This Week in Science

Science  23 Feb 2001:
Vol. 291, Issue 5508, pp. 1443
  1. ENSO Through the Ages

    The El Niño-Southern Oscillation (ENSO) is Earth's largest interannual quasi-periodic climate event. Although it is still incompletely understood mechanistically, many researchers believe that it may be sensitive to global warming. Tudhope et al. (p. 1511; see the Perspective by Cole) have analyzed corals from Papua New Guinea (located at the heart of the Western Pacific warm pool and thus perfectly situated to record ENSO-related variability) to determine how ENSO has varied in response to both glacial and interglacial conditions during the past 130,000 years. They find that it is stronger now than during any of the other seven intervals of time they examined, and they suggest that its variability is a dual function of some type of glacial dampening and solar forcing.

  2. Electrons Make the Long Jump

    Direct electron transfer over distances of a few nanometers may be quite useful in molecular electronics, but the barriers that must be overcome to transfer electrons between donor and acceptor groups generally increase with increasing distance. In many cases, long-distance electron transfer requires an intermediate site, and the electrons actually “hop” rather than fire directly. Sikes et al. (p. 1519) present data from a thermal-jump experiment that indicates that direct electron transfer can occur over distances as great as 28 angstroms. In this case, electron transfer in self-assembled monolayers occurred through oligophenylenevinylene bridges that tethered ferrocene groups in solution to gold electrodes. The authors eliminate other possible mechanisms for electron transfer and suggest that these bridges may maintain greater planarity and thus greater orbital overlap than similar bridging groups that have been used previously.

  3. Unlocking Rare Clues

    The Permian-Triassic (P-T) extinction, the largest in Earth's history, was also abrupt, lasting perhaps a few hundred thousand years. The cause of the extinction has been widely debated; unlike the Cretaceous-Tertiary extinction, there has been no compelling evidence for an asteroid impact, such as an iridium anomaly or shocked mineral grains. Becker et al. (p. 1530; see the news story by Kerr) have now isolated and analyzed fullerenes from two separate locations at the P-T boundary that seem to contain an extraterrestrial signature. Fullerenes can trap rare gases in their cages, and analyses of the gases in the fullerenes from the P-T boundary show that their isotopic composition is unlike that of Earth's atmosphere but reflects an extraterrestrial signature. Fullerenes were not identified in sediments above or below the boundary and might have been delivered to the Earth in a large comet or asteroid.

  4. Form Follows Functional Cost

    Traditional explanations for elaborations of sexually selected structure (such as the elaborate plumage of birds and the horns on beetles) focus on the reproductive benefits of specific ornament or weapon types. In a comparative study of beetle horns, Emlen (p. 1534; see the Perspective by Harvey and Godfray) illustrates instead how functional costs of producing these structures can lead to the development of novel morphologies. The functional costs of horn expression vary with the type of horn produced, and this factor—rather than the reproductive benefits—may drive the evolutionary diversification of the beetle horns.

  5. Antigen Recognition Cuts Both Ways

    B cells appropriate help from T cells by expressing of class II molecules, which display antigenic peptides to specific receptors expressed by the T cells. These receptors are wired up to complex intracellular signaling pathways that launch a program of activation and differentiation in the T cell upon the recognition of appropriate antigen. Lang et al. (p. 1537) provide evidence that the interaction between the T cell receptor and class II molecule is not simply a one-way exchange of information. Signals could be delivered back to the B cell via the class II molecules. This process depended upon their association with the signaling chains of the B cell antigen receptor. This process may play a role in regulating T cell-B cell cooperation during immune responses to antigen.

  6. New Receptors for B Cells

    Expression of self-reactive receptors by B lymphocytes generally leads to their demise through clonal deletion, yet some B cells appear to escape death by swapping their existing receptors for less dangerous ones. This process of receptor editing was first revealed through the transgenic expression of self-reactive receptor chains, which artificially forced some cells to express new receptors. However, these studies could not predict the extent to which this process contributed to the B cell repertoire. By tagging one of the antibody loci in mice with a human version of the light chain κ gene, Casellas et al. (p. 1541; see the Perspective by King and Monroe) tracked which cells might undergo receptor editing during normal development. The results of these experiments suggest that revision of receptor specificity by B cell may be much more frequent than previously predicted.

  7. Automating Carbohydrate Synthesis

    Nucleic acids and many polypeptides can be readily synthesized by automated solid-phase synthesizers. Carbohydrate synthesis has been more labor intensive, and each step, whether in solution or taking advantage of solid-phase supports, is normally done by hand. Plante et al. (p. 1523; see the news story by Service, 2 Feb.) now report that several carbohydrates can be synthesized in an automated fashion with glycosyl phosphate and trichloroacetimidate building blocks supported on an octenediol-functionalized resin.

  8. Centrosomes Center Stage

    The centrosome of mammalian cells is thought to function in the organization of microtubules, particularly those that form the spindle during mitosis. Two reports provide evidence from time-lapse video-microscopy for substantial new roles for the centrosome (see the Perspective by Murray). When a cell divides, each daughter cell inherits one centrosome, which contains two centrioles: The daughter centriole, which assembled during S phase, and a mother centriole passed along from the previous cell cycle. Piel et al. (p. 1550) saw dramatic movement of the mother centriole from the center of the cell to the small intercellular bridge that connected daughter cells at the end of cytokinesis. Their observations, including experiments in which centrosomes were removed microsurgically, indicate that the final separation of the daughter cells and severing of the intercellular bridge requires the presence of the mother centriole, which stimulates disassembly of microtubules in the bridge. Hinchcliffe et al. (p. 1547) surgically removed the centrosome from African green monkey kidney cells to create karyoplasts and followed their progress through the cell cycle. Cells committed to the cell cycle did not require an intact centrosome to progress through mitosis. However, cells lacking centrosomes failed to initiate a new round of DNA synthesis.

  9. Flys, Mice, and Men

    Toll-like receptors (TLRs) are an ancient class of proteins utilized in antimicrobial defense. Mammalian TLRs can respond to microbial products by activating immune response genes, including those for cytokines and costimulatory molecules. Unlike Toll receptors from insects, however, a direct antimicrobial function has not been demonstrated for mammalian TLR. Thoma-Uszynski et al. (p. 1544) observed that macrophages from humans and mice could directly kill the intracellular bacterium Mycobacterium tuberculosis after stimulation through TLR-2. However, while antimicrobial activity in mice was achieved by induction of nitric oxide (NO), killing by human macrophages was mediated by a mechanism independent of NO.

  10. Leaving a Bitter Taste

    A large family of bitter-taste receptors has recently been characterized and cloned. The expression of multiple messenger RNAs of members of this family in individual cells lead to the speculation that there is only one general bitter taste and that single gustatory cells respond broadly to bitter compounds. However, Caicedo and Roper (p. 1557; see the news story by Brown) show that, in the majority of cases, individual taste cells can discriminate among different bitter stimuli. These results are in agreement with earlier psychophysical data and the results from taste-nerve recording and indicate that there is much heterogeneity even within one subpopulation of sensory cells.

  11. Attention Leads to Synchronization

    When we look around, we perceive a multitude of stimuli in a typical visual scene. When we focus our attention onto a particular object, the other stimuli get ignored and their input is suppressed. Fries et al. (p. 1560; see the Perspective by Stryker) investigated how selective attention operates in the visual processing pathways. They found that high-frequency oscillations in area V4 are enhanced in neurons that encode attended stimuli. These results support the hypothesis that attention modulates synchronized neuronal activity.

  12. Electron-Hole Symmetry in Cuprates

    The most studied high-temperature cuprate superconductors to date have been hole-doped. There is now a large body of experimental evidence pointing toward a pairing mechanism that involves the formation of a d-wave superconducting gap. For electron-doped cuprates, however, the picture is not so clear; some work suggests s-wave symmetry and studies suggest d-wave. Sato et al. (p. 1517 have performed a high-resolution, angle-resolved photoemission spectroscopic study on high-quality samples that provide direct evidence for a d-wave pairing state. The results suggest that superconductivity in both types of superconductors have a similar underlying pairing mechanism.

  13. Oxidation State of Mars

    The red soil of the martian surface is attributed to abundant ferric iron-rich minerals, but the redox state of the martian mantle is more difficult to determine. Wadhwa (p. 1527) has now estimated the redox state of the crust and the mantle by measuring the distribution of trivalent and divalent europium in pyroxene minerals from martian meteorites. The mantle is more reduced than the crust, which suggests that the crust may have been oxidized by a process such as aqueous alteration, and that this oxidized crust was never recycled into the mantle.

  14. Letting Go

    Members of the molecular chaperone 70-kilodalton heat shock protein (Hsp70) family bind and release substrate in a manner dependent on their adenosine triphosphate (ATP) activity. In the bacterial Hsp70 homolog, DnaK, the protein GrpE promotes substrate release by enhancing nucleotide exchange. Sondermann et al. (p. 1563) have now solved the structure at 1.9 angstroms of the Hsp70 ATPase domain bound to a Bag domain that promotes ATP-dependent release of substrate in vitro. Although the Bag domain is structurally unrelated to GrpE, the conformations of the bound ATPase domains of Hsp70 and DnaK are similar. It appears that the nucleotide-release mechanism has been conserved through convergent evolution of the eukaryotic Bag domain.

  15. Polyploidy and Gender Dimorphism

    In a study of the North American plant genus Lycium, Miller and Venable (Reports, 29 Sep. 2000, p. 2335) proposed that polyploidy may trigger gender dimorphism “by disrupting self-incompatibility and leading to inbreeding depression.” Brunet and Liston argue that the analysis by Miller and Venable “does not support the claimed associations,” suggest possible shortfalls in their method for measuring self-compatibility, and hold that showing the importance of the claimed associations in the evolution of gender dimorphism requires a demonstration “that gender dimorphism arises more frequently via this pathway than via other pathways.” Miller and Venable cite additional phylogenetic work supporting their analysis and suggest some additional statistical tests for the notion of polyploidy as a trigger for gender dimorphism. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/291/5508/1441a