Catálogo de publicaciones - revistas

<jats:p> Gallium nitride nanorods were prepared through a carbon nanotube–confined reaction. Ga <jats:sub>2</jats:sub> O vapor was reacted with NH <jats:sub>3</jats:sub> gas in the presence of carbon nanotubes to form wurtzite gallium nitride nanorods. The nanorods have a diameter of 4 to 50 nanometers and a length of up to 25 micrometers. It is proposed that the carbon nanotube acts as a template to confine the reaction, which results in the gallium nitride nanorods having a diameter similar to that of the original nanotubes. The results suggest that it might be possible to synthesize other nitride nanorods through similar carbon nanotube–confined reactions. </jats:p>

<jats:p>Coronae are volcanotectonic features that are unique to Venus and are interpreted to be small-scale upwellings. A model in which upwelling causes delamination at the edge of the plume head, along with deformation of a preexisting depleted mantle layer, produced the full range of topographic forms of coronae. If half of the coronae are active, delamination of the lower lithosphere could account for about 10 percent of Venus' heat loss, with another 15 percent due to upwelling. Delamination may occur in other geologic environments and could account for some of Venus' heat loss “deficit.”</jats:p>

<jats:p>Annually dated ice cores from Siple Dome, West Antarctica, and central Greenland indicate that meridional atmospheric circulation intensity increased in the polar South Pacific and North Atlantic at the beginning (∼1400 A.D.) of the most recent Holocene rapid climate change event, the Little Ice Age (LIA). As deduced from chemical concentrations at these core sites, the LIA was characterized by substantial meridional circulation strength variability, and this variability persists today despite strong evidence for an end to LIA cooling. Thus, increased late 20th century storm variability may be in part a result of the continuation of these climatic fluctuations.</jats:p>

<jats:p>Island area is frequently a major determinant of the species composition of biological communities; community structure, in turn, often has important effects on ecosystem-level properties. Fifty islands of varying area were selected in an archipelago in the northern Swedish boreal forest zone, in which larger islands burn more frequently than smaller ones through wildfire arising from lightning strike, thus inducing a significant relationship between island area and plant species composition. This relationship was found to be a major factor in determining several ecosystem-level properties of these islands, including standing biomass, plant litter decomposition, nitrogen mineralization, terrestrial carbon partitioning, humus accumulation, and plant nitrogen acquisition.</jats:p>

<jats:p>Humans are modifying both the identities and numbers of species in ecosystems, but the impacts of such changes on ecosystem processes are controversial. Plant species diversity, functional diversity, and functional composition were experimentally varied in grassland plots. Each factor by itself had significant effects on many ecosystem processes, but functional composition and functional diversity were the principal factors explaining plant productivity, plant percent nitrogen, plant total nitrogen, and light penetration. Thus, habitat modifications and management practices that change functional diversity and functional composition are likely to have large impacts on ecosystem processes.</jats:p>

<jats:p>The relative effects of plant richness (the number of plant functional groups) and composition (the identity of the plant functional groups) on primary productivity and soil nitrogen pools were tested experimentally. Differences in plant composition explained more of the variation in production and nitrogen dynamics than did the number of functional groups present. Thus, it is possible to identify and differentiate among potential mechanisms underlying patterns of ecosystem response to variation in plant diversity, with implications for resource management.</jats:p>

<jats:p>Newly assembled major histocompatibility complex (MHC) class I molecules, together with the endoplasmic reticulum chaperone calreticulin, interact with the transporter associated with antigen processing (TAP) through a molecule called tapasin. The molecular cloning of tapasin revealed it to be a transmembrane glycoprotein encoded by an MHC-linked gene. It is a member of the immunoglobulin superfamily with a probable cytoplasmic endoplasmic reticulum retention signal. Up to four MHC class I–tapasin complexes were found to bind to each TAP molecule. Expression of tapasin in a negative mutant human cell line (220) restored class I–TAP association and normal class I cell surface expression. Tapasin expression also corrected the defective recognition of virus-infected 220 cells by class I–restricted cytotoxic T cells, establishing a critical functional role for tapasin in MHC class I–restricted antigen processing.</jats:p>

<jats:p> Activation domains are functional modules that enable sequence-specific DNA binding proteins to stimulate transcription. The structural basis for the function of activation domains is poorly understood. A combination of nuclear magnetic resonance (NMR) and biochemical experiments revealed that the minimal acidic activation domain of the herpes simplex virus VP16 protein undergoes an induced transition from random coil to α helix upon binding to its target protein, hTAF <jats:sub>II</jats:sub> 31 (a human TFIID TATA box <jats:bold>–</jats:bold> binding protein-associated factor). Identification of the two hydrophobic residues that make nonpolar contacts suggests a general recognition motif of acidic activation domains for hTAF <jats:sub>II</jats:sub> 31. </jats:p>

<jats:p> The <jats:italic>SGS1</jats:italic> gene of yeast encodes a DNA helicase with homology to the human <jats:italic>WRN</jats:italic> gene. Mutations in <jats:italic>WRN</jats:italic> result in Werner's syndrome, a disease with symptoms resembling premature aging. Mutation of <jats:italic>SGS1</jats:italic> is shown to cause premature aging in yeast mother cells on the basis of a shortened life-span and the aging-induced phenotypes of sterility and redistribution of the Sir3 silencing protein from telomeres to the nucleolus. Further, in old <jats:italic>sgs1</jats:italic> cells the nucleolus is enlarged and fragmented <jats:bold>—</jats:bold> changes that also occur in old wild-type cells. These findings suggest a conserved mechanism of cellular aging that may be related to nucleolar structure. </jats:p>