This Week in Science

Science  23 Aug 2002:
Vol. 297, Issue 5585, pp. 1233
  1. In Brevia ...

    An analysis by Castro et al. (p. 1299) shows that the transdifferentiation potential for mouse bone marrow cells may be more limited than previously thought.

  2. Large Archean Impact Event

    Cratering left after the early solar system bombardment of Earth has long been eroded, but impact ejecta still remain in parts of the Archean cratonic crust. Byerly et al. (p. 1325) obtained lead-uranium ages for zircons from the lowermost ejecta layers found in South Africa and Australia. Consistent ages of 3.470 billion years for these widely separated sites suggest that a large impact event created a potentially global ejecta layer and probably altered any early inklings of terrestrial life.

  3. Freeing BeH2

    Theorists have been intrigued by BeH2 because this molecule has only six valence electrons instead of the usual octet. Confirming their predictions has been hampered by the difficulties spectroscopists have had in generating the free molecule. The solid compound is formed from BeH4 species, and spectra of isolated molecules have been obtained only after stabilization within a matrix. Bernath et al. (p. 1323) have now generated the free molecule with an electrical discharge in a high-temperature furnace. Analysis of infrared emission spectra reveal a symmetric, linear structure and provides highly precise bond distances.

  4. The Mane Explanation

    The mane of the male African lion is a sexual dimorphism whose adaptive function has been enigmatic. West and Packer (p. 1339; see the news story by Withgott) combine field experiments and 30 years of long-term data to address virtually every aspect of the biology of the lion's mane. They found that the darkness of a lion's mane is a reliable indicator of the male's hormonal and nutritional condition, that other lions respond to mane color, and that dark-maned males can better protect their offspring. Males grow darker manes in cooler months of the year and in cooler habitats, and shorter manes in hotter climates.

  5. Developing a Taste for Toxins

    In order to understand the underlying mechanisms of natural selection, individual differences upon which selection can act need to be observed. Geffeney et al. (p. 1336; see the Perspective by Huey and Moody) integrate studies of adaptive radiation in predator-prey arms races with an investigation of the neurophysiological mechanisms underlying a complex organismal trait. Garter snakes in western North America have developed resistance to the deadly neurotoxin possessed by their newt prey through the evolution of tetrodotoxin-resistant sodium channels. This adaptation has evolved multiple times within the framework of a geographic mosaic of coevolution.

  6. An Amorphous Continuum

    At first glance, it would seem that there could be only one amorphous form of a material, but distinct amorphous phases with different connectivities and density can exist. For example, at low temperatures and high pressures, a high-density amorphous water ice can be created that, upon heating, expands to form a low-density amorphous ice. This transformation was believed to be a single-step process, but x-ray and neutron diffraction studies by Tulk et al. (p. 1320; see the Perspective by Soper) indicate that this transition occurs continuously. At each incremental stage of heating, a distinct metastable form was obtained that had its own structure factor.

  7. Second Vertebrate Sequenced

    The small size of the genome of the pufferfish, Fugu rubripes, about 1/10 that of the human genome, makes it a valuable model for studying the evolution of the vertebrates. Aparicio et al. (p. 1301; see the cover and the Perspective by Hedges and Kumar) present their results of whole-genome shotgun sequencing of Fugu. Although the genome of Fugu is much more compact than that of humans, the two have a comparable number of genes, and Fugu has some giant genes that resemble mammalian structures. However, about 25% of human genes do not appear to have counterparts in Fugu. There have also been extensive rearrangements during the 450 million years since mammals and teleost fish diverged.

  8. X Marks the Merger

    According to Einstein's theory of general relativity, gravitational waves should be produced by the merger of two supermassive black holes. Astronomers have been trying to determine where, when, and how often these mergers could occur. Merritt and Ekers (p. 1310) have found that a merger will reorient the spin axis of the more massive black hole, which in turn will alter the direction of the associated and observable jet ejected along its spin axis. Several X-shaped radio emissions observed from different galaxies, where the radio lobes from the old jet crosses the radio lobes from the reoriented jet, fit well with this model. The estimated event rate suggests that astronomers should be able to observe gravitational waves within decades.

    CREDIT: MERRITT AND EKERS
  9. Making C-O, Too

    Polyketides, natural products that include important drugs such as the antibiotic erythromycin and the anticancer drug epothilone, are biosynthesized from acyl coenzyme A (CoA) precursors by polyketide synthases (PKSs). Known PKSs have differing structures and mechanisms, but all have a β-ketoacyl synthase (KS) domain that catalyzes the formation of C-C bonds in the polyketide backbone. Kwon et al. (p. 1327) have now identified and characterized the minimal genes required for forming C-O bonds in the synthesis of a cyclic polyther, nonactin, from Streptomyces griseus. Two KSs, NonJ and NonK, catalyze sequential condensation of CoA precursors by forming C-O rather than C-C bonds. This activity could potentially be engineered into other PKSs to create novel polyketides.

  10. Interfering with Receptors

    The family of receptors for epidermal growth factor (EGF) transmits signals critical for growth and differentiation of cells in a wide variety of tissues during development. Inappropriate expression of these receptors occurs in many human cancers, and Herceptin, an antibody against one of these receptors (HER2), is used in the treatment of breast cancer. Cho and Leahy (p. 1330) present the 2.6-angstrom crystal structure of the entire extracellular portion of one member of this receptor family, HER3. One of the L-shaped halves sits atop the other and assumes a toroidal shape with a protruding spur. Previously mapped regions of the EGF binding site, on the spur and the torus, must be brought together in order to interact productively with the ligand. Interference with this large-scale conformational change might offer a promising route to therapeutics.

  11. Putting the Heart at Risk

    About 450,000 people in the United States die each year from cardiac arrhythmia. A genetic study by Splawski et al. (p. 1333; see the news story by Marx) reveals that the risk of arrhythmia is slightly increased in individuals who carry a specific variant allele of the cardiac sodium channel gene SCN5A, a group that includes an estimated 13% of African Americans. Although this allele alone does not cause life-threatening arrhythmias, it increases the risk of arrhythmia in the setting of other, acquired risk factors such as the use of certain medications. This allele may be a valuable marker for identifying individuals in the general population who should avoid these additional risk factors.

  12. Starts and Stops

    In the mammalian ovarian follicle, several layers of somatic granulosa cells surround the oocyte and support its development. Within this structure, the oocyte begins meiosis, but then halts until luteinizing hormone triggers its resumption. How is meiosis halted? Studies to answer this question have been hampered in that removal of the follicle from an oocyte reinitiates meiosis. However, Mehlmann et al. (p. 1343) now present a technique in which oocytes can be injected while still surrounded by the follicular cells. The authors show that activity of the Gs G-protein is required to maintain the meiotic arrest in oocytes, and they suggest that a signal from granulosa cells may be acting through a receptor to activate the G protein.

    CREDIT: MEHLMANN ET AL.
  13. Persistent Pores in Insulin Secretion

    During stimulated secretion, a secretory granule fuses with the plasma membrane to release secretory granule contents. Takahashi et al. (p. 1349) have used two-photon excitation imaging to reveal the dynamics of the fusion pore during insulin secretion from pancreatic islets. The lifetime of the pore, which was made up mostly of membrane lipids, was much longer than anticipated from other studies with single cells.

  14. Prince of Dendrites?

    Neurons obtain signaling information through their dendrites, which may range in structure from simple thin extensions of the cell body to complex branched outreaches. Studying the ventral pore sensory organs of fruit fly, Moore et al. (p. 1355) find that the complexity of the dendritic arbor is regulated by a single gene, hamlet. Genetic manipulations that raised or decreased hamlet protein expression from its normal levels caused the local progenitor cell to produce neurons with single dendrites or with complex dendritic arbors during the early stages of neuronal development. Analyses of the gene sequence and subcellular localization suggest that hamlet might encode a transcription factor.

  15. Spin the Bottleneck

    The addition of electrons to a quantum dot gives rise to Coulomb blockade effects—the electrons that occupy the dot exert a repulsive electrostatic force that must be overcome before another electron can be added. The occupation of the electronic states and transport through them should also exhibit a spin dependence that arises through the Pauli exclusion principle. Ono et al. (p. 1313) report transport measurements through a coupled pair of quantum dots in which the spin on one of the dots is fixed. The asymmetric transport properties through the coupled dot system are consistent with the spin-blocking scenario expected from Pauli exclusion.

  16. Follow the Jumping Oxygen

    In applications such as fuel cells and sensors, the mobility of oxygen anions in ceramics is crucial for performance. Such mobility is generally seen only at elevated temperatures, and efforts to find materials with lower operating temperatures will likely depend on a better understanding of conduction mechanisms. Kim and Grey (p. 1317; see the Perspective by Stebbins) were able to use magic angle spinning nuclear magnetic resonance of oxygen-17 and vanadium-51 to determine the sites involved in oxygen anion mobility in the amorphous anionic conductors -Bi4V2O11 and -Bi4V1.7Ti0.3O10.85, even at temperatures where the mobilities are quite low. Oxygen conductivity in these bismuth vanadates occurs through oxide-ion jumps between different sites in the vanadate layers and through the Bi2O22+ layers. A double-resonance method was used to detect motion between equivalent sites in the lattice.

  17. Signal Recognition Particle Sites in the Ribosome

    In eukaryotes, signal recognition particle (SRP) interacts with the ribosome-nascent chain complex (RNC) of secretory proteins and targets them to the endoplasmic reticulum (ER) through interaction with its receptor (SR), an ER membrane protein. Now, using protein cross-linking studies, Pool et al. (p. 1345) have analyzed the interaction between an SRP subunit, SRP54, and the ribosome at different functional stages. When SRP54 is bound to the signal peptide, it is near two ribosomal proteins at the exit site, L23a and L35, which allows recognition of the nascent chain as it emerges from the ribosome. Proximity to L23a is lost when SRP binds to SR. The SR-induced changes in binding may allow the translocon to dock with the ribosome so that the signal peptide can be easily inserted into the translocation channel.

  18. First Things First

    Cells responding to cytotoxic stresses like DNA damage are thought to activate apoptosis-inducing caspases only after the mitochondria are permeabilized and release their cytochrome c, which functions as a caspase activator. Lassus et al. (p. 1352; see the Perspective by Kumar and Vaux) present experimental results challenging this view. They monitored apoptosis caused by DNA damage in human fibroblasts transformed with the adenoviral oncogene E1A and in other human tumor cell lines. When small interfering RNA (siRNA) was used to block expression of caspase-2, release of pro-apoptotic factors from the mitochondria was inhibited and the percentage of cells undergoing apoptosis was reduced. Apoptosis was restored by the expression of mutant caspase-2 RNA that was designed not to interact with siRNA. Thus, even “intrinsic” death-inducing pathways activated by DNA damage includes caspase activation as an early event in order to permeabilize the mitochondria.