This Week in Science

Science  14 May 1999:
Vol. 284, Issue 5417, pp. 1085
  1. Holding a Ribosomal RNA Fold

    Unlike most enzymes, the ribosome, which translates genetic information into peptide sequences, contains several RNA subunits in addition to many proteins. Although it might be expected that the RNAs would play a greater structural than catalytic role, a crystal structure determined by Conn et al. (p. 1171) shows how the protein L11 stabilizes an unusual fold of a piece of the large subunit ribosomal RNA and exposes highly conserved bases. This RNA piece contains the site where elongation factor G binds reversibly during peptide bond formation and where the antibiotic thiostrepton acts to inhibit protein synthesis. Helical portions of the RNA are brought together through base stacking and hydrogen bonds between three base triplets, and the protein helps to stabilize the intertwined helices through precise interactions with extrahelical bases.

  2. Sculpting Faint Jovian Rings

    Jupiter has three faint rings (the gossamer ring, the main ring, and the inner halo) composed mainly of micrometer-sized particles that have been difficult to observe. Burns et al. (p. 1146) have analyzed and modeled data from the Galileo spacecraft and the Keck telescope in Hawaii and determined that the gossamer ring is actually two rings, and the roughly rectangular cross sections that these rings present when viewed edge on (as opposed to a lens-shaped cross section) are controlled mainly by the production of ejecta from the small satellites Amalthea and Thebe. Absorption of solar radiation causes a subsequent inward reduction of the particles' orbit through momentum loss. The main ring of the Jupiter system is probably also derived from ejecta from the satellite Adrastea. These observations and models suggest that the Jovian planets and possibly Mars have faint rings associated with their small satellites.

  3. Switching Superconductivity

    The electronic properties of a high-temperature superconductor are sensitive to the amount of doping of positive charge carriers (holes) in the material. The ability to control this dopant density in a reversible manner is crucial for developing useful devices. Ahn et al. (p. 1152) describe how they switch a thin-film cuprate between a superconducting state and insulating state by biasing a ferroelectric layer located adjacent to the cuprate layer. The electric field provided by the ferroelectric is sufficient to modify the dopant density in the cuprate layer by pulling in or repelling holes from the layer, depending on its polarization state. The ability to program the polarization state of the ferroelectric allows nonvolatile switching of the cuprate layer.

  4. Tougher Vesicles from Polymers

    The combination of hydrophilic and hydrophobic interactions that allow lipids to form membranes and vesicles can be built into synthetic polymers. Discher et al. (p. 1143) found that they could replicate the flexibility of lipid vesicles in short polyethyleneoxide (PEG)-polyethylethylene copolymers but achieve almost an order of magnitude improvement in toughness (rupture strength) and at least a 10-fold decrease in water permeability. Such tunability may prove useful in designing vesicles for delivery applications.

  5. Famine of the Deep

    A 7-year comparison of sinking particulate organic carbon and oxygen consumption by a sediment community at a site in the eastern North Pacific by Smith and Kaufmann (p. 1174; see the Perspective by Druffel and Robison) indicates that the demand for food by this deep-sea community currently outstrips its supply from the ocean surface. The discrepancy results from a decrease in plankton biomass at the surface that may be caused by increases in sea-surface temperature. Such trends could impact deep-sea communities and geochemical cycling.

  6. Capturing CO2

    Concentrations of carbon dioxide (CO2) are not increasing as fast as they should based on known inputs. The world's forests may be consuming this excess CO2, but detection has been difficult because the forests have several carbon pools (including wood, foliage, litter, and soils) that also rapidly exchange carbon with each other. Delucia et al. (p. 1177) present results from an experiment in an intact ecosystem designed to evaluate these pools and fluxes. The experiment has run for 2 years and involves forest plots exposed to ambient or to high CO2 concentrations. During this time, total net primary production increased by 25% in the CO2-enriched plots. Such an increase globally would extract about one-half of the anthropogenic CO2 emissions in 2050.

  7. Replenishing Epithelial Lines

    The oval cells of the liver are thought to be precursors (stem cells) for the major cell types that comprise the liver, including hepatocytes and bile ductal epithelium. These cells are a potential source for regeneration of the organ after acute liver failure. Their origin has been obscure, however. Petersen et al. (p. 1168) report that such cells can be found in the bone marrow. Thus, the bone marrow, source of stem cells for blood and mesenchymal cells, may also be a source of stem cells of an epithelial lineage.

  8. Moving Pictures

    How do we decide which bits and pieces of a visual scene go with which other bits and pieces? The usual suspects would be common features, such as color, brightness, or direction of motion. Lee and Blake (p. 1165; see the news story by Barinaga) have created a new kind of visual display in which patches containing contours of a particular orientation (called Gabor elements) change orientation unpredictably. Observers perceived a figure distinct from the background if the changes of direction within a small area were synchronized under conditions where the usual cues for figure-ground segmentation do not contribute. These results raise the question of how the visual system detects fine temporal correlations and identifies boundaries where these local correlations differ.

  9. Disrupting Sperm for Gene Transfer

    Current methods of transferring foreign genes into embryos all have different drawbacks. One approach, the use of sperm to carry DNA on their external surfaces into eggs, had fallen into disrepute because of irreproducibility. Perry et al. (p. 1180) found that they could get a high frequency of transfer of two different plasmids into mouse oocytes by disrupting the membranes of sperm heads. A transgene for green fluorescent protein was expressed in live offspring, which were then capable of passing the transgene to their progeny. This procedure has the potential to be applicable to species in which micronuclear injection has not been easy and for the transfer of larger pieces of DNA.

  10. Hippocampal Development

    In the mammalian brain, higher cognitive functions such as learning and memory depend upon the circuits that run through the hippocampus. Zhao et al. (p. 1155; see the cover) have identified a LIM homeobox gene, Lhx5, that is required for normal hippocampal development. In mice carrying a deletion in the Lhx5 gene, development of the hippocampus began relatively normally as the precursor cells proliferated. However, in the mutant mice, the cells destined to form the hippocampus migrated aberrantly and failed to differentiate. Comparison with phenotypes generated by other mutations affecting the hippocampus now delineates an intermediate phase in hippocampal development, after proliferation but before final placement and differentiation of the critical cells, for which Lhx5 function is critical.

  11. GAD and Type I Diabetes

    In type I diabetes, the immune system destroys the insulin-producing β cells of the pancreas. T cells to specific antigens develop—those recognizing glutamic acid decarboxylase (GAD) appear early in disease. Transfer of these T cells to nondiabetic animals can initiate disease. Yoon et al. (p. 1183; see the Perspective by Boehmer and Sarukhan) report that expression of GAD in β cells is required for the development of diabetes in NOD mice, the mouse model of type I diabetes. A series of mice were developed that expressed increasing amounts of an antisense gene to GAD messenger RNA. When expression was high, little GAD was detected on the β cells, and the NOD mice did not develop diabetes. Transfer of β cells from these mice into diabetic NOD mice cured NOD diabetes. Thus, not only was GAD necessary for the development of diabetogenic T cells, but the β cells were not destroyed in mice with ongoing disease unless they expressed GAD.

  12. Hitting the Immune System Twice

    Although it is accepted that CD8 T cells need continued engagement of their T cell receptor (TCR) for survival, it is not clear what role CD8 actually plays. Pestano et al. (p. 1187) report that if CD8 cells do not simultaneously engage both their TCR and CD8 proteins, the CD8 gene is remethylated and the cells eventually die through the Fas-FasL pathway. It seems that blockade of this default death pathway, rather than being provided with an external signal, may be most important for survival. Thus, if CD8 T cells bind to targets that do not express enough of the ligand for CD8 (major histocompatibility complex class I proteins), such as some virally infected cells and many tumor cells, not only may the targets live, but the CD8 T cells may die as well.

  13. Ancient Australian Climate

    The vegetation of central Australia has varied greatly over the past 65,000 years. Johnson et al. (p. 1150) trace the expansion and contraction of grasslands by measuring carbon isotopes in emu eggshells from the Lake Eyre Basin. The carbon isotopes reflect changes in diet of the emu and thus the abundance of grasses, which in turn reflect the vigor of the monsoon. The data imply that the monsoon in Australia weakened after about 45,000 years ago, which is near the time that humans are thought to have arrived. One possibility is that human modification of the landscape by burning had an impact on the Australian climate.

  14. Deciding Where to Look

    How does the brain decide where the eyes should look next? Horwitz and Newsome (p. 1158) made neuronal recordings from the superior colliculus of monkeys in response to moving groups of visual stimuli; in this area of the brain, many neurons fire just before an eye movement. They found that some cells appear to be involved in selecting the direction of the eye movement, whereas others appeared to be more involved in the motor tasks that actually execute the eye movement.

  15. Controlling Protein Synthesis

    Regulation of localized messenger RNAs (mRNAs), such as those found in the dendrites of neurons, is thought to contribute to control of many biological processes, such as the long-term changes in signaling between neurons. Sabatini et al. (p. 1161) report that gephyrin, a protein that is localized to synaptic sites in dendrites and interacts with glycine receptors and proteins of the cytoskeleton, also interacts with a protein that regulates protein synthesis, RAFT1, a member of a family of kinases that have sequence similarity to phosphoinositide kinases but appear to act as protein kinases. The RAFT1 protein (for rapamycin and FKBP12 target 1; FKBP12 being the cellular receptor for rapamycin) functions to mediate the effects of the immunosuppressant rapamycin and promotes signals that enhance mRNA translation. Mutants of RAFT1 that could not bind gephyrin failed to produce signals that contribute to activation of translation. The results indicate that protein synthesis in neuronal dendrites may be controlled through RAFT1.

  16. Considering a Neoproterozoic Snowball Earth

    P. F. Hoffman et al. (Reports, 28 Aug. p. 1342) found “carbon isotope anomalies in carbonate rocks bracketing Neoproterozoic glacial deposits in Nambia” and calculated “estimates of thermal subsidence history.” Their results support the proposal that a “global glaciation (that is, a snowball Earth)” occurred about 700 million years ago (Ma), stifling life in the oceans. They concluded that “volcanic outgassing” created “extreme greenhouse conditions,” which finally melted the ice.

    N. Christie-Blick et al. comment that evidence of “gradual retreat of the ice front is recorded in many areas” of Australia and North America, contrary to the “catastrophic” melting posited in the report. They also state that isotopic depletion data indicate that the maximum point of “collapse of primary productivity” in the oceans likely happened after the time at which the ocean would have been frozen.

    In response, Hoffman and Schrag describe a “complex interplay” of forces, stating that “conventional glaciation” probably preceded—and that “a limited hydrologic cycle” would still have existed during—a snowball Earth. They interpret the isotopic values as reflecting the “relative amounts of carbonate carbon and organic carbon burial in sediments,” which thus supports the snowball Earth scenario.

Stay Connected to Science