This Week in Science

Science  10 Dec 1999:
Vol. 286, Issue 5447, pp. 2041
  1. Martian Seashores

    Previous studies have suggested that early Mars experienced a warmer and wetter climate, and that liquid water may have been abundant. Head et al. (p. 2134) have analyzed Mars Global Surveyor altimeter data to refine the detailed topography of the martian surface and present evidence that a large ocean existed in the northern lowlands. Their analysis suggests a possible shoreline with smooth topography within the shoreline boundaries, the termination of six outflow channels at the elevation of the shoreline, and the presence of several terraces that may be related to shoreline recession. The estimated volume of water that would have filled this proposed ocean is consistent with estimates of Mars' water budget.

  2. El Niño and CO2

    During El Niño events, the tongue of warm surface water that spreads eastward across the equatorial Pacific Ocean reduces upwelling of cold, carbon dioxide (CO2)-rich water and thus reduces the amount of CO2 that is transferred to the atmosphere. Conversely, during La Niña years, when the eastern equatorial Pacific contains colder-than-average surface water, the CO2 flux from atmosphere to the ocean is high. Chavez et al. (p. 2126; see the cover) have collected data from sea surface sensors, ships, and satellites to construct a detailed picture of the chemistry and biology of the –1998 El Niño, one of the strongest ever observed. These data have allowed them to quantify the effects of zonal winds and ocean circulation on primary productivity and air-sea CO2 fluxes.

  3. Life Below the Ice

    Lake Vostok, located about 3743 meters below glacial ice near the center of East Antarctica, may be home to a long-isolated microbial ecosystem (see the Perspective by Vincent). This possibility has been investigated by drilling an ice core to within about 120 meters above the lake (to avoid contaminating this environment). Jouzel et al. (p. 2138) have analyzed the ice below 3500 meters and determined from the oxygen isotopic concentrations that this ice is probably refrozen lake water rather than glacial ice. Priscu et al. (p. 2141) and Karl et al. (p. 2144) have analyzed different depth sections of this ice core and confirmed that some of the ice is refrozen lake water. Using different techniques, they have found evidence for microorganisms in this ice. Priscu et al. found phylotypes related to extant members of the alpha- and beta-Proteobacteria and the Actinomycetes. Karl et al. determined that the bacteria may be viable based on respiration rates during incubations. Thus, Lake Vostok may support a low nutrient and low mass microbial population, despite being isolated from the atmosphere for more than 1 million years and may provide an analog for biogenic conditions during a snowball Earth event (complete glaciation of Earth's surface) or the possible ocean beneath Europa, a moon of Jupiter.

  4. Getting to Grips with Carbon Nanotubes

    The ability to manipulate and move nanoparticles in three dimensions is a key requirement for nanotechnology. Kim and Lieber (p. 2148; see the Perspective by Mirkin) introduce a manipulation device constructed from two carbon nanotubes attached to either side of a glass rod. They are able to open and close a gap between the tubes by applying a voltage to the nanotubes, rather like a miniature pair of tweezers, and use this device to move around clusters of small particles. In addition, the conducting nanotubes can be used to probe the electronic properties of the clusters they hold.

  5. Tale of Tubby

    One approach to characterizing genes of unknown function is structure-based functional genomics, in which the three-dimensional structure of the encoded protein or of one of its domains is determined and then used to design experiments to search for function. Boggon et al. (p. 2119) applied this approach to the gene tub, which has been linked to obesity and retinitis pigmentosa. The crystal structure of its carboxyl-terminal domain reveals a β-strand barrel containing a central helix that presents a putative DNA binding groove on its surface. Further studies indicate a propensity for binding double-stranded DNA, localization to the nucleus of neurons, and capacity to serve as a transcriptional activator. Reconciling these qualities led to the working hypothesis of tubby as a cell-type specific regulator of gene expression in the mammalian brain.

  6. Assembling Transcription Complexes

    A key step in eukaryotic transcription is the recognition of specific sequences in the promoter region by multisubunit transcription factors, which nucleate the assembly of the pre-initiation complex that includes general transcription factors and RNA polymerase II. Structures of the multisubunit transcription factors TFIID and TFTC are described at 35 angstrom resolution in two reports. TFIID is composed of TATA binding protein (TBP) and TBP associated factors (TAFs). TFTC does not contain TBP but does regulate transcription of promoters containing the TATA sequence. Andel et al. (p. 2153) describe the structure of a TFIID complex alone and in complex with the transcription factors TFIIA and TFIIB, and Brand et al. (p. 2151) compare the structures of a TFIID and a TFTC complex. Both TFIID and TFTC form horseshoe-shaped structures with a cavity large enough to accommodate DNA.

  7. Antibodies that Herd Pathogens

    Even mice that are raised in sterile environments have antibodies. The physiologic role of “natural antibodies” is not clear. Ochsenbein et al. (p. 2156) report that natural antibodies contain specificities for various pathogens. When mice lacking these antibodies are infected, the viruses or bacteria become widely disseminated throughout multiple organs. Addition of antibody-containing normal mouse serum to these mice changes the localization of the pathogens, which are then concentrated in the spleen and lymph nodes. Thus, natural antibodies seem to prevent wide dissemination of the virus and target it to the organs of the immune system, where an immune response is initiated.

  8. Finding the Right Place to Work

    How do B cells and T cells coordinate their movements so that lymphocytes with the appropriate specificities are at the right place at the right time? Specific interactions do occur, and Cyster (p. 2098) reviews how the chemokine highway helps direct traffic. Chemokines act as attractants and are expressed in temporally and physically discrete fashion. The receptors for chemokines are expressed only by those cells best able to mount an immune response. A specific case of immune cell localization is presented by Randolph et al. (p. 2159), who report that different subsets of T cells tend to be in discrete locations in the spleen, an organ that also hosts B cells. This T cell localization depends on the chemokine receptor CCR7, a receptor for the chemokine SLC, which TH1 cells express. If TH2 cells are forced to express CCR7, not only do they migrate to the wrong spots in the spleen, but they no longer provide help for B cells. Thus, CCR7 seems critical for appropriate localization of T cells to ensure an adequate immune response.

  9. A Minimalist Approach to Life

    What is the minimum number of genes needed to make a living organism? Hutchison et al. (p. 2165) have been exploring this question by using transposon insertions to knock out genes of the fully sequenced organism, Mycoplasma genitalium, which has only 470 genes, and its relative, M. pneumoniae. They found only 255 to 340 of those genes, many with unknown function were required for growth under laboratory conditions. The ethics of using this information to construct a living organism in vitro are discussed by Cho et al. in a Policy Forum.

  10. Creating Corneas

    An anatomically complete human corneal equivalent has been constructed by Griffith et al. (p. 2169; see the news story by Ferber) in vitro from immortalized cells and an artificial matrix. This artificial cornea responded like a natural cornea to osmoregulation, changes in gene expression in response to a surfactant, and in changes of corneal transparency in response to chemicals. Immediate applications include drug testing and research, and the long-term goal would be to create corneas for implants or transplantation.

  11. Not Gone to Seed

    Supra-annual synchronization of seed production, or mast-fruiting, occurs in many plant species, but its causes and adaptive significance have been a matter of much debate. In an extensive and long-term study of more than 50 species of dipterocarps—the dominant tree family in Bornean rainforests—Curran et al. (p. 2184; see the Perspective by Hartshorn and Bynum) demonstrate that seed production in these trees occurred only during the El Niño-Southern Oscillation (ENSO) event and that satiation of seed predators is the probable selective force favoring mast-fruiting. Disturbingly, the authors also report a total failure of dipterocarp seedling recruitment in a major Bornean national park caused by too extensive logging in the neighboring unprotected forest. This finding implies that southeast Asian rainforests might not persist even in protected areas.

  12. Slow Earthquakes

    Slow earthquakes are rarely observed and poorly understood events where the rupture propagates much more slowly than for ordinary earthquakes. Crescentini et al. (p. 2132) used a laser interferometer located beneath the Apennines mountain range in central Italy to record the changes in extension of the interferometer's two orthogonal baselines during several seismic swarms, including the swarm that proceeded the Umbria-Marche earthquakes in 1997. They observed about 180 slow earthquakes and derived a novel scaling law between the strength of the events (seismic moment) and the duration of the rupture. These observations and relations should improve our understanding of the rupture mechanism and the physics of slow deformation of the crust.

  13. Class Action

    Proteasomes generate the appropriate peptides for display on major histocompatibility complex (MHC) class I proteins. The protein PA28 binds to proteasomes and increases their activity. Preckel et al. (p. 2162) made mice that were deficient in PA28b, one of the two subunits of PA28, and found that neither subunit was detected in these mice and that the types of peptides that bound to MHC class I had changed. The lack of PA28 seemed to affect the assembly of “immunoproteasomes,” those protease complexes that contain the interferon-γ-regulated subunits and that efficiently produce peptides that bind class I.

  14. Improving Mouse Models of Cancer

    Neurofibromatosis type I (NF1) is a prevalent inherited cancer syndrome caused by germ line mutations in the NF1 tumor suppressor gene. Patients with NF1 develop neurofibromas (benign peripheral nerve sheath tumors) that can progress to malignancy. Mice bearing heterozygous mutations in NF1 do not develop neurofibromas and thus have provided little insight into the disease. Cichowski et al. (p. 2172) and Vogel et al. (p. 2176) have developed more sophisticated mouse models that better mimic human NF1, one based on a chimerism strategy and the other on combined mutations in NF1 and p53. These models will be useful tools for studying NF1 pathogenesis and for testing new therapies.

  15. Gene Expression on the Fly

    DNA microarrays have been used for monitoring gene expression in single-cell organisms and tissue culture analyses. White et al. (p. 2179) have now applied this approach to a multicellular organism, the fruit fly Drosophila, which undergoes dramatic changes through metamorphosis that are largely triggered by two pulses of the hormone ecdysone. The up- and down-regulation of genes involved in processes as varied as metabolism, myogenesis, nervous system development, and apoptosis occurred during these ecdysone exposures. This analysis has revealed many unexpected fluctuations in the expression of known genes as well as newly identified genes that show ecdysone-affected expression patterns.

  16. Atmospheric CO2 in the Eocene

    Pearson and Palmer (Reports, 11 June, p. 1824) used boron isotopes to infer past ocean pH during the Cenozoic. Because ocean pH is related to carbonate chemistry in the ocean, which is affected by equilibrium with atmospheric carbon, they then related the pH to past atmospheric CO2 levels. One of their conclusions was that atmospheric CO2 levels during the Eocene, a time when Earth's climate was much warmer than it is today, may have been similar to modern levels or slightly higher. In a pair of Technical Comments, Caldiera and Berner, and Sundquist critique some of the assumptions involved in using ocean pH to assess atmospheric CO2 levels and conclude that the inferred atmospheric CO2 levels were likely too low. In response, Pearson and Palmer note that their assumptions were conservative but highlight the uncertainties involved in this calculation and the need for additional information. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/286/5447/2043a

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