This Week in Science

Science  31 Aug 2001:
Vol. 293, Issue 5535, pp. 1553
  1. Lighting a Route to Hydrogen Production

    In principle, it should be possible to create small molecules that catalyze the conversion of their hydrogen-containing solvents into H2 in the presence of sunlight without relying on any heterogeneous mediators. Heyduk and Nocera (p. 1639; see the Perspective by McCusker) show that a dirhodium compound dissolved in pure hydrohalic acids (HX, such as condensed HCl with no water present) can photogenerate H2. In the catalytic cycle, ultraviolet (UV) light removes a CO ligand so that HX can bind in a two-electron step to create a mixed-valence Rh0-RhII species. Two of these molecules react to add an additional HX and then to liberate H2. Finally, UV light eliminates X to regenerate the catalyst. This system is still short of the goal of photoproduction of H2 from H2O, but it shows that important steps, such as halide removal, can be catalyzed by two-electron processes.

  2. Source of Galactic Plane Plasma

    Hard x-ray emissions from the Galactic plane of the Milky Way form a ridge-like structure. The hard x-rays are from ionized elements such as silicon, sulfur, and iron, and the source of this plasma has been a mystery. Ebisawa et al. (p. 1633) completed a deep and high-resolution x-ray survey of part of the Galactic plane using the Chandra X-ray Observatory and found 36 new point sources. The majority of these point sources are extragalactic, and thus the diffuse plasma is not from Galactic sources. Instead, the authors suggest that supernova remnants may be the source consistent with theoretical models for producing this extremely high-temperature, high-density plasma.

  3. Buoyant Reykanes Ridge

    Iceland sits atop a hot spot plume and a slow spreading mid-ocean ridge. The intersection of these two magmatic systems provides a natural laboratory to study plume-ridge interactions. Gaherty (p. 1645) studied the difference in the travel time of vertically-polarized Rayleigh waves versus transversely-polarized Love waves along the Reykanes mid-ocean ridge, just south of Iceland (and the center of the hotspot plume). The Love waves traveled more slowly than the Rayleigh waves, a difference attributed to an extended region of vertical flow along the ridge created by the plume. Thus, the enhanced buoyancy and magmatism along the Reykanes Ridge is a consequence of the hotspot plume.

  4. All Patterns Great and Small

    The interaction of chemical reactions and molecular diffusion can create reaction fronts on length scales far larger than the molecules themselves. Sachs et al. (p. 1635; see the Perspective by Jaeger) have now visualized reaction fronts at the atomic scale for the oxidation of hydrogen on the (111) surface of platinum. Scanning tunneling microscopy revealed that an underlying autocatalytic process, which generates diffusing OH, affects patterns that form on the scale of tens of nanometers. Modeling studies show that a simple reaction-diffusion model cannot quantitatively describe the results and that more complex interactions between surface species need to be taken into account.

  5. The Grass That Grows

    It has been proposed that atmospheric CO2 has been the dominant influence on the relative abundance of C3 plants, such as trees, and C4 plants, such as grasses, in the geologic past. These two types of plants are distinguished by their use of different photosynthetic pathways, and they respond differently to higher or lower CO2 concentrations. Huang et al. (p. 1647; see the news story by Kerr) examine the generality of this proposal by comparing two lake sediment cores from Mesoamerica. They find that moisture variations over the past 25 thousand years have had a large effect on the expansion and contraction of C4 species, and that the two sites have responded differently depending on the climatological control of aridity. These results indicate that compiling C4 plant histories based solely upon CO2 concentrations overlooks an important variable.

  6. Information with a Twist

    DNA microarrays show tremendous power for yielding information. By sorting on the basis of fluorescent protein expression, Furlong et al. (p. 1629) acquired enough biological material to do DNA microarray analysis on Drosophila embryos of known developmental stages and known mutant status. With a particular view toward mesoderm development, the data yield information on hundreds of genes that are turned on and off during normal mesoderm development, seemingly orchestrated by a particularly important transcription factor, Twist. Analysis of embryos lacking Twist and embryos overexpressing Twist, when compared to expression profiles from normal embryos, leads to insights into the function of individual genes as well as groups of co-regulated genes.

  7. Ubiquitin Stimulation of Transcription

    The herpes simplex virus transcription factor VP16 has long been studied for its ability to activate transcription. Similar to other transcription activators, VP16 has a transcription activation domain that overlaps with a domain that signals its degradation. Is there a mechanistic link between transcription and proteolysis? Salghetti et al. (p. 1651) show that in yeast the Met30 subunit of a ubiquitin ligase is not only required for the ubiquitin-proteasome pathway but is also required for transcriptional activation by VP16. The process of transcription activation was separated from proteolysis by artificially ubiquinating VP16. Ubiquitin may be fulfilling this additional role, separate from that in protein degradation, by recruiting the proteasome.

  8. Afterimages and Illusions

    At what stage in the visual pathway does the production of afterimages with illusory filling-in occur? Shimojo et al. (p. 1677) tried to dissociate this type of afterimage from traditional afterimages that are thought to occur within the retina due to bleaching of photoreceptors. They observed a clear dissociation of aftereffects associated with illusory filling-in from those associated with actual stimuli, and they conclude that the former has a cortical rather than a retinal locus.

  9. Maintaining Fluid Homeostasis

    Natriuretic peptides are hormones that are involved in the regulation of fluid balance, with particular relevance for cardiovascular function and blood pressure. This family of peptides is recognized by a family of cell surface receptors (NPR-A, -B, and -C). He et al. (p. 1657) have determined the crystal structures of the extracellular portion of NPR-C with and without natriuretic peptide C bound. One molecule of the peptide binds to a receptor dimer; binding appears to close the cleft between the monomers.

  10. Extracellular Signaling Pathways

    Heparan-sulfate proteoglycans (HSPGs) are important mediators of extracellular signaling by Wnt, Fgf, and hedgehog proteins. Dhoot et al.(p. 1663) identify a member of a family of conserved sulfatases, QSulf1, which is responsive to Sonic hedgehog (Shh) signaling in the somite and neural tube. However, Qsulf1 does not function directly in the Shh signaling pathway. Instead, HSPG mediates Wnt signaling during avian myogenesis. This work connects heparan sulfate proteoglycans (HSPGs) to signaling events in cell fate determination and identifies a sulfatase enzyme as a signal modulator.

  11. Strengthening Blood Vessels

    After an injury, the coagulation cascade goes into action to repair damaged tissue. When coagulation factors are eliminated in mouse embryos, they die at mid-gestation and display extensive bleeding. Whether this is a result of defective platelet function or vascular defects has been unclear. Griffin et al. (p. 1666; see the Perspective by Carmeliet) have addressed this question by examining the role of Par1, a G protein—coupled receptor that binds to and is cleaved by thrombin, a serine-protease coagulation factor. Par1 is shown to be an endothelial factor and is needed for normal vascular development and integrity.

  12. Sweet View of Heart Development

    Cardiac valves are essential for normal heart function. Formation of the atrioventricular valve in the developing vertebrate heart involves complex but poorly defined signaling interactions between myocardial and endocardial cells at the boundary of the atrium and ventricle. Walsh and Stainier (p. 1670) provide molecular insight into this process through their analysis of a zebrafish mutant called jekyll, which is severely defective in the initiation of heart valve formation. The mutated gene encodes UDP-glucose dehydrogenase (UGDH), an enzyme required for production of hyaluronic acid and proteoglycans. Loss of UGDH appears to disrupt the events that mark the valve-forming region as distinct from atrium and ventricle.

  13. Aspirin for Diabetics

    Resistance of cells to the actions of insulin is a prominent feature of type 2 diabetes. As early as 1876, reports emerged that high doses of aspirin could help decrease concentrations of glucose in the blood of diabetic patients, an action that could reflect enhanced insulin action in target cells. Yuan et al. (p. 1673) are finally able to provide a mechanism that may explain this effect. Their studies show that aspirin in high doses inhibits the protein kinase IKKβ, which mediates the actions of pro-inflamatory cytokines. Furthermore, heterozygous mice that had lost one allele encoding IKKβ were protected against insulin resistance when fed a high-fat diet. Thus, IKKβ may be a useful target for therapeutic agents designed to ameliorate insulin resistance.

  14. A Flexible Biosensor

    Benson et al. (p. 1641) have developed a versatile approach for converting a molecular recognition event into an easily measured electrochemical signal by exploiting the large hinge-bending motions that signal molecular recognition in the bacterial periplasmic binding protein superfamily. The proteins are tied to a modified electrode surface and are tagged with a ruthenium complex. The distance of the complex to the surface, and hence the electrochemical response, changes when a ligand binds. A protein modified to bind zinc as well as the parent maltose-binding protein were used as detectors. This approach may be extended to a wide range of analytes and may be used to develop medical or environmental sensors.

  15. Acetylation and Nuclear Transport

    The transcription factor NF-κB functions to control gene expression in immune and inflammatory responses and to control susceptability of cells to apoptosis. It is therefore not surprising to find that multiple layers of regulation exist to control the activity of NF-κB and its movement in and out of the nucleus. NF-κB is normally held in an inactive state in the cytoplasm through interaction with its inhibitory protein (IκB). Signals that activate NF-κB cause degradation of IκB and translocation of active NF-κB to the nucleus. Now Chen et al. (p. 1653) indicate that another layer of regulation is superimposed on this regulation by IκB. They show that interaction of NF-κB with IκB is disrupted when the NF-κB subunit RelA is acetylated. In the nucleus, deacetylation of the RelA protein by histone deacetylase 3 (HDAC3) leads to its reassociation with IκB and consequent transport out of the nucleus. Thus, HDAC3, which also regulates transcription through deacetylation of histones, influences both the activity and cellular localization of the NF-κB complex.

  16. Gene Duplication and Evolution

    Two comments examine a study by Lynch and Conery (Reports, 10 Nov. 2000, p. 1151) that estimated the “birth rate” and half-life of gene duplications for several eukaryotic species over evolutionary time. Long and Thornton argue that the measure for duplicate-gene-pair age used in the study, substitutions per silent site (S), may not be “a suitable proxy,” that the half-life calculation rests on the “untested, hidden assumption” of a constant long-term rate of gene duplication, and that analysis of the statistical data suggests alternatives to the interpretation of Lynch and Conery. Zhang et al. hold that the study's conclusions “are compromised by the fact that their data … included many redundant records” and raise several other issues. Lynch and Conery defend their use of S, note that the assumption of long-term rate constancy was “stated explicitly” and “was not relevant to our birth rate estimates,” and address the statistical questions raised by Long and Thornton. They also “present some reanalyses … that take into consideration the concerns raised by Zhang et al.” The full text of these comments can be seen at