This Week in Science

Science  30 Jan 2004:
Vol. 303, Issue 5658, pp. 585
  1. Coupling Ferroelectrics to Ferromagnets

    Materials that are either ferroelectric (having a spontaneous electric dipole) or ferromagnetic (having a spontaneous magnetic moment) are well studied and can be readily fabricated. However, the combination of these properties into a single multiferroic materials in which the two properties are strongly coupled presents a challenging materials design problem. Zheng et al. (p. 661) have fabricated a multiferroic material through the self-assembly processes in which ferromagnetic CoFe2O4 nanoparticles form pillars inside a ferroelectric BaTiO3 ferroelectric matrix. This geometry couples to the magnetic and electric order parameters and allows the interconversion of energy stored in electric and magnetic fields.

  2. A Trace of Technetium

    The discovery of technetium in the atmospheres of stars about 50 years ago proved that heavy elements were produced by stars and initiated the development of nucleosynthesis models. Savina et al. (p. 649; see the Perspective by Nittler) have detected extinct 99Tc by measuring its excess 99Ru daughter product in stellar SiC dust grains trapped and protected in the matrix of the Murchison meteorite. The isotopic character of the stellar grains provide direct samples for comparison with astronomical observations and could help refine nucleosynthesis models.

  3. Silence of Paternal Xs

    Many species have different male and female sex chromosomes, and the dosage of genes on these chromosomes has to be balanced between the two sexes during dosage compensation. Failure to do so can have severe or even fatal effects. In mammals, one of the two X chromosomes in the female (XX) is shut down to match the dosage of the single X in the male (XY). In the female mammalian embryo, the X chromosomes have been presumed to be initially active, and then one is randomly selected for inactivation for the life of the organism. In the extraembryonic tissues of the embryo, X inactivation is imprinted, and the X chromosome inherited from the father is always inactivated. Okamoto et al. (p. 644) and Mak et al. (p. 666) now show instead that every paternal X chromosome in the mouse embryo is initially inactivated (see the Perspective by Hajkova and Surani). Within the embryo, proper paternal X chromosomes are subsequently reactivated and lose all of their normal heterochromatin markers before being randomly inactivated in each cell. Thus, the inactive, imprinted state is surprisingly labile in the embryo.

  4. Up, Up, and Away

    Molecules in liquid crystalline phases tend to orient in the same direction, and this orientation can be controlled or disrupted by the presence of surfaces or embedded particles. If the particles are asymmetric, then the disruptive forces between the molecules and the particles will also be orientation dependent. Lapointe et al. (p. 652) measured these forces as they magnetically rotated nickel nanowire within a liquid crystal. The measured forces agree with theoretical predictions that had not been previously verified. By continuously rotating the wire, they could cause the liquid crystal orientation to uniformly twist though the height of the sample, which then caused the wire to levitate.

  5. On Top of Water

    Recent experimental studies have provided evidence for a substantial fraction of “single-donor” and “acceptor-only” water molecules at aqueous liquid-vapor interfaces. In these water molecules, one or two hydrogen atoms do not form hydrogen bonds. Kuo and Mundy (p. 658; see the Perspective by Marx) modeled the aqueous liquid-vapor interface in a Car-Parrinello molecular dynamics study of a slab containing 216 water molecules. The slab is sufficiently thick to reproduce bulk water properties in the center. The authors find evidence for both single-donor and acceptor-only water molecules at the interface. The calculated electronic properties of the water molecules suggest that the interface should be more reactive than bulk water.

  6. Tipping the Scales

    Effective immune responses depend on the precise activation and deactivation of signaling cascades by protein tyrosine kinases (PTK) and phosphatases (PTPases). The PEST-domain enriched tyrosine phosphatase (PEP) is one such PTPase, which opposes T cell receptor signaling. Hasegawa et al. (p. 685) report that the loss of PEP activity in mice produced unchecked effector and memory T cell responses. Although this stopped short of causing overt autoimmunity in PEP-deficient mice, excessive B cell responses were evident, suggesting a role for the phosphatase in regulating diverse aspects of T cell coordinated immunity.

  7. In Vitro Synthesis of Lantibiotics

    Unlike glycopeptides, such as vancomycin, and penicillins for which bacteria have developed and disseminated methods of counterattack, nisin and other lantibiotics have been used for many years without having induced widespread mechanisms of resistance. One practical barrier to developing artificial members of this class of ribosomally synthesized peptide antibiotics has been in-built barriers to modifying the lantibiotic precursors. For example, tinkering with the nisin gene can slow subsequent posttranslational processing steps to the point where intracellular proteolysis destroys the peptide before it can be secreted. An alternative approach has been to attempt to isolate the synthetic enzymes that can then be fed designed peptide substrates in vitro, thus bypassing any cellular constraints. Xie et al. (p. 679) report success for lacticin 481, a peptide produced by Lactococcus lactis, which has potential uses as a food preservative. The lacticin 481 synthase carries out three dehydrations and three cyclizations that together convert a propeptide into a tricyclic antibiotic. This enzyme exhibits a relaxed substrate specificity that may ease the production of lantibiotic variants.

  8. Blocking Low-Level Signals

    Development of the vulva in the nematode Caenorhabditis elegans has been used as a model system for examining the patterning of cell fate in an animal. Two different intercellular signals contribute to distinguish cell fate among vulval precursor cells (VPCs): a long-range spatial gradient of epidermal growth factor (EGF) mediated by the EGF receptor and a cell-to-cell lateral signal mediated by the Notch-like receptor LIN-12. Yoo et al. (p. 663; see the Perspective by Sternberg) now find that VPCs activated by a low level of EGF are blocked from adopting a particular cell fate by a LIN-12 lateral signal from a neighboring cell.

  9. Hit and Run

    Bdellovibrio bacteriovorus is a fast-moving, flagellated predatory bacterium that seeks a diversity of prey bacteria. Rendulic et al. (p. 689) present its genome sequence, which reveals molecular aspects of different stages of its life cycle. This bacterium has a specialized prey interaction locus called hit. Although the eponymous protein has not yet been identified, hit is part of a transcriptional unit with genes coding for a cell wall protein and flagellar and pilus genes. Using pili, Bdellovibrio attaches to the host cell wall, punctures it with a suite of hydrolytic enzymes, and squeezes into the periplasm. Despite the intimacy of the predator-prey interactions, there are no signs of horizontal gene transfer. Bdellovibrio then uses a variety of transport systems to digest and ingest molecules from the prey's cytosol; it can only synthesize 11 amino acids de novo. After septation and flagellar growth, the progeny escapes from the prey's ghost by secreting more enzymes to dissolve the remaining peptidoglycan.

  10. RNAi and Chromatin Packaging

    Chromatin, the histone-packaged form of DNA found in the cell nucleus, plays a vital role in the regulation of gene expression. Critically, genes packaged into heterochromatin are switched off, often for many cell generations. Genetic and molecular analyses have defined the components that constitute heterochromatin and some of the mechanisms that underlie its formation. Recent studies in fission yeast and plants have indicated that the RNA interference (RNAi) machinery is involved in heterochromatin formation. Pal-Bhadra et al. (p. 669) now show that components of the RNAi machinery are also involved in the formation of heterochromatin in animals, and specifically in the process of position effect variegation in the fruit fly Drosophila melanogaster, suggesting that RNAi is probably required for heterochromatin formation in all eukaryotes.

  11. RNA-Driven Targeting of Heterochromatin

    The RNAi pathway regulates transcriptional gene silencing in fission yeast and multicellular eukaryotes. What is the link between RNAi and the establishment of heterochromatin? Verdel et al. (p. 672) have now characterized a three-protein complex from fission yeast that is involved in RNA-driven targeting of heterochromatin. The complex contains Ago1, a component of the miRNA/RNAi RISC effector machinery. It also contains Chp1 and Tas3, both of which localize to regions of heterochromatin. Most significantly, the complex contains small RNAs from these very same regions of heterochromatin. In the absence of these RNAs, the complex no longer associates with, nor drives, the formation of heterochromatin.

  12. Factor Cascade

    During the middle part of the mammalian reproductive cycle, a surge of luteinizing hormone (LH) is followed by a series of changes in the ovarian follicle, including resumption of oocyte meiosis and increased gene expression in cells surrounding the oocyte. The end point of these changes is follicle rupture and ovulation of a fertilizable egg. It has been unclear how LH participates in this series of events, because the oocyte and surrounding cumulus cells do not respond directly to LH and do not express LH receptors. Park et al. (p. 682) show that LH operates via intermediary factors of the epidermal growth factor family to trigger cumulus expansion and oocyte meiotic maturation. Elucidation of LH action on the ovary may be useful in manipulating follicular function in the treatment of infertility.

  13. Nanotube Quantum Dots on the Double

    Measurements on a pair of quantum dots fabricated in a single carbon nanotube by Mason et al. (p. 655) show that transport through each dot can be manipulated independently. The coupling of the dots can be electrostatically controlled from the weak-to-strong coupling regime, providing the possibility of gated applications in quantum computing architectures.

  14. Kinesin Walking with an Uneven Gait

    Kinesin is an adenosine triphosphate (ATP)-dependent, double-headed motor. It moves processively along microtubules and takes an 8.3-nanometer (nm) step for each ATP hydrolyzed. However, Yildiz et al. (p. 676) now show that the size of the steps taken by individual heads alternates between 17 nm and 0 nm. These results provide direct evidence that kinesin moves by a hand-over-hand mechanism, where the leading head is stationary and the trailing head moves past it, rather than by an inchworm mechanism in which one head always leads. The data also suggest that both kinesin heads are bound to the microtubule between steps.