This Week in Science

Science  02 Jun 2000:
Vol. 288, Issue 5471, pp. 1545
  1. Coupling Excitations

    Light harvesting is a key step in photosynthesis in which numerous chromophores are coupled to collect light and then funnel energy to the reaction centers. Lidzey et al. (p. 1620) have studied an analogous system in which spatially separated layers of different organic dye molecules are contained within an optical microcavity. Electronic excitations of the dyes (excitons) can couple to the photon mode of the microcavity to form new modes that are mixtures of all three states. Coupling can be seen between the excitons at room temperature despite an energy separation of 60 millielectron volts. These model systems of energy transfer may also have applications in optoelectronic devices.

  2. Light-Driven Liquids

    Whether a liquid will wet a surface or form droplets depends in part on the free energy of surface. Ichimura et al. (p. 1624) have modified surfaces with a compound that contains photochromic azobenzene units. The photoinduced cis-trans isomerization of these groups changes the contact angles that droplets make with the surface. For hydrophobic liquids and even some nematic liquid crystals, millimeter-sized droplets could be propelled by creating surface gradients with asymmetric light pulses. The authors used this effect to move glass beads through derivatized glass tubes.

  3. Galaxy Metamorphosis

    Recent observations of distant clusters of galaxies suggest that older clusters are dominated by lenticular galaxies (S0 galaxies) that have lost all of their atomic hydrogen gas and that show no evidence for recent star formation. Quilis et al. (p. 1617) present results from a high-resolution, three-dimensional model which show that S0 galaxies can form when spiral galaxies move through a hot, ionized intracluster medium. Their simulations, which included shock-wave effects and viscous stripping of the galactic halo, produced S0 galaxies within 100 million years from spiral galaxies like the Milky Way and provide a mechanism that can explain the changing galaxy morphology in clusters.

  4. How a Bacterium Invades an Immune Cell

    The tick-transmitted bacterial infection, human granulocytic ehrlichiosis, is a disease of emerging public health importance, as the same tick also transmits Lyme disease and the two diseases can occur together. Apart from this interest, this is the only bacterial pathogen known to enter and survive within phagocytic cells of the immune system called neutrophils. Herron et al. (p. 1653) have discovered a molecular interaction that offers a promising drug and vaccine target for this problematic disease. The bacterium can produce a mimic of a cell-adhesion molecule called P-selectin, which allows the bacterium unimpeded host-cell adhesion and entry. The authors can also cause the bacterium to enter nonhost cells by engineering them to produce a P-selectin receptor.

  5. Post-Stishovite in the Shergotty Meteorite

    Examinations of the Shergotty meteorite, thought to originate on Mars, have revealed a checkered history of dynamic deformation from repeated impacts that led to its ultimate transport to Earth. Study of its mineralogy and textures are important to unravel its complex origin to improve our understanding of planetary interactions and evolution. El Goresy et al. (p. 1632) have characterized an additional silica phase more dense than stishovite (a post-stishovite phase) that has a monoclinic structure interwoven with another post-stishovite phase and silica glass. Post-stishovite phases form at pressures above 48 gigapascals (GPa) from stishovite, and the new post-stishovite phase should be stable between 70 to 85 GPa. Such high pressures are inconsistent with other observations of Shergotty that suggest a peak impact pressure of about 30 GPa. The post-stishovite phases may form through a metastable reaction, which may require a peak pressure of as low as 40 GPa, however, and thus Shergotty may have been shocked to an intermediate pressure range.

  6. Iron Vibrations

    Earth's core is thought to be composed predominantly of hexagonal close-packed (hcp) iron. To explain the seismic properties of the core, experimentalists and theorists have tried to extrapolate the elastic properties of hcp iron to the high pressures and temperatures, yet discrepancies remain. Merkel et al. (p. 1626) have measured the one Raman-active mode of hcp iron up to pressures of 150 gigapascals; ultrapure diamond anvils were used to avoid background luminescence and scattering that could obscure the Raman spectrum. Their results suggest that the shear wave anisotropy at core conditions should vary by about 35%, which should provide geophysicists with a useful parameter to distinguish structural versus chemical changes reflected in varying seismic waves that sample the core.

  7. Flowers Empowered

    In order to better understand the transition from vegetative growth to flowering, Samach et al. (p. 1613; see the Perspective by Devlin and Kay) investigated genes directly regulated by the gene CONSTANS (CO), itself known to promote flowering of Arabidopis in response to longer days. With the use of a conditional overexpression system, four genes were identified that are directly regulated by CO. Of these four, FT and SOC1 are regulated by a balance of CO and FLC activity, itself part of the vernalization response. The other two genes affect patterns of development in the stem and the flower. Thus, a network of regulatory pathways is revealed that not only promotes development of floral structures but that also integrates day length, response to chilling, and age of the plant into the ultimate developmental decision of when to begin flowering.

  8. Stem Cell Secrets

    Despite an apparent lack of phenotype, stem cells are able to give rise to many other types of cells. The types of cells they can become, and some genetic insights into how a stem cell undergoes its transformations, are the subject of two reports. Recent analyses have shown that stem cells can generate cells of a type far removed from the tissues from which the original stem cells were derived. Clarke et al. (p. 1660; see the cover and the news story by Vogel) have explored the limits of stem cell multipotentiality. When isolated adult stem cells are given an opportunity to experience a more embryonic environment, either by coculture with embryonic stem cells or by culture within an early embryo, they display even greater developmental diversity than expected. Neuronal stem cells gave rise to cells of muscle and liver, as well as some of the outcomes previously seen. Thus adult stem cells may show greater potential, and share greater commonality, than so far thought. Phillips et al. (p. 1635) have produced an exhaustive description of the molecular phenotype of a hematopoietic stem cell. Roughly half of the identified transcripts are related to known proteins, and categories of cell signaling, RNA synthesis, and cellular metabolism are well represented. The database offers numerous intriguing expression profiles for further study; the details of their data are presented on an interactive Web site.

  9. Adding to RNA's Repertoire

    The role RNA plays in biology continues to expand—its originally recognized roles in translation now include several enzymatic functions. Peluso et al. (p. 1640) now show that the 4.5S RNA component of the signal recognition particle (SRP) accelerates both association and dissociation of the guanosine triphosphatases Ffh and FtsY, the protein components of the SRP and the SRP receptor, respectively. Because a part of the RNA is thought to interact directly with signal sequences recognized by SRP, this finding raises the possibility that a consequence of that interaction is regulation of the binding of SRP to its receptor, just as protein-protein interactions mediate allosteric regulation.

  10. Replication: A One-Shot Deal

    For all organisms, regulating the replication of their genomes is of paramount importance, as any mistakes can have profound and most often deleterious effects. Perhaps the most basic requirement is ensuring that only one new copy of the genome is made in a cell division cycle. Labib et al. (p. 1643) show that the six so-called minichromosome maintenance (MCM) proteins of Saccharomyces cerevisiae play a central role in preventing runaway copying of the chromosomes. The MCM proteins, in addition to the initiation of replication, turn out to be absolutely critical for the continued synthesis of the new genetic blueprint. Thus, by preventing the reloading of MCM proteins onto DNA during replication, the cell ensures that only one copy of the genome can be completed.

  11. How Cholesterol Aids Mycobacterial Invasion

    Mycobacteria, including the agent that causes tuberculosis, must invade cells and then multiply in specially adapted vacuoles within the host cell. Gatfield and Pieters (p. 1647) examined the mechanism of pathogen invasion and found a surprising requirement for cholesterol in the host macrophage cell membrane for successful mycobactrerial invasion and colonization. Membrane cholesterol accumulates around invading mycobacteria and appears to be important in ensuring the formation of a vacuole that protects the invading organism from intracellular digestion by the host cell.

  12. You Can Keep Your Iron

    Iron is an essential nutrient for most microbes and pathogens. Posey and Gherardini (p. 1651) looked at the iron requirements of the spirochete responsible for Lyme disease, Borrelia burgdorferi, and discovered that the organism lacks any iron-containing proteins. Instead of iron, the metalloenzyme systems of the microbe appear to utilize manganese. Thus, this human pathogen can circumvent one of the host's natural defense systems—the severe lack of free iron in the host environment.

  13. Demarcating Cognition

    In comparison to sensory and motor modalities, cognition uses more anterior portions of the human brain. These areas also are less well defined than the sensory and motor cortices in terms of subserving distinct functional duties, partly because cognitive tasks tend to involve multiple abilities and partly because it has been difficult to design tasks to separate these abilities. Rowe et al. (p. 1656) designed a task that separated the maintenance of items in working memory and the selection from among those items for response. They were able to associate area 46 of the dorsolateral prefrontal cortex with the selection phase and area 8 with the maintenance function.

  14. Quantifying Protein Softness

    Proteins offer a striking illustration of how a collection of weak interactions (hydrogen bonds, salt bridges, and van der Waals forces) support precise three-dimensional structures, as assessed by x-ray crystallography, while providing sufficient flexibility for enzyme catalysis and conformational changes. Zaccai (p. 1604) reviews recent developments in the use of neutron scattering to measure the resilience of two proteins, myoglobin and bacteriorhodopsin, under a variety of conditions and environments. Neutron scattering has the advantage of not needing crystalline samples and can be combined with specific deuteration to spotlight particular regions of a protein.