This Week in Science

Science  09 Jul 2004:
Vol. 305, Issue 5681, pp. 143
  1. Photonic Crystals Keep It In


    Three-dimensional (3D) photonic crystals can expel or trap electromagnetic radiation of particular wavelengths and can be used to control the light-emitting properties of optically active materials. However, the growth of photonic crystals of sufficient quality with incorporated optically active centers has proved problematic. Ogawa et al. (p. 227, published online 3 June 2004, see the Perspective by Hillebrand and Gösele) fabricated a 3D photonic crystal, which has a structure known to exhibit a full 3D band gap, that also has an emitting layer buried inside. Spontaneous emission from the optically active layer was suppressed in regions surrounded by the 3D photonic crystal, but emission was seen from artificial defects fabricated into the photonic crystal.

  2. Bilateral Beginnings

    The earliest unequivocal evidence for multicellular life includes tiny embryos found in the Doushantuo Formation that were deposited about 580 to 600 million years ago in China. The chicken that formed these “eggs” has been uncertain, and it is not clear if these fossils represent an early bilateral animal; only a few bilateral animals are seen in the Edicaran (about 560 million years ago) before radiating greatly in the Cambrian (after about 540 million years ago). Chen et al. (p. 218, published online 3 June 2004 with the news story by Stokstad) now describe what may be small fossils with bilateral symmetry from the Doushantuo Formation. Although these possible animals are not quite 200 micrometers across, several internal organs can be discerned.

  3. Reactionary Radicals

    Ribonucleotide reductase (RNR) uses radical-based chemistry to synthesize deoxyribonucleotides, the building blocks of DNA. In class I RNR, a diiron site in the R2 protein generates a stable radical on a conserved tyrosine. Högbom et al. (p. 245) show that the R2 protein from the intracellular pathogen, Chlamydia, uses a different radical generation system. The structure of R2 from C. trachomatis reveals that the conserved tyrosine is replaced by a phenylalanine, and electron paramagnetic resonance studies show that there is no tyrosyl radical. Instead, the reconstituted protein yields an iron-coupled radical that is probably involved in the RNR mechanism. Chlamydia R2, and similar R2s from several other microbial pathogens including Mycobacterium tuberculosis, may thus be less sensitive to the release of NO during host defense.

  4. Scoring Goals


    The firing patterns of cortical motor neurons—the neurons that normally activate our muscles and enable movements—can be recorded and used to control similar movements in artificial situations, either a cursor on a computer screen or a mechanical arm. Musallam et al. (p. 258; see the news story by Wickelgren) record from goal-oriented neurons in monkey parietal cortex and show that the activity of “cognitive” neurons—which fire during the planning stage of a reaching movement but are silent during the movement itself—can also provide the necessary information. Monkeys learned how to generate spatially accurate signals more consistently with time (which might represent formulating one's intentions more clearly) and in a fashion that reflected the expected reward for correctly specifying the goal (which might correspond to the motivational basis for one's intentions).

  5. Himalayan Squeeze a Puzzle

    When India collided with Eurasia, the complex continent-to-continent collision created the Himalayan mountain range. Several large strike-slip faults, such as the Karakoram and the Altyn Tagh, extend across much of the plateau. These faults distribute the deformation, and in one popular model, the plateau is being extruded to the east; that is, it is squeezed out as one rigid body between these major strike-slip faults. Wright et al. (p. 236; see the news story by Kerr) measured the amount of slip along the Karakoram and western Altyn Tagh and found low rates of slip that would not be consistent with the extrusion model. Instead, the deformation is distributed along many faults like a jigsaw puzzle, and the pieces move at lower rates either together or at different times, which suggests that a weak crust is present. Surface wave data shows that a layer of lower crust beneath the Himalayas is weak and becoming thinner. Shapiro et al. (p. 233) suggest that their results are consistent with this weak layer flowing along a channel between the stronger upper crust and lowermost crust to upper mantle.

  6. Recrystallization Revisited

    When metals are heavily deformed, the energy imparted into the metal creates excess disclinations and point defects. Upon annealing, the density of the defects is reduced through a number of mechanisms, including the recrystallization of small grains and grain growth. It was thought that the recrystallization process was homogeneous and led to the spherical growth of new grains. Using a three-dimensional x-ray diffraction microscope, Schmidt et al. (p. 229, see the Perspective by Offerman) followed the recrystallization process in situ in a deformed sample that was being annealed. They find that the growth pattern was in fact highly heterogeneous.

  7. Modifying Erythropoietin Activity

    The cytokine erythropoietin binds to the classical erythropoietin receptor to mediate hematopoiesis and has independent tissue-protective bioactivities. Leist et al. (p. 239, see the Perspective by Ehrenreich) generated a distinct isoform of erythropoietin that separates its two activities. A carbamoylated derivative did not bind to the classical erythropoietin receptor, and lost the hematopoietic capacity of natural erythropoietin. However, the derivative was neuroprotective in animal models of stroke, spinal cord injury, experimental autoimmune encephalomyelitis, and diabetic neuropathy.

  8. Mitochondrial Protection by Frataxin

    Friedreich's ataxia is a genetic disorder characterized by a deficiency in frataxin, the mitochondrial iron-binding protein. Bulteau et al. (p. 242) identify a role for frataxin as an iron chaperone protein that is required for the reversible modulation of mitochondrial aconitase activity in response to pro-oxidants. By protecting Fe-S clusters from disassembly, frataxin can prevent iron accumulation and production of the highly reactive and toxic hydroxyl radical. Alterations in the level, structure, and chaperone function of frataxin may participate in the progression of degenerative disorders associated with declines in aconitase and mitochondrial activity.

  9. Going Back to the Source

    Although the thymus is known as the major source of T cells, many lines of evidence point to other organs, such as the liver and intestine, as significant alternative sites of T cell development. In particular, intraepithelial lymphocytes, the abundant T cells that reside between the epithelial cells of the intestinal mucosa, are thought to derive largely from specific developmental islands within the mucosa. Using a fate-mapping approach, Eberl and Littman (p. 248, see the Perspective by Guy-Grand and Vassalli) provide direct evidence that intraepithelial lymphocytes are, instead, the direct descendants of thymocytes. Extra-thymic development is thus unlikely to contribute significantly to the T cell pool.

  10. Condensin DNA


    All organisms condense their chromosomes into compact structures so that they can be accurately segregated during cell division. The highly conserved protein machines that carry out this condensation are called condensins, members of the Structural Maintenance of Chromosomes (SMC) protein family, whose members are also involved in sister chromatid cohension and dosage compensation. Little is known about how condensins function. Case et al. (p. 222, published online 3 June 2004; see the Perspective by Zhuang) probe the mechanism of DNA condensation by the bacterial condensin MukBEF using single-molecule analysis. DNA molecules are condensed by MukBEF in an ordered, repetitive manner that requires nucleotide binding as a source of energy. The highly stable interaction with DNA is maintained even in the decondensed (stretched) state. Condensin molecules do not slide along DNA. Instead, they interact cooperatively to fold up DNA.

  11. Fixing Spinal Injury in Fish

    The tragedies of spinal cord damage in humans would be much alleviated if spinal cord neurons could regenerate, but human neurons show great reluctance to do so. Restricting factors seem to be both intrinsic to the damaged neuron and as well as extrinsic to the surrounding central nervous system (CNS) tissue. In zebrafish, the CNS seems to have less of the general repressive nature, but Mauthner neurons still cannot regenerate reliably. Bhatt et al. (p. 254) now find that application of cAMP (adenosine 3', 5'-monophosphate) to the cell body of the Mauthner neuron in zebrafish induced its regeneration and restored Mauthner neuron-dependent behavioral responses.

  12. Self-Defense in the Spineless

    A hallmark of vertebrate immune systems is the ability to generate a huge repertoire of distinct antigen receptors through somatic shuffling of immune receptor gene segments. Although invertebrates also possess proteins encoded by members of the immunoglobulin superfamily (IgSF), it has not so far been apparent that they have evolved similar means of generating receptor diversity. Exploring the immune defenses in a species of snail, Zhang et al. (p. 251) uncovered diverse IgSF genes, termed fibrinogen-related protein genes, or FREPs. Closer scrutiny revealed FREP diversity to be generated by point mutation and by a process similar to gene-conversion—two mechanisms that help to generate antibody diversity in vertebrates.