This Week in Science

Science  07 Jan 2000:
Vol. 287, Issue 5450, pp. 9
  1. Solitons in Bose-Einstein Condensates

    The atoms that constitute a Bose-Einstein condenstate all occupy the same quantum state. In other words, a single wave function can be used to describe the entire macroscopic pool of atoms. Denschlag et al. (p. 97) now demonstrate the ability to engineer and manipulate the quantum wave function of a sodium atom condensate. They use a patterned laser beam to stamp, or imprint, a region of the condensate with a specific phase pattern. In analogy to optical solitons, which propagate through a nonlinear medium without spreading out, they show that the modified region can propagate through the rest of the condensate while maintaining its imprinted phase pattern.

  2. Connecting the Eye

    We see the world as a reasonable approximation of reality at least in part because axons extending from the retina search out specific regions of the optic tectum in the brain, thus mapping the visual world to the perceived world. Koshiba-Takeuchi et al.(p. 134) show that the gene Tbx5 is responsible in the chick for generating dorsal-ventral polarity in the eye and for directing the targeting of retinal axons. Tbx5 seems to be a regulator of other genes previously implicated in the process.

  3. Watching Diamond Grow

    Diamond synthesis by chemical vapor deposition onto diamond or nondiamond substrates enables the use of relatively low pressures and temperatures. However, many details of the growth process are not well understood, especially for heteroepitaxial growth. Lee et al. (p. 104) have used high-resolution electron microscopy to image the interfaces between small diamond crystallites and the underlying silicon substrate. Ideal epitaxial alignment between the crystallites and the substrate was observed at steps on the silicon substrate. This identification of the diamond nucleation site may help in devising protocols for growing heteroepitaxial diamond films.

  4. A Light Universe

    The density of non-relativistic particles in the universe (characterized by the parameter W) can be estimated from the Hubble parameter (the speed at which the universe is expanding) and the relative distances between galaxies. Unfortunately, neither of these parameters are well constrained or easily determined. Juszkiewicz et al. (p. 109) developed a method for determining the relative velocities between galaxies that does not require assumptions concerning mass distributions over specific volumes of space. They can derive W from the relative velocities of galaxies and have tested this method by estimating W from data gathered on thousands of galaxies in the Mark III survey. They find that W is about one-third, which is relatively low compared to the value of 1 for the standard cold dark matter model (the Einstein-de Sitter model). This approach could be used to refine independently the density of the universe by including other observations and ultimately to refine cosmological parameters that depend on W.

  5. Growing Bigger and Slower

    The supernova SN1993J discovered in the nearby galaxy M81 in 1993 has been observed extensively to determine the nature of the stellar explosion and its interaction with the surrounding medium. Bartel et al. (p. 112) obtained 20 radio images of the expanding shell of shocked material that has been exiting from the explosion. After about 2 months, the shell was a uniform “bull's-eye” with a radius 13 times that of our solar system, but after 5 years it has expanded to a complex shell about 20 times larger. Two hot spots now form a rotating horseshoe that has moved from the east to the south. In addition, the expanding shell has been decelerating during the last 4 years. These observations suggest that the supernova is changing from an isotropic to an adiabatic expansion.

  6. Charon, a Fountain of Youth

    The small moon Charon orbits Pluto so closely that it was not discovered until 1978, and only recently has Charon's surface spectra been isolated from that of Pluto with ground-based observations. Brown and Calvin (p. 107; see the Perspective by Young) obtained a separate spectrum of Charon with the Keck telescope at infrared wavelengths and identified crystalline water ice and possible ammonia ices on the moon's surface. Neither of these ices have been identified on Pluto, but the authors suggest that Pluto is massive enough to retain a nitrogen-rich atmosphere and frost that may be covering a water-rich surface. Crystalline water ice suggests a young (renewed) surface on Charon that has not been rendered amorphous by solar radiation. The presence of ammonia would lower the melting temperature of water ice and allow it to flow.

  7. Smads About You

    Smad proteins play a key role in transforming growth factor-β (TGF-β) signaling pathways, and structural studies can help unravel how related members of the Smad family can induce different signaling outcomes. The receptor-regulated Smads (R-Smads) are each involved in a specific signaling pathway, and the activation of specific TGF-β receptors mediates phosphorylation of specific R-Smads. Phosphorylated R-Smads heterodimerize with co-mediator Smads (co-Smads), and such complexes translocate into the nucleus and activate target genes. Smad2, an R-Smad that acts as a tumor suppressor in humans, is recruited to its TGF-β receptor by the protein SARA (Smad anchor for receptor activation). SARA does not interact with the R-Smads Smad1 or Smad5 despite an 80% sequence identity with Smad2. Wu et al. (p. 92) have determined a structure of the Smad2 MH2 domain, which is involved in receptor recognition, with the Smad binding domain of SARA that reveals the molecular basis for the specificity of the Smad2 interaction with SARA. Comparison of R-Smad and co-Smad structures provide insight into how R-Smads are recognized by receptors.

  8. Catalyst or Substrate?

    Although heme complex may be the most familiar iron-containing proteins, carboxylate-bridged species can play key roles in respiration and the oxidation of organic species, as well as in iron storage. Hwang et al. (p. 122) have studied the peroxodiferric intermediates of ferritin, which appeared to be spectroscopically similar to those in enzymes. X-ray absorption and Mössbauer studies reveal that in ferritin, the iron-iron distance in the peroxodiferric complex is unusually short—2.53 angstroms, instead of the expected 3 to 4 angstroms. This short distance should favor release of hydrogen peroxide and the formation of biominerals over oxidation of organic species.

  9. Sex Ratios in Malaria

    For the malaria parasite, a single haploid cell gives rise to a clone producing both females and males. The ratio of males to females is important in understanding and controlling disease because transmission of malaria in nature depends on sexual union of the parasites and because more males are formed in lethal infections. Paul et al.(p. 128) have found that the frequency of the sexes is affected by the host hematologic state. Treatments that induce erythropoiesis result in a shift to male parasites, which leads to decreased reproductive success. This finding may provide new approaches in malaria control as well as new considerations in therapy, as the antimalarial drug chloroquine inhibits erythropoiesis.

  10. Pulling Out Proteins

    Genome sequencing is producing a huge number of putative gene sequences, many of unknown function. Walhout et al. (p. 116, see the Perspective by Kim) now report the feasibility of using large-scale yeast two-hybrid analysis to examine the proteins that interact during vulval development in the worm Caenorhabditis elegans. They selected 29 proteins known to be crucial for vulval development as “bait” and then scanned the entire C. elegans genome. They found 992 proteins that interacted with the bait proteins. The investigators also characterized an important interaction between one of the bait proteins LIN-37 and a worm homolog of LIN-35, a component of a tumor suppressor transcription complex.

  11. DNA Replication and Neuronal Development

    Topoisomerases, which allow DNA strands to cross one another, are critical for successful DNA replication, yet cells with defective topoisomerase II-b (IIb) proliferate normally. Yang et al. (p. 131) now show that mice with IIb mutations have very specific defects in the nervous system—motor neurons develop and differentiate but fail to extend axons that reach their normal targets. These results may indicate a particular sensitivity of nondividing cells to deficiencies in DNA repair systems, or, alternatively, a specialized function of IIb in sustaining transcriptional programs.

  12. Mix-and-Match Protein Function?

    Voltage-gated H+ channels can be found in many different cell types throughout the body, but these channels have until now eluded all attempts at cloning. Banfi et al. (p. 138) report the identification of a gene that encodes a protein homologous to the enzyme that oxidizes the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH oxidase). A proton channel is generated through alternative splicing of messenger RNA derived from this gene, which suggests that a gene product is converted from a putative enzyme to an ion channel.

  13. Loss of Appetite

    Leptin is a hormone produced by fat cells that promotes weight loss by suppressing food intake and stimulating metabolism. The hypothalamus has been identified as the neuroanatomical target of leptin, and several molecular mediators of leptin action have been identified, but the neurobehavioral mechanisms underlying leptin's effects on food intake are unclear. In experiments with a rat model of intracranial self-stimulation, Fulton et al.(p. 125)show that leptin modulates brain reward circuitry by decreasing the appetitive value of food and increasing the value of other as yet unidentified behaviors that lead to increased energy expenditure. These opposing effects of leptin may help explain its role in energy balance.

  14. Recovering Equilibrium

    Statistical mechanics provides a powerful method for understanding systems at or near equilibrium. Egolf (p. 101) now shows that a theoretical system far from equilibrium, a chaotic, coupled-map lattice, can display equilibrium-like properties. The length scales associated with chaotic behavior in this system are much smaller than those used in the averaging processes needed to recover macroscopic properties. Thus, a dissipative chaotic system, observed on a sufficiently coarse scale, can recover features associated with equilibrium, such as Gibbs distributions, ergodicity, and detailed balance.

  15. G protein Orders Growth

    Heterotrimeric guanine nucleotide-binding proteins (G proteins) couple activation of G protein-coupled receptors on the cell surface to modulation of a large array of diverse biological effects. The subunits of the G proteins interact with effectors to bring about such responses. A member of the Gα subunit family, Gαo, when mutated to remain constantly in an active form (Gαo*), causes transformation of NIH 3T3 cells. Ram et al. (p. 142) explored the signaling pathways that mediate this effect of Gαo. They find that Stat3 (for signal transducer and activator of transcription 3) is activated in cells expressing Gαo*and appears to be necessary for the growth-promoting action of Gao*. Stats are transcription factors activated by tyrosine phosphorylation in response to cytokine receptors (which are not coupled to G proteins). In this case, however, Gαo seems to stimulate the tyrosine kinase c-Src, which in turn activates Stat3 and contributes to abnormal cellular proliferation.

  16. Non-Molecular Carbon Dioxide (CO2) Solids

    Iota et al. (Reports, 5 Mar., p. 1510) reported the high-pressure synthesis of a new quartzlike phase of CO2, which suggested that non-molecular (polymeric) CO2 phases could be produced at high pressure. Subsequently, Serra et al. (Reports, 30 Apr., p. 788) supported this result using molecular dynamics simulations.

    Dong et al. comment that, on the basis of other molecular dynamic simulations, the high-pressure phase is not quartzlike, but cristobalite-like, and that this structure is also more consistent with the experimental data also.

    In response, Yoo and Cavazzoni et al. indicate that because of the similarity in energies, other structures, including tridymite-like, may also be possible and should be considered.

    The full text of these comments can be seen at