This Week in Science

Science  21 Jan 2000:
Vol. 287, Issue 5452, pp. 389
  1. The Big (Wobbly) Top

    True polar wander is thought to result from a shift of Earth's spin axis related to a major change in the distribution of mass in the mantle. In most cases, shifting paths of paleomagnetic poles are called apparent polar wander because the shifting is related to plate motions and not changes in the orientation of the axis. Sager and Koppers (p. 455; see the news story by Kerr) have found evidence for a true polar wander event in polar paths calculated for the Pacific plate in the Late Cretaceous. They suggest that about 84 million years ago Earth's rotation axis shifted at a rate of 3° to 10° per million years. This large and rapid shift is correlated in time with major plate reorganizations and a magnetic field reversal, and it may have been caused by a complete overturn of the mantle.

  2. Robust Polyethylene Catalysts

    Polyolefins represent the bulk of all commercial polymers made each year, but both the traditional catalysts for Ziegler-Natta polymerization and the newer cationic metallocene catalysts are readily deactivated if “heteroatoms” such as oxygen, nitrogen, or sulfur are present or if even traces of impurities are present in the starting materials. These requirements greatly limit the incorporation of monomers with functional groups that could modify the polymer's properties. Neutral nickel catalysts, while more tolerant of functional groups, usually produce shorter oligomers. Younkin et al. (p. 460; see the cover and the Perspective by Jacobson and Breinbauer) now report that nickel salicylaldimine complexes form high molecular weight, linear polyethylene. These catalysts require no co-catalysts, remained active after air or water exposure, and could tolerate the presence of ethers, ketones, and esters.

  3. Guiding the Light

    Optical circuitry requires a method for directing light just as electronics requires wires. Silica-based optical waveguides formed on a substrate via a sol-gel process and patterned by soft-lithographic techniques exhibit amplified stimulated emission (ASE, a kind of mirrorless lasing) and have the potential for incorporation into such integrated optical circuits. So far, the intensity needed for ASE has been rather high. Yang et al. (p. 465) show that the threshold intensity can be decreased by an order of magnitude by using mesoporous silica doped with rhodamine 6G dye. The combination of these patterned waveguides showing ASE may provide a route for one-step processing of microlaser arrays and integrated optical circuits.

  4. A Stunning Phenotype

    Myocardial stunning is a common form of heart damage that occurs when blood flow to the heart is temporarily stopped and then re-established. Murphy et al. (p. 488) show that a proteolytic fragment of the myofilament protein troponin I (TnI) has a causal role in pathogenesis. Transgenic mice expressing this fragment in the heart developed the characteristic features of stunned myocardium. In addition, myocardial biopsies from coronary artery bypass patients were found to contain TnI proteolytic fragments. These results indicate that aberrant processing of contractile proteins can cause heart failure.

  5. A Compact Database

    Quantum mechanical approaches to information storage can speed up the retrieval of specific items from a database because, unlike ordinary data bits, extra information can be encoded in the quantum bits (“qubits”) as the phase difference between the two states that represent 0 and 1. For example, Grover has recently shown theoretically that a single operation on all of the items in a quantum register should be able to retrieve an entry in which the state of one qubit is “flipped.” Ahn et al. (p. 463; see the Perspective by Knight) have stored information in N Rydberg states (N, either 6 or 8) in a beam of cesium atoms. They show that they can prepare the single-flip states and read them out with single laser pulses.

  6. Not Your Usual Magnetic Resonance

    Magnetic resonance is normally induced with magnetic fields and radio frequency pulses, but in semiconductors optical pumping or detection of magnetic resonance has been achieved. Kikkawa and Awschalom (p. 473) now report on the creation and detection of magnetic resonance in gallium arsenide through an all-optical route. Circularly polarized light created electron spins that interacted with nuclear spins through hyperfine interactions. Optical pumping induced local magnetic fields up to 0.4 Tesla in a period of 250 seconds, which is a relatively long time scale for electronic processes and more typical of nuclear spin polarization. Resonant depolarization was observed that corresponded to gallium-69 but was also observed at a frequency that did not correspond to any nuclear moment present in the sample, which suggests that the phenomena observed may be more complex.

  7. Histone Acetylation and V(D)J Recombination

    In thymocytes, recombination of the V(D)J gene segments of the T cell receptor (TCR) is developmentally regulated. What signals this developmental change? It appears that chromatin structure plays a role in changing the accessibility of recombination signal sequences to the recombinase, but the identity of the particular chomatin modification that functions in this case has been unknown. McMurry and Krangel (p. 495; see the Perspective by Schissel) examined a transgenic human V(D)J recombination reporter construct and an endogenous mouse TCR α/δlocus, and they show that acetylation of histone H3 provides this chromatin modification. Regions displaying H3 hyperacetylation corresponded with those displaying accessibility to recombinase. Thus, histone acetylation is linked to V(D)J recombination.

  8. Treating Rotavirus Infections

    Although rotavirus-induced diarrhea kills nearly 1 million children each year in developing countries, the mechanism of pathogenesis has not been clear. Lundgren et al. (p. 491; see the news story by Winkelgren) have evidence that the enteric nervous system is responsible for changes in the intestinal secretory response (potential differences across the epithelium and fluid transport) that results from rotavirus infection. They tested drugs known to act specifically on the nervous system and found that the local anesthetic lidocaine could decrease diarrhea in live baby mice if they were treated after infection.

  9. Paying for Too Much Complement

    A developing embryo is at the mercy of the natural and specific immune systems of the mother. Because the complement system is designed to lyse cells and to alert phagocytes to the presence of intruders, it may well be that the system needs to be regulated closely so that the embryo can survive. Lack of regulation of the complement system in humans can lead to chronic inflammatory conditions. Xu et al. (p. 498; see the news story by Hagmann) report that the loss of the Crry protein, a major complement inhibitor in the mouse, leads to complete inability of embryos to survive to birth caused by spontaneous complement activation and the subsequent inflammatory infiltration. Regulation of complement is thus critical for prevention of the destruction of embryos in the maternal environment.

  10. Moonlighting Enzymes

    The existence of a one-to-one correspondence of specific amino acid “charging” activity to aminoacyl transfer RNA (tRNA) synthetase enzyme (which is responsible for aminoacylating a respective tRNA molecule) has been thought to be an essential element of accurate protein synthesis. However, the complete genome sequences of two thermophilic methanogenic archaea failed to identify genes for cysteinyl-tRNA synthetase. Stathopoulos et al. (p. 479; see the Perspective by Yarus) now show that, in Methanbacterium thermoautotrophicum and M. jannaschii, proline aminoacyl tRNA synthetase is endowed with dual functionality during protein synthesis in which the enzyme can specifically charge the tRNAs for proline and cysteine with their respective amino acids.

  11. Fighting Stomach Acid

    The bacteria Helicobacter pylori (associated with gastritis, ulcers, and stomach cancer) has had to adapt to the acidity of the stomach, which acts as a defense against many pathogens. It survives low pH from gastric acids by using a urease to produce ammonia. The activity of urease depends on UreI, which Weeks et al. (p. 482) show is a proton-gated urea channel that lets urea into the cell at low pH for generation of ammonia.

  12. Beating the Heat (and the Cold)

    Many plant varieties can adapt to colder or warmer climes. By limiting expression of a desaturase enzyme through antisense technology, Murakami et al. (p. 476; see the Perspective by Sharkey) have demonstrated that the ability of the tobacco plant to tolerate higher growing temperatures is limited by their trienoic fatty acid content. The effect is particular to the fatty acids of the chloroplast membranes. Trienoic fatty acids, already correlated with cold-tolerant plants, may alter critical membrane characteristics.

  13. Infectious Diseases in Wildlife

    Communicable diseases in wildlife not only impact the health of humans and domesticated animals but also can adversely affect effort to maintain species and biodiversity. Daszak et al. (p. 443) reviews emerging infectious diseases in wildlife, domestic animals, and humans, the complex interactions between them, and their impact on the health of all three groups.

  14. Condensed Pair Dynamics

    Condensed pairs in a superconductor are in a state of change—they constantly break and reform. However, most spectroscopic information gleaned from superconductors has been static and revealing little of the dynamics of these interactions. Using a femtosecond pulse-probe techniques, Kaindl et al. (p. 470) depopulated the superconducting condensate and monitored the dynamics of the excited state as it relaxed back into its superconducting state. In the high-transition temperature (Tc) superconductor yttrium-barium-copper oxide, they identified a slow relaxation process associated with the condensed pairs below Tc and a fast process associated with excitations above Tc. The results suggest that the charge carriers strongly couple to spin excitations.

  15. There's the Rub

    Mechanical devices are getting smaller and smaller—examples include read-writes heads in magnetic hard drives and the many developments in microelectromechanical devices. As the separation of two surfaces decreases, the tribological behavior (friction and wear) of the two surfaces separated by a monolayer of lubricant depend not only on the topography and morphology of the surfaces but also on the chemical characteristics of the surfaces and lubricant. Eisert et al. (p. 468) used a nonlinear optical technique (second-harmonic generation) to look at how the molecules between the two surfaces respond to an applied load in situ and show how the molecules align along the direction of the shear force.

  16. A Critical Role for Histone Ubiquitination

    DNA is packaged in eukaryotic cells by histone proteins. Gene expression requires the histone-bound DNA, or chromatin, to first be “remodeled” in order for regulatory proteins to access specific sites on the DNA. Histones can be modified by processes such as acetylation, phosphorylation, or ubiquitination. Although histone ubiquitination has been reported to regulate transcription, replication, and meiosis in vertebrates, it has not been shown to occur in yeast cells. Robzyk et al. (p. 501) now examine histone mutants of yeast and show that histone H2B is ubiquitinated by the Rad6/Ubc enzyme. Modification of a specific lysine residue in H2B, or elimination of Rad6 function, results in yeast cells that are defective for mitosis and meiosis. Hence, histone H2B ubiquitination by Rad6 is required for critical cellular activities in yeast.

  17. The Host Is Not in the Clear

    Vesicular stomatitis virus (VSV) is an economically important livestock disease, but how the insects that transmit the disease become infected has been unclear; the virus is not present in the bloodstream of infected animals; that is, they do not display viremia. Mead et al. (p. 485) allowed infected and non-infected black flies to feed on the same host, the deer mouse, which also becomes infected with VSV. Although the mice did not develop viremia, the non-infected flies co-feeding on the mice became infected. Thus, the absence of viremia in a host is not sufficient to exclude it from playing a role in propagating a virus.

  18. Activation and Inhibition of the Staphylococcus agr System

    Balaban et al. (Reports, 17 Apr., p. 438) showed results which suggested that immunization with RNAIII activating protein (RAP) can inhibit Staphylococcus aureus infection in mice and that treatment with a RAP-derived linear heptapeptide can likewise suppress infection. Both the RAP antibodies and the peptide presumably inhibit activation of virulence genes at the agr locus, the encoding locus of RNAIII; however, Balaban et al. found that RAP purified from an agr-null S. aureus strain was equally protective relative to RAP purified from wild-type S. aureus. This result implied that RAP is independent of agr.

    Novick et al. comment that they were unable to produce agr-activating supernatants from agr-null strains of S. aureus, despite repeated attempts, and conclude that agr activation or inhibition activity is more likely tied to autoinducing peptides than to RAP or its derived peptide.

    In response, Balaban et al. attribute the different results to different purification procedures used by Novick et al. (which, Balaban et al. suggest, may not have taken adequate account of RAP's sensitivity to boiling), and reiterate their view that RAP uses a different signal transduction pathway to regulate RNAIII synthesis. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/287/5452/391a

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