This Week in Science

Science  23 May 1997:
Vol. 276, Issue 5316, pp. 1169
  1. Warming the early Earth

    Earth's atmosphere 4 billion years ago has been thought to have had a neutral oxidation state, and given estimates of the sun's energy output, the average surface temperature of Earth would have been near or below freezing for water. However, there is evidence that methanogens appeared on Earth's surface about 3.8 billion years ago, suggesting a warmer planet with liquid water. Sagan and Chyba (p. 1217; see the Perspective by Kasting, p. 1213) attempt to resolve this paradox by suggesting that Earth's early atmosphere was more reducing, and that gases such as methane created a greenhouse effect. Methane photolysis may have created an organic haze in Earth's upper atmosphere that shielded ammonia in the lower atmosphere from ultraviolet photodissociation.

  2. Third-order efforts

    Symmetry effects limit the materials that can generate second-order harmonic signals (combining two photons, such as in frequency doubling). Such restrictions do not apply to third-order effects, which also have important uses in optics, but the second-order hyperpolarizability γ of most materials needed for third-order effects are usually quite small. Garito and co-workers have suggested a way to increase γ in organic molecules, and now Marder et al. (p. 1233; see the news story by Service, p. 1195) have tested these ideas experimentally. Linear π -conjugated molecules that had increasing intramolecular charge transfer showed enhanced three-photon resonances.

  3. Smoke signals

    When wildfires burn across a natural landscape, the original vegetation may be completely destroyed, but new vegetation soon takes its place, usually with a predictable pattern. Keeley and Fotheringham (p. 1248; see the news story by Malakoff, p. 1199) studied the germination of an annual plant typical of the California chaparral. Unlike some post-fire plants, whose germination is stimulated by high heat, seeds of Emmenanthe are stimulated by chemicals in the smoke. Whether the smoke is fresh or stale, the active components, which can independently induce germination of Emmenanthe, are nitrogen oxides.

  4. Europa, inside and out

    The Galileo orbiter had two close encounters with Europa on 19 December 1996 (E4) and on 20 February 1997 (E6). Kivelson et al. (p. 1239) analyzed perturbations in Jupiter's magnetosphere measured during E4 and suggest that Europa has a dipole moment of about 240 nanoteslas oblique to the jovian magnetosphere and possibly related to a core dynamo. Anderson et al. (p. 1236; see the cover) analyzed the external gravity field obtained during E4 and E6 and suggest that Europa has an icy to liquid water exterior shell of about 100 to 200 kilometers thick with a rocky to metallic interior. A metallic core is favored if Europa does have a magnetic field.

  5. Rab role

    Transport of proteins within the cell occurs in vesicles that bud from one compartment and specifically recognize and fuse with another compartment. The recognition is mediated by vesicle proteins (called v-SNAREs) that interact with proteins in the target membrane (called t-SNAREs). The small guanosine triphosphatases known as Rabs are required in this process, but their precise role has been unclear. Lupashin and Waters (p. 1255; see the Perspective by Rothman, p. 1212) studied the process in yeast cells and obtained evidence that the t-SNARE is held in an inactive state (in which it cannot bind the v-SNARE) by another protein called Sly1p. The Rab protein then transiently interacts with the t-SNARE, which relieves the inhibitory effect of Sly1p and allows the formation of a t-SNARE-v-SNARE complex and subsequent fusion of the membranes.

  6. Not easily fooled

    Learning at times can seem effortless, but how does it happen? Berns et al. (p. 1272) imaged areas of the brain that were activated when subjects were first trained on a number sequence based on a definite grammar and then confronted with a second, slightly different grammar—so slight that the subjects are not consciously aware of the difference. Activity in one of these areas, the ventral striatum, peaked during learning of the first grammar, and then peaked at a significantly higher level during training on the second. Other studies have suggested that the striatum participates in encoding probabilistic expectations, and these results suggest that it may do so through detecting novelty beneath awareness.

  7. Tired cats

    Why do we fall asleep? Porkka-Heiskanen et al. (p. 1265) demonstrate that, in cats, adenosine mediates the inclination toward sleep brought on by prolonged wakefulness. The extracellular concentration of adenosine increases in proportion to the duration of wakefulness and diminishes during subsequent periods of recovery sleep. Neuronal circuits known to support arousal, as measured by electroencephalograms, are known to be inhibited by adenosine, which is a by-product of metabolic activity.

  8. Cell profiles

    Understanding the differences between cancer cells and normal cells will not only add to our fundamental knowledge of cancer, but it can also lead to new therapeutics and markers to aid in diagnosis. Zhang et al. (p. 1268) used the technique they had previously developed, serial analysis of gene expression (SAGE), to examine more than 300,000 transcripts from 45,000 genes. More than 500 transcripts were expressed at significantly different levels in neoplastic cells from gastrointestinal tumors as compared to normal cells.

  9. Day in, day out, nitrogen in, oxygen out

    The colonial marine cyanobacterium Trichodesmium has puzzled marine biologists because of its unusual ability to fix atmospheric nitrogen, usually an anaerobic process, while it generates oxygen through daytime photosynthesis. Capone et al. review the state of our understanding of this important organism, which helps provide biologically available nitrogen, a limiting nutrient in the open ocean. Its diurnal pattern of nitrogen fixation represents an important example of a simple organism with an endogenous rhythm.

  10. Improving the angles

    In nuclear magnetic resonance studies of proteins, angles between bond vectors are usually determined by measuring scalar coupling constants and calibrating them with an empirical curve. However, this approach is constrained to adjacent bonds (that is, torsion angles), and multiple solutions are possible. Reif et al. show that this ambiguity can be reduced by measuring the cross-correlated relaxation rates for the dipolar coupling between the four nuclei that participate in two different bonds. Empirical calibration is not needed, and the bonds need not be adjacent.

  11. Soft diamonds and strong metals

    Diamond-anvil cells are routinely used in high-pressure experiments to determine the properties of materials, particularly the properties of rocks at pressures of hundreds of gigapascals that correspond to Earth's lower mantle and core. At these high pressures, there is much concern about the condition of the diamonds and associated metal gaskets that are squeezing the sample. Hemley et al. have determined the three-dimensional stress-strain distributions of the diamonds and the metal gaskets by developing a technique to observe synchrotron x-ray beams along different cell geometries. They found that the diamonds deform elastically at high pressures, and they then used this information to determine that the yield strengths of tungsten and iron increased beyond theoretical expectations at 200 to 300 gigapascals.

  12. Oxygen diffusion and melt structures

    The diffusivity of oxygen in molten rock is related to its viscosity, a key dynamical property. The variation of melt structure and viscosity with pressure has been difficult to measure and is important for understanding the mobility and behavior of melts in Earth's mantle. Poe et al. measured oxygen diffusion at high pressures in several silicate melts to infer the effect of increasing pressure on viscosity. The results show that oxygen diffusivities in a polymerized aluminosilicate melt pass through a maximum between 5 and 8 gigapascals, corresponding to a change in melt structure.

  13. Correcting mistakes

    During protein synthesis, the enzyme tRNA synthetase attaches an amino acid to its corresponding tRNA molecule, a process called aminoacylation. The difficulty in discriminating between amino acids of similar structure can cause errors in acylating the tRNA that are corrected by a translational editing reaction. By constructing a series of chimeric tRNA molecules, Hale et al. showed that the tRNA nucleotides involved in aminoacylation are distinct from those needed for translational editing.

  14. Dividing into sets

    A critical aspect of all cell divisions is the separation of chromosomes into two equivalent sets, one for each daughter cell. Meiosis is further complicated by recombination and separation of the genetic complement into haploid sets. Failure in these events can lead to non-disjunction of chromosomes, such as occurs in human Down syndrome. Conrad et al. have identified a protein in yeast, the nondisjunction 1 protein (Ndj1p), that seems to facilitate interactions of homologous yeast chromosomes, thereby allowing chromosomal recombination to occur. Yeast cells lacking Ndj1 are unable to divide the chromosomes efficiently into equivalent sets during meiosis. Ndj1 is abundant at chromosomal telomeres.

  15. Ties to promoters

    One of the first steps of transcription initiation involves the separation of the duplex DNA strands so that the DNA template can be exposed to RNA polymerase. Marr and Roberts examined the protein-DNA interactions involved in transcription initiation at the late gene promoter PR′. Escherichia coli RNA polymerase bound specifically to the nontemplate hexameric sequence centered at −10, below the transcription start site. Mutagenesis of either this −10 sequence element or conserved region 2 of sigma factor 70, a subunit of E. coli RNA polymerase, reduced binding of polymerase to the promoter. The binding of RNA polymerase to the nontemplate promoter sequence highlights the specificity of this protein-DNA interaction.

  16. Pliable porins

    The ferric enterobactin receptor, FepA, acts as a pore through which iron and antimicrobial agents are transported through the outer membrane of Gram-negative bacteria. In vitro studies had suggested that porins like FepA form rigid, nonmoving channels. However, Jiang et al. used site-directed spin labeling and electron spin resonance spectroscopy to look at FepA in living cells and found that the molecule can shift between different conformations.

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