This Week in Science

Science  07 Nov 2003:
Vol. 302, Issue 5647, pp. 945
  1. Martian Soil Analogs

    The Viking landers' analysis of martian soil detected no organic species and rapid release of O2. It was concluded that the martian soil was dry and that any organic species were rapidly oxidized by inorganic oxidants. Navarro-González et al. (p. 1018) have found a terrestrial analog for martian soil in the Atacama desert, Chile. The Atacama soil is dry, has minute traces of organic species below the level of detection of the Viking landers, and is an effective inorganic oxidant. Repetition of the Viking experiments with the Atacama soils indicates the landers may have missed minute traces of organics because of a lower temperature setting for the analyses. Experiments with the Atacama soils may also help identify the still-unknown soil oxidant.

  2. Coupling Molecular Magnets

    One of the proposed architectures for quantum computing, the use of nanomagnets as the spin-based building blocks for qubits, is a step closer experimentally. Hill et al. (p. 1015) show that pairs of single molecular magnets (SMMs) can be assembled to form coupled quantum systems that exhibit coherent superposed states and sufficiently long coherence times that could enable quantum computing applications. The SMM approach may also offer a bottom-up, self-assembled approach to fabricating a quantum computer architecture.

  3. Nitrate Under the Desert

    Desert soils, which cover nearly one-third of the land area of the world, are normally thought to be relatively low in nitrogen. Walvoord et al. (p. 1021; see the news story by Stokstad) discovered a large pool of nitrate below the soil zone in a variety of deserts in the United States. This nitrogen, which has apparently accumulated during the last 10,000 to 16,000 years, accumulated when these regions have been arid. This nitrogen represents a bioavailable pool that could be flushed out by land management practices, such as irrigation and the construction of reservoirs, or by the increased precipitation that is expected to accompany global warming. Such removal would have important consequences for ecosystems and for regional groundwater quality, particularly given the steep increases in water use and development in arid and semiarid areas worldwide.

  4. Flexible Sex and Suspended Animation in Nematodes

    There are two Caenorhabditis elegans sexes—male and self-fertilizing hermaphrodite, and males generally represent about 0.1% of a population. Sex is generally determined at fertilization by the ratio of X chromosomes to autosomes. Prahlad et al. (p. 1046) now show that sex determination in cross-fertilized C. elegans is plastic and that sexual development can be altered postembryonically by exogenous factors. This ability to switch sexual phenotype and karyotype might allow the worm to optimize its development for the environmental conditions encountered. In another example of physiological flexibility, C. elegans can endure adverse environmental conditions by entering into an extreme, but temporary, state of quiescence called suspended animation. Nystul et al. (p. 1038) report that for C. elegans to survive through this state in response to severe oxygen deprivation, two components of the mitotic spindle checkpoint pathway are required. In the absence of either of these factors, cells proceeded through metaphase but exhibited chromosomal missegregation. By engaging this mechanism of cell cycle arrest, the organisms ensure genome stability and survival until environmental conditions improve.

  5. Not All Our Fault

    Human-induced environmental degradation or meteorological effects resulting from sea surface temperature (SST) variations have been thought to have caused the persistent drought in the African Sahel during the 1970s and 1980s. The first hypothesis blames local precipitation decreases on increased land surface albedo, caused by changes in vegetation cover resulting from the expansion of farming and herding, and the second invokes processes far beyond human control (see the Perspective by Zeng). Giannini et al. (p. 1027; see the 10 October news story by Kerr) reconstruct the precipitation history of the Sahel between 1930 and 2000, using an atmospheric general circulation model forced with only observed SSTs. A weaker continental monsoon, caused by a warming trend in the distant western equatorial Indian Ocean, is the primary reason for lower-than-normal rainfalls during the 1970s and 1980s. Land-atmosphere interactions are of second-order importance as amplifiers of the effects of remote ocean conditions. Dust production and climate are closely interrelated, and it is typically assumed that variations in the amounts of atmospheric dust during the past several hundred years are mainly the result of human activity. Prospero and Lamb (p. 1024) analyzed a record of dust transport out of Africa for the past 30 years and found that dust emissions from this region have been unusually intense because of widespread and persistent drought. The increase in dust emissions during the 1970s and 1980s was much greater than the increase in pollution emissions from the United States and Europe during the same period. Additionally, the annual variability was very large. These results indicate that natural processes are the major sources of atmospheric dust variability.

  6. In the Shadow of Mass Extinctions

    The biota that survive mass extinctions often undergo significant evolutionary change. For some of the extinctions of the Phanerozoic Eon, recovery taxa developed a broader geographic distribution. Macroevolutionary theory suggests that widely distributed taxa should persist for longer geological intervals. Miller and Foote (p. 1030), using a genus-level compendium of fossil marine animals, now test the hypothesis that recovery taxa, because of their wider distribution, should persist for longer times. For certain cohorts of species, recovery taxa did indeed persist for longer intervals, a finding which might be more generally true for taxa that have recovered from other periods of elevated extinction.

  7. Third Part of a Light Motif

    Three integral membrane proteins, the photosystem reaction centers I and II (PSI and PSII) and the cytochrome b6f complex, work together to generate the transmembrane electrochemical proton gradient that is used to transduce energy in photosynthesis. Kurisu et al. (p. 1009) have determined the 3.0 angstrom crystal structure of the dimeric b6f complex from a thermophilic cyanobacterium. Three-dimensional structures have already been elucidated for PSI and PSII, so this study provides the missing piece of structural information about the components of oxygenic photosynthesis. The core of the b6f complex is similar to the analogous respiratory cytochrome bc1 complex. However, the structure reveals differences, including a new heme cofactor.

  8. Junk DNA Passed Down Through the Ages

    How important are the noncoding regions of the human genome in evolution? Dermitzakis et al. (p. 1033; see the Perspective by Johnston and Stormo) examined a set of 191 nongenic regions on human chromosome 21 and found that they were even more conserved than protein-coding regions in 14 mammalian species. Patterns of nucleotide substitution were different from that seen in protein-coding and noncoding RNA genes, which suggests that particular evolutionary constraints may have been in operation. The extent of conservation indicates that there is some function for these regions that remains to be determined.

  9. Heparan Sulfate in Brain Development

    Heparan sulfate binds to a number of growth factors and morphogens and is highly expressed in the developing mammalian central nervous system (CNS). In order to elucidate heparan sulfate's role in brain development, Inatani et al. (p. 1044) selectively knocked out heparan sulfate synthesis in the developing mouse central nervous system. Mutant mice exhibited malformations in specific regions of the brain that corresponded to disrupted distribution of fibroblast growth factor and decreased cell proliferation. Axon pathfinding in the brain and retina was also disrupted, pointing to additional regulatory functions of heparan sulfate.

  10. Allocating T Cell Fate

    The attributes of helper T cells are governed by two master transcriptional regulators, GATA-3 and T-bet, which coordinate the expression of specific cytokine genes. However, neither factor appears critical in CD8+ T cell function, leading Pearce et al. (p. 1041; see the Perspective by Hatton and Weaver) to explore other pathways that might specify the transcriptional program in these cells. This trail ended in the identification of Eomesodermin (Eomes), a T-box transcription factor related to T-bet and already characterized as a key regulator of mesodermal differentiation. Eomes was specifically up-regulated in activated CD8+ T cells and forced cellular expression of the factor conferred CD8+ characteristics on cells that had already differentiated into T helper 2 type cells. Disruption of expression significantly reduced the CD8 cytolytic program and interferon gamma expression. Thus, Eomes is likely to cooperate with T-bet in regulating distinct aspects of cell-mediated immunity.

  11. Ozone and Atherosclerosis

    Atherosclerosis is now recognized as encompassing a complex inflammatory etiology, in which mediators of atherosclerotic plaque formation are induced by products of oxidized cholesterol. Wentworth et al. (p. 1053; see the news story by Marx) identify novel oxidized sterol species from atherosclerotic plaque tissue and provide evidence that these are formed by ozone-mediated oxidation of cholesterol. The two oxysterols displayed activity consistent with a pathogenic role in vivo and were present in sera of patients with severe atherosclerosis. Ozone production was detected in plaque tissue after exposure to mitogen, which suggests a direct link between immune activation and the generation of the novel oxidant and its products.

  12. Inheritance of Circadian Clocks in Plants

    The circadian clock plays a role in the physiology of plants in their responses to light-dark cycles and to changes in season. Michael et al. (p. 1049) examined the genetic inheritance of the circadian clock. Circadian clock parameters varied among a group of Arabidopsis plants collected from diverse geographic origins. Circadian clock period length correlated with source latitude, suggesting functional importance of clock period in adaptation to the environment. One set of plants displayed no overt circadian rhythm in leaf movement and may represent a naturally occurring clock loss of function mutant. Two common laboratory forms of Arabidopsis display quite similar circadian parameters, probably as a result of the additive effects of multiple loci. The correlation of period length and latitude is consistent with a role for the clock in adaptation to the local environment.

  13. Understanding Killing

    The tumor supressor p53 acts in part by causing cell death or apoptosis of tumor cells. p53 appears to act through transcriptional activation of certain genes, but those that specifically mediate the apoptotic response have not been identified. Villunger et al. (p. 1036) suspected two genes, puma and noxa, because they are targets of p53 regulation and encode proteins with structural similarity to other pro-apoptotic proteins. Their analysis of knockout mice lacking the Puma or Noxa proteins shows that death induced by agents that cause DNA damage (and thus activation of p53) is inhibited in fibroblasts lacking Noxa and in fibroblasts and lymphocytes lacking Puma. Lack of Puma also conferred resistance to other death-inducing stimuli that do not act through p53. The findings help to explain both how the p53 tumor suppressor functions and how certain chemotherapeutic agents help battle cancer cells.

  14. HIV Hits Back

    The cytidine deaminase, APOBEC3G, defends cells against retroviruses like the virus responsible for AIDS, human immunodeficiency virus type-1 (HIV-1), by converting cytidine residues to uracil in the minus strand of DNA copied from viral RNA. The resulting virions are unfit for future replication. As a countermeasure, HIV-1 has exploited the viral protein Vif, which interferes with APOBEC3G through mechanisms that have not been clear. Yu et al. (p. 1056) observed that Vif forms a complex with some of the cell's own ubiquitin-protein ligases and thus targets APOBEC3G for proteolytic degradation. By ensuring the destruction of APOBEC3G before it can be incorporated into virions, Vif effectively helps HIV-1 avoid damage to its own genome.