This Week in Science

Science  04 Jul 2003:
Vol. 301, Issue 5629, pp. 13
  1. The Makeup of the Moon

    The Moon is believed to be made up of material from both the nascent Earth and a Mars-sized impactor. Münker et al. (p. 84) constrained their relative contributions to the Moon by re-examining the niobium and tantalum ratios in a variety of terrestrial and planetary-body samples. These elements have been thought to be retained in a constant and primordial (or chondritic) ratio in Earth's mantle, but the authors found a subchondritic ratio for the bulk silicate mantle of Earth and the Moon. The authors conclude that the Moon has less than 65% by weight of the impactor material, and that the Giant Impact occurred 4.553 billion years ago.

  2. A Model of Efficiency

    Nitrogenases reduce dinitrogen to ammonia at ambient conditions, and chemists have tried with limited success to mimic this reaction with simple metal complexes. The reaction is difficult even for the enzymes—the best nitrogenases are only about 75% efficient because some of the reducing agents supplied by the cell are spent in the unproductive evolution of hydrogen. Yandulov and Schrock (p. 76; see the Perspective by Leigh) re-examined a class of molybdenum complexes that can react to form ammonia that have large ligands that shield their single metal centers. The authors found conditions that made this nonaqueous system catalytic and show that the slow addition of protons and reducing agents led to efficiencies of 66% in terms of reducing agent.

  3. Sorbing Sulfur from Fuels

    The catalytic converters in automobiles can tolerate only trace amounts of sulfur impurities in fuels. Oil refiners use a hydrodesulfurization step to remove aliphatic sulfur compounds, but this process is less effective at removing aromatic sulfur species such as thiophene and its derivatives. Yang et al. (p. 79) show that zeolite molecular sieves containing copper or silver ions reduced the amount of sulfur impurities in a diesel fuel to 0.2 parts per million by weight, which a factor of 30 less than the fuel standard to be imposed in the United States in the next few years.

  4. Firing and Wiring

    Molecular cues guide the general direction of axon travel between brain areas, but correlated activity likely refines the final connections. To test whether neurons that “fire together, wire together,” Ruthazer et al. (p. 66) redirected axons growing from the frog eye so that both eyes would innervate the same optic tectum. In this competitive situation, eye-specific regions were set up in the tectum. Thus, higher correlation among same-eye axons led to connections being formed in adjacent territory. Axons in regions dominated by opposite-eye innervation showed a higher rate of elimination of new branches. A computer simulation reproduced the generation of eye-specific territories resulting from these differing addition and elimination rates.

  5. Slipping Under the Ice

    Fast slip occurs in glaciers that are melting at their bases. What prevents such glaciers from accelerating rapidly down-slope like a melting ice block on an inclined plane? For glaciers that rest on hard rock beds, bumps on the bedrock surface are believed to exert drag on basal ice, and for glaciers that rest on soft rock beds of unlithified sediments, the slip rate is supposedly limited by how quickly the bed can shear. Iverson et al. (p. 81) present results from experiments conducted beneath the Svartisen ice cap in northern Norway, where tunnels in the rock beneath the glacier provide access to the bed beneath 210 meters of sliding ice, that may prompt re-thinking of glacial movement. For hard beds, friction between debris in the ice and the underlying bedrock was high enough to account for most of the total resistance to basal movement of the glacier. For a soft bed, when pore-water pressure in the sediment bed was high, the ice slipped across the bed rather than shearing it.

  6. Climate Change on the Rocks

    The removal of atmospheric CO2 by the chemical weathering of silicate rocks is the ultimate arbiter of atmospheric CO2 concentrations over time scales of millions of years, but this process is invoked less often to explain possible anthropogenic effects on climate. Raymond and Cole (p. 88; see the Perspective by Ittekkot) show that export of alkalinity from the Mississippi River is responding to increased rainfall, and that alkalinity export rates for the Mississippi basin are linked to patterns of land use. Chemical weathering is responding to increased rates of precipitation that also affect global climate change.

  7. How Tumors Keep Their Blood Vessels Flowing

    Solid tumors usually feed their own growth by producing factors that stimulate the formation of new blood vessels. However, the tumor microenvironment also harbors factors that should promote the apoptotic death of the endothelial cells (ECs) that comprise these new vessels. Alavi et al. (p. 94) find that ECs are protected from both intrinsic and extrinsic pathways of apoptosis because they activate Raf-1 kinase. Given its critical role in cell survival, the Raf-1 kinase is a potentially target for anti-angiogenesis drugs.

  8. Perceptual Learning and Brain Reorganization

    In Hebbian learning, repeated events (such as feeding and the ringing of a bell) should produce increases in synaptic strength. Dinse et al. (p. 91) analyzed the cortical changes underlying such perceptual learning by using a lengthy period of finger tip stimulation at randomly assigned intervals of 100 to 3000 ms in human subjects. As expected from previous studies, tactile two-point discrimination was improved after this intense experience on the side that was stimulated but not on the unstimulated side. Improvement was blocked by memantine, an NMDA-receptor blocker, and enhanced by the psychostimulant amphetamine, which operates through the increased release of modulatory neurotransmitters. By combining measurements of somatosensory evoked potentials in primary somatosensory cortex with tactile discrimination thresholds, the authors showed a close correlation between the amount of coactivation-induced perceptual improvement and the degree of individual cortical reorganization.

  9. Comparing the Noncoding Parts of Genomes

    Comparative genomics has often focused on the long coding regions of genes, but the shorter noncoding elements hold important clues to gene regulation. Cliften et al. (p. 71) generated two-to threefold shotgun sequence coverage of five Saccharomyces species and compared them to the S. cerevisiae genome sequence with the hope of identifying conserved noncoding motifs such as gene regulatory elements. Certain conserved motifs were associated with particular patterns of gene expression, and it was possible to predict the binding sites for certain transcription factors. Despite the large number of transcription factors in these organisms, many of the sequences of regulatory regions found were already known, although some new candidates were identified.

  10. Hair-Trigger Responses

    Underlying the sense of hearing and balance are hair cells that transduce the mechanical deflection of their hair bundles into electrical signals through mechanosensitive ion channels, which open and trigger an inward transduction current. Sidi et al. (p. 96) have identified a transient receptor potential cation channel called NompC in zebrafish sensory hair cells as a core component of this mechanosensory machinery. Mutants lacking functional NompC no longer responded to acoustic stimuli. Invertebrate orthologs of NompC were previously implicated in hearing, but a vertebrate mechanosensor has remained elusive.

  11. Taken to the Limit?

    Can organisms always evolve in response to selective pressure? Hoffmann et al. (p. 100; see the Perspective by Roff) found that resistance to desiccation in a rainforest fruit fly did not budge after 30 generations of intense selection, even though this trait is easily selected in other species and despite ample genetic variation in this fly. Thus, specialist species already at their environmental limits may not be able to adapt to further change. This result raises concerns for conservation priorities and may affect models for how populations adapt to climate change.

  12. Profiling Cell Networks via Perturbations

    The properties of a regulatory network have been determined without the exhaustive examination of each experimental parameter. Gardner et al. (p. 102) explored a network of nine genes in the SOS pathway in Escherichia coli that regulates cellular responses to DNA damage and other stresses. Relatively small changes made near a physiological steady-state point (like that of a cell maintained in culture) allowed these nonlinear cellular processes to be modeled linearly. The transcription activity of each component of the network was altered, and the effects on the abundance of messenger RNA transcripts of the other components were measured. After making some assumptions about the limits of network connectivity and regulatory inputs for each gene, they could identify most of the known regulatory connections in this well-studied pathway.

  13. Sealed Off from Immune Surveillance

    Recurring bladder infections can be caused by certain strains of Escherichia coli. Anderson et al.(p. 105; see the cover and the news story by Ferber), working in a mouse model, found that these pathogens cause infections within the epithelial cells of the bladder. The bacteria multiply and differentiate to form distinct “pods” filled with bacteria that protrude from the bladder wall. The pods are sealed off from host immune responses and, like the bladder epithelium, are coated with uroplakin, which probably renders them impermeable.