Editors' Choice

Science  09 Dec 2005:
Vol. 310, Issue 5754, pp. 1587
  1. GEOCHEMISTRY

    Emulsifying the Crust

    1. Brooks Hanson

    Not quite 2 billion years ago, a large asteroid stuck itself into what is now eastern central Ontario, forming the Sudbury impact crater. The energy of the impact melted a large amount of the continental crust, producing a thick melt sheet that was initially about 1700°C, well above the liquidus for norite (~56% silica) and for granophyre (~70% silica).

    Zieg and Marsh describe the subsequent evolution and cooling of this molten body as a natural experiment that can be compared to the formation of magma bodies in igneous intrusions such as those underlying volcanoes. The superheated Sudbury melt sheet began as an emulsion containing droplets of silica-rich and silica-poor magma; the less dense, silica-rich drops separated within months and coalesced into an upper melt sheet. Vigorous convection in both sheets occurred until they cooled to the liquidus, at which time crystals began to form and convection ceased. The combined melt layers solidified from the top and bottom. Aside from the initial separation of the two liquids, the solidified sheet shows little compositional gradations. Early formed crystals are dispersed throughout, and layers are not apparent. These textures contrast with those of many igneous magma bodies, suggesting that the latter may not have originated as large hot chambers at an instant in time. — BH

    Geol. Soc. Am. Bull. 177, 1427 (2005).

  2. BIOCHEMISTRY

    Ribosomal Logic

    1. Gilbert J. Chin

    The recently acquired appreciation of metabolic and regulatory pathways as an immensely complicated wiring diagram has been accompanied by attempts to reroute and redefine these circuits by adding and subtracting switches and connectors. One challenge, of course, is to maintain cell viability while tinkering with macromolecular components whose interactions may not yet be completely specified. Rackham and Chin have developed an orthogonal approach—building a parallel metabolism within a cell—by selecting for modified Shine-Dalgarno sequences that bind to correspondingly modified 16 Sribosomal RNAs (rRNAs) and that no longer bind to wild-type 16 SrRNAs. Amazingly, these orthogonal 16 SrRNAs still assemble into competent ribosomes, and placing the cognate Shine-Dalgarno sequence in front of a reporter gene results in faithful translation of an active enzyme independently of the endogenous protein synthesis machinery. Introducing several pairs of orthogonal messenger RNAs and rRNAs allows for the construction of AND and OR gates within otherwise unperturbed Escherichia coli. — GJC

    J. Am. Chem. Soc.10.1021/ja055338d (2005); Nat. Chem. Biol. 1, 159 (2005).

  3. CELL BIOLOGY

    A Good Amyloid

    1. Stella M. Hurtly

    Amyloids are an insoluble fibrous form of protein aggregates and are generally associated with a variety of neurodegenerative disease states. Fowler et al. find that in melanocytes, intracellular amyloid is not a pathological aberration but instead plays a productive role in melanin formation. Melanin is a tyrosine-based polymer that protects organisms from some toxins and ultraviolet radiation. In mammalian melanocytes, melanin is produced within membrane-bound organelles known as melanosomes, with the aid of the protein Pmel17. During this process, it appears that Pmel17 adopts an amyloid-like structure that provides a template for the assembly of melanin precursors, and recombinant Pmel17 amyloid was observed to accelerate melanin production in vitro. Within the cell, the Pmel17-containing amyloid could also serve to sequester highly reactive intermediates in melanin biosynthesis. — SMH

    PLoS Biol. 4, e6 (2006).

  4. SURFACE SCIENCE

    Subsurface Manipulation

    1. Phil D. Szuromi

    The movement of hydrogen into and out of the bulk regions of metals is important in hydrogen storage, metal embrittlement, and fuel cell reactions. Sykes et al. used voltage pulses delivered via a scanning tunneling microscope tip to manipulate subsurface hydrogen atoms. They applied bias pulses of >0.5 V to a Pd(111) surface held at 4 K that had had hydrogen removed from its near-surface region by oxygen treatment. These bias pulses were able to excite residual hydrogen atoms in the bulk (which has a population of one H atom per 2000 Pd atoms) and allowed these atoms to move into more energetically favorable subsurface sites. The subsurface hydride depleted the surface Pd atoms of charge and caused an outward surface relaxation of Pd atoms of 0.1 to 0.6 Å. Surface hydrogen also tended to move away from these regions to leave behind ordered arrays of overlayer vacancies. — PDS

    Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.0506657102 (2005).

  5. CHEMISTRY

    All in the Dope

    1. Marc S. Lavine

    Cadmium selenide nanoparticles are used in light-emitting diodes, lasers, and sensors and for biological labeling. However, the toxicity of cadmium is a major concern. Zinc chalcogenides, such as ZnSe, doped with transition metal ions may offer as much flexibility and dynamic range as CdSe, but it has been difficult to dope particles uniformly. Recent success in separating the nucleation and growth phases in making high-quality nanoparticles prompted Pradhan et al. to consider whether efficient and controlled doping could be introduced. For growth-stage doping, seed ZnSe particles were quenched, and copper was then added as a dopant. Overgrowth with additional ZnSe shifted the photoluminescence (PL) toward the red wavelengths. For the nucleation strategy, Mn was added to shift the PL even further toward the red. The nanoparticle syntheses were performed as one-pot reactions so control of the doping relative to the nucleation or growth could be achieved by varying the reactivity of the precursors and the temperature. — MSL

    J. Am. Chem. Soc.10.1021/ja055557z (2005).

  6. ECOLOGY

    Fisheries Failures

    1. Caroline Ash

    Some collapsed fisheries fail to recover even when harvesting has stopped for more than a decade. Fishing usually targets the largest, oldest, and fastest-growing individuals and hence favors the survival of smaller, younger, and slower-growing fish. Walsh et al. have chosen the Atlantic silverside, a commercially exploited fish with an annual life cycle, for harvesting experiments under a variety of regimens. They found that selecting out the largest individuals affected multiple traits in subsequent generations, with significant reductions in vertebral number, egg size and subsequent viability; rates of growth and growth efficiency; and foraging and fecundity. It is still not clear why some fish stocks fail to recover and others are more resilient, although duration and intensity of exploitation may be a factor. The authors are continuing to monitor rates of recovery of the experimental silverside populations. — CA

    Ecol. Lett. 8, 10.1111/j.1461-0248.2005.00858.x (2005).

  7. STKE

    Remember That Gradient?

    1. L. Bryan Ray

    During early development, morphogen gradients instruct the differentiation of distinct cell types in proper spatial order. Exposure of cells to a specific concentration of morphogen can specify cell fate, but the exposure does not need to last for the several hours needed to complete execution of the gene expression program that drives the cell's response. Jullien and Gurdon explored how cells remember a brief exposure to morphogen by studying responses of Xenopus embryo cells to activin. Exposure for 10 min resulted in changes in gene expression several hours later. This response appeared to require continuous receptor signaling, because it could be inhibited at later stages by a pharmacological inhibitor of kinase activity of the activin receptor. Continued signaling also appeared to require receptor internalization, because a dominant-negative form of dynamin that prevents internalization of receptors from the plasma membrane inhibited activin-dependent gene expression when injected into embryonic cells. Expression of mutant Rab proteins that increase trafficking of membrane proteins through the lysosomal pathway (and thus increase the rate at which they are degraded) did not affect the memory of the activin signal, and the authors concluded that the signaling receptors have not yet entered the degradation pathway. Rather, it seems that the persistence of vesicles as they move from the plasma membrane to the lysosome accounts for the signal, and the authors propose that receptors activated by brief exposure to activin provide a prolonged signal. — LBR

    enes Dev. 19, 2682 (2005).

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