slower and longer wavelength) laser light field. The confinement of the packet to a particular region of the light field not only allows the acceleration and deceleration of the electron packets, but also provides a powerful probe for use in attosecond metrology.

  4. Marine Light Alkyl Nitrates

    The light alkyl nitrates in the atmosphere play an important role in controlling tropospheric ozone levels, and their sources have been assumed to be exclusively anthropogenic. Chuck et al. (p. 1151; see the Perspective by Ballschmiter) show that assumption to be erroneous. They report measurements of methyl and ethyl nitrate in seawater which indicate that large areas of the Atlantic Ocean are supersaturated for both compounds, thereby providing direct evidence for an oceanic source. Upper-ocean depth profiles suggest that biological processes are involved in their production.

  5. Electrons Swimming on Surfaces

    Electron solvation in layers of organic molecules adsorbed on the Ag(111) surface has been studied by Miller et al. (p. 1163). A probe laser pulse was used to excite electrons from the metal into discrete energy levels (image potential states) in overlayers of acetonitrile or butyronitrile, and a later pulse then probed the state of the electron. They observed a constant energy difference between the first and second levels which suggests that the primary effect of solvation is to lower the local work function. They also observed the localization of the image electron, to a region about the size of a single absorbate, on a time scale of a few hundred femtoseconds.

  6. Nitrogen Balance

    The nitrogen isotopic ratios of the mantle and the crust are significantly different, but some volcanic rocks derived from the mantle show a crustal nitrogen signature. Subduction has been suggested as a vehicle for recycling nitrogen. Fischer et al. (p. 1154) measured the nitrogen and helium isotopic concentrations of gaseous emissions from arc volcanoes in Guatemala and Costa Rica, where the arc volcanism is caused by the subduction of the Cocos plate beneath the Caribbean plate. Heavy nitrogen (15N) from the crust is carried down into the mantle wedge and then recycled back to the surface in the arc magmas, rather than into the mantle. If similar recycling occurs at other subduction zones, then methods of balancing nitrogen recycling and volatile fluxes need to be reconsidered.

  7. Survey Says...

    The “public information hypothesis” (or “habitat copying hypothesis”) proposes that individual animals cue on the local reproductive success of other members of their species to assess and choose their breeding site. Doligez et al. (p. 1168; see the news story by Withgott) performed a long-term manipulation of reproductive success of the collared flycatcher in Sweden at the scale of numerous entire patches of breeding habitat. From studies of more than 1000 nesting attempts, they found that birds used public information to decide whether to emigrate from, and settle in, a patch of breeding habitat.

  8. Keep Those Platelets Rolling

    At sites of vascular damage, von Willebrand factor (VWF) that is bound to the cell matrix interacts transiently with platelet receptor glycoprotein Ibα (GpIbα and slows down the platelets so that they can respond to the injury. Huizinga et al. (p. 1176; see the Perspective by Sadler) have determined structures of the GpIbα amino-terminal domain and its complex with the VWF A1 domain at 1.9 and 3.1 angstrom resolution, respectively. In the complex, an area of weak electrostatic interaction bridges two contact areas that are potential targets for antithrombotic drugs. Conformational changes in GpIbα and the A1 domain are required for interaction at the two contact sites, and mutations that cause gain-of-function diseases favor the bound conformation at both sites. The authors propose a model based on these structures for shear-induced activation.

  9. Transverse Tubule Biogenesis

    Amphiphysins are part of the cellular machinery that facilitates invagination of the plasma membrane during endocytosis. Lee et al. (p. 1193) report that a muscle-specific form of amphiphysin 2 (also called Bin1) directs invagination of the membrane into tubules and is critical for transverse (T) tubule biogenesis during mammalian muscle development. This amphiphysin localizes to the plasma membrane through a domain that recognizes specific phosphoinositides that are highly concentrated in the T-tubule system. Thus, the amphiphysins appear to have a conserved role in membrane deformation and morphogenesis.

  10. Controlling Population Expansion

    Several genes that function in eye development display a surprising level of conservation in gene function between mammals and invertebrates. One of these genes, mammalian Six6, is expressed early in development of the retina and also in the pituitary gland. Li et al. (p. 1180; see the 19 July news story by Vogel) investigated whether this factor is involved in cell-type determination or cell proliferation of precursor cells. In Six6−/− mice, all neuronal cell types in the retina and all pituitary cell types were found, but the cell types were decreased in number, and defects were observed in both organs. Six6 associates with the corepressor Dach to block cyclin-dependent inhibitors and allow for early progenitor cell proliferation. Hence, a tissue-specific repressor and corepressor regulate the cell cycle in an organ-specific manner.

  11. The Ups and Downs of Expression

    Gene expression varies substantially from one cell to another, and depending on the history of the cell, its interactions, the state of its regulatory machinery, and so on. Elowitz et al. (p. 1183; see the cover and the Perspective by Fedoroff and Fontana) have focused on the noise in gene expression that is left when all other regulatory influences are equal. To measure this “intrinsic noise,” they used strains of Escherichia coli in which two alleles encoding distinguishable green fluorescent proteins were controlled by identical promoters. In these single bacterial cells, genes have essentially the same intracellular environment, so the variation in their expression is a measure of intrinsic noise. Intrinsic noise accounted for a substantial amount of the total variation in gene expression and amount of noise varied with changes in other factors like transcription rate.

  12. Redundant Pathways Against Neuronal Regeneration

    In vertebrates, severed nerves in the limbs can regrow and re-innervate their targets, restoring movement and sensory input. Similar injuries to the brain or spinal cord are not repaired, however, because of two inhibitory molecules in the myelin sheaths of central nerves that prevent neural regeneration—Nogo and myelin-associated glycoprotein (MAG). The glycosyl-phosphatidylinositol-linked receptor for Nogo has no intracellular domain, but the receptor for MAG is not definitively known, although it does bind sialic acid. In a screen designed to find other membrane proteins that might mediate the transmembrane signal of the Nogo receptor, Liu et al. (p. 1190; see the Perspective by Woolff and Bloechlinger) identified MAG as a primary ligand. This binding is of an appropriately high affinity to control the growth-cone-collapsing effects of MAG and to ensure that expression of the Nogo receptor confers MAG sensitivity to embryonic neurons. Thus, both MAG and Nogo activate the Nogo receptor and act as redundant signals to inhibit neural regeneration.

  13. Split Fault Ends

    Subduction zones are major plate boundaries where one plate is driven beneath another. Many of the most destructive earthquakes occur along these boundaries or trigger tsunamis. Park et al. (p. 1157) obtained reflected wave profiles of the fault structure along the Nankai subduction zone where the Philippine Sea plate is subducted beneath the Eurasian plate (along the southeastern coast of Japan). They imaged a splay fault that branches from the subduction zone to the oceanic crustal surface at a submarine topographic high called the outer ridge. This splay fault can accommodate deformation from subduction-zone earthquakes, it can cause tsunamogenic events, and it provides an outlet for fluids as evidenced by the cold seep at the outer ridge.

  14. Spotting Hot Metal Dots

    Optical studies of labeled molecules that require long imaging times, such as diffusion studies, can be hampered by the labels going dark. Fluorescent dyes can photobleach, and semiconductor nanoparticles can temporarily “blink” and go dark. Metal particles do not bleach, but imaging methods such as Rayleigh scattering require somewhat large particles because scattering from similarly sized particles can dominate the signal. Boyer et al. (p. 1160) have developed a method that allows them to image metal particles as small as 2.5 nanometers in diameter through their photothermal signal. Particles of different sizes can be discriminated through their saturation at different heating powers.

  15. Fickle Charges

    Complexes of weakly bound neutral complexes are more readily made than are their charged analogs, but the charged complexes are in many ways more related to the reactive intermediates seen in many chemical reactions. Linnartz et al. (p. 1166) have now been able to make sufficient amounts of the cationic complex of Ar and N2 in a supersonic jet expansion of a plasma that they could obtain high-resolution infrared spectra. Based in part on their determination of the N-N stretching frequency, they show that a [Ar-N2]+ complex in which most of the positive charge is on the Ar atom is the favored ground state. These studies confirm previous photodissociation and reaction dynamics results and provide structural data that can guide theoretical calculations.

  16. Syntheses That Share Source Code

    The enediynes antibiotics, whose characteristic motif is a pair of triple bonds joined by a double bond, are structurally complex natural products that have antitumor potential because of their ability to cleave DNA. Their toxicity often limits their use as clinical agents, but the manipulation of their biosynthetic pathways could produce effective but less toxic derivatives. Liu et al. (p. 1170) and Ahlert et al. (p. 1173) describe the cloning and characterization of gene clusters for the biosynthesis of the enediynes C-1027 and calicheamicin, respectively. C-1027 is a chromoprotein comprising an apoprotein and the C-1027 chromophore, whereas calicheamicin is a nonchromoprotein enediyne. Surprisingly, the gene clusters that synthesize C-1027 and calicheamicin have a conserved polyketide synthase that is distinct from known bacterial polyketide synthases, which suggests that chromoprotein and nonchromoprotein enediynes are generated by similar biosynthetic pathways.

  17. When NO Means No to a Neuron

    Nitric oxide can modify cysteine residues on proteins and produce an S-nitrosylated derivative. Gu et al. (p. 1186) report that such a modification of matrix metalloproteinase-9 (MMP-9) activates the enzyme. MMP-9 nitrosylation and activation were observed in rodent brain tissue upon stroke, and treatment of cultured neurons with NO-activated MMP-9 caused apoptosis. This activation pathway may contribute to neuronal cell death that is associated with the extracellular matrix disruption observed in cerebral ischemia and neurodegenerative diseases.

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