Editors' Choice

Science  06 May 2005:
Vol. 308, Issue 5723, pp. 759

    Reduced Nitrogen

    The growth of phytoplankton is limited by the loss of fixed nitrogen from the world's oceans. This loss occurs predominantly in zones of low oxygen (< 25 μM), such as the Black Sea, Chilean waters, and the Benguela upwelling off the Namibian coast. Classically, N2 was thought to be produced by denitrification—the reduction of nitrate to N2 by heterotrophic bacteria—but Kuypers et al. show that a large contribution may come via the anammox process: the anaerobic oxidation, carried out by bacteria known as Planctomycetes, of ammonium by nitrite. They present five corroborating strands of evidence. First, concentrations of nitrate drop at the bottom of the oxic zone; second, ammonium concentrations in the suboxic zone are low; third, water samples doped with [15N]nitrate and [14N]ammonium produced significant amounts of 14N15N; fourth, ladderane lipids, characteristic of the anammoxosome membrane, were present; fifth, fluorescence in situ hybridization and ribosomal RNA sequence analysis revealed an abundance of Planctomycetes in the suboxic zone. One unknown is why there are anammox bacteria in the Benguela upwelling at depths where there is free oxygen (9 μM). Either these cells are quiescent, or there may be a suboxic microenvironment available, such as marine snow. — CA

    Proc. Natl. Acad. Sci. U.S.A. 102, 6478 (2005).


    A Mitotic RNP

    The mitotic spindle is an arrangement of cellular microtubules that acts as the physical scaffold used to partition chromosomes into the daughter cells during mitosis. Blower et al. find that an RNA-binding protein, Rae1, already known to be involved in the export of mRNA from the nucleus during interphase, also has a role in spindle assembly. Rae1 was isolated from Xenopus egg extracts as an activity required for spindle assembly. When it was depleted from egg extracts or from cells, mitotic spindle assembly was inhibited, and purified Rae1 stabilized microtubules in the presence of its nuclear import/export partners, the small GTPase Ran and importin β. Rae1 appears to be part of a large ribonucleoprotein (RNP) complex that controls microtubule dynamics; the association of RNA with the mitotic spindle is unanticipated but appears to be due to a structural requirement, perhaps as a second kind of scaffold. — SMH

    Cell 121, 223 (2005).


    Nanoparticle Films

    Anti-reflective coatings are used in ophthalmic lenses, solar cells, optical data storage, and other applications in which reflections hamper device performance. Coatings are usually applied expensively via vacuum processes, and recently developed sol-gel methods still require multiple steps, including a thermal or chemical curing stage.

    Krogman et al. have developed a simple process for applying anti-reflective films by spin-coating polymer substrates with metal oxide nanoparticles. Ceria or silica particles were added to water-based solutions of a pentafunctional acrylate monomer to increase or decrease the refractive index, respectively, and were then deposited onto an acrylate substrate. In order to make thin, strong, and uniform films, the monomer solutions were doped with a second solvent to stabilize the colloidal particles and to enhance evaporation rates. By varying the concentration of nanoparticles, the authors tuned the refractive indices of the cured two-layer films and were also able to adjust the wavelength of minimum reflection. The nanoparticles hardened the films, too, making the coatings more resistant to wear. — MSL

    Nanotechnology 16, S338 (2005).


    Reactions That Float

    Solvents are generally thought to accelerate bimolecular reactions by increasing the mixing of the substrates and by stabilizing key structural changes along the pathway. Both factors would seem to rely on intimate contact between the solvent and the reactants. For over a half-century, water has been known to accelerate some organic coupling reactions, such as Diels-Alder cyclization, but the effect has remained largely unexploited because of the poor aqueous solubility of most reagents.

    Narayan et al. have achieved rate enhancements for a wide range of cycloadditions and ring-opening reactions simply by stirring the insoluble reaction partners in an aqueous suspension. Remarkably, several reactions involving azodicarboxylates are accelerated beyond the rate achieved by solvent-free mixing of miscible liquid reagents: Coupling of neat quadricyclane and dimethyl azodicarboxylate takes 2 days as compared to only 10 min “on water.” Hydrogen bonding appears to increase the reaction rate, yet heterogeneity is a surprisingly important factor. When a suspension was homogenized by adding methanol, the reaction slowed down. A molecular explanation for the phenomenon is elusive, but the authors have encouraged those who make related observations to share their thoughts. — JSY

    Angew. Chem. Int. Ed. 10.1002/anie.200462883 (2005).


    Silencing Mini-μ

    Premature stop codons (PTCs) result in truncated proteins, species that would be extremely injurious. A post-transcriptional monitoring system—nonsense-mediated decay (NMD)—has evolved in eukaryotes to remove PTC-containing mRNAs before they can be translated. Immunoglobulin (Ig) genes are rearranged as part of normal lymphocyte development, and alleles containing PTCs are generated as nonfunctional byproducts of the process. Transcripts from these alleles are destroyed by NMD, but features of their extirpation suggest that something else is also suppressing these rogue mRNAs.

    Bühler et al. have introduced PTCs into mouse Ig-μ minigenes and assayed their expression in tissue culture cells. They find that posttranscriptional NMD accounts for a 50% reduction in their expression. But they also find that 50% of the suppression occurs at the level of transcription and is mirrored by chromatin features associated with gene silencing: the loss of histone acetylation and an increase in methylation of histone H3 on the lysine-9 residue in the vicinity of the PTC-containing minigenes. Repression of putative small interfering RNAs (siRNAs) by overexpression of the siRNase 3'hExo abrogates the PTC-suppression effect, suggesting that RNA interference-related mechanisms may be involved. — GR

    Mol. Cell 18, 307 (2005).


    Happiness in the Civil Service

    It is not surprising that negative emotional states, such as stress or depression, are associated with a higher risk of unhealthy conditions, such as cardiovascular disease. We can assess stress (cortisol) and depression (psychiatric diagnosis) in objective ways, but how can we ascertain whether positive affect (happiness) is healthful? In beginning to address this question, Steptoe et al. have collected two data sets from over 200 British civil servants (mostly happy and healthy). One contains aggregate measurements (35 time points in a working day) of physiological (cortisol) and psychological (self-ratings) status, and the other contains similar measurements recorded in a laboratory mental stress test (modified Stroop task). [See also the Day Reconstruction Method of Kahneman et al., Reports, 3 December 2004, p. 1776.] They find that cortisol and plasma fibrinogen (a predictor of coronary heart disease) levels were inversely related to happiness and that these correlations were independent of psychological distress, supporting the idea that positive affect may be associated with neuroendocrine and cardiovascular indicators of well-being. — GJC

    Proc. Natl. Acad. Sci. U.S.A. 102, 6508 (2005).

  7. STKE

    Vascular Effects of Stress

    Atherosclerotic plaques, which develop in response to a localized inflammatory response, occur at regions of disturbed blood flow. Fluid shear stress stimulates the binding of endothelial cell integrins to the subendothelial extracellular matrix (ECM), leading to activation of the nuclear factor-κB (NF-κB) signaling pathway and transcription of target genes. Noting that endothelial cells express multiple integrins that bind to matrix proteins and that inflammation promotes the deposition of fibronectin and fibrinogen into the subendothelial ECM, Orr et al. found that changes in subendothelial matrix composition and activation of NF-κB target genes occurred at regions of disturbed flow in vivo before other atherosclerotic changes and were most pronounced in atherosclerosis-prone mice fed a high-fat diet. Fluid shear stress promoted phosphorylation and translocation to the nucleus of NF-κB in bovine aortic endothelial cells cultured on fibrinogen or fibronectin. In contrast, shear stress, acting through integrin α2β1, promoted activation of the p38 protein kinase in cells grown on collagen, leading to reduced NF-κB activation. Intriguingly, NF-κB activation in cells grown on fibronectin could be blocked by treatment with a peptide that alters matrix structure and stimulates p38, suggesting that modification of the ECM with external factors (and localized activation of p38 at integrin adhesion sites) could provide a novel approach to treating atherosclerosis. — EMA

    J. Cell Biol. 169, 191 (2005).