Catálogo de publicaciones - revistas

<jats:title>Improving Lithium Batteries</jats:title> <jats:p> Lithium-oxygen batteries have similar volumetric energy densities to lithium-ion batteries, but, because the oxygen part of the battery can be extracted from the air, they have a significant advantage in their gravimetric energy densities. One of the fundamental problems plaguing the nonaqueous Li-O <jats:sub>2</jats:sub> system is that the Li <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> that forms on discharge must be completely reversed on charging, but for most systems, a range of side products form instead of Li <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> . <jats:bold> Peng <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="563" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1223985">563</jats:related-article> , published online 19 July) show that by using dimethyl sulfoxide as the electrolyte, and a porous gold cathode, they can get reversible production and removal of Li <jats:sub>2</jats:sub> O <jats:sub>2</jats:sub> during discharge and charge cycles. Furthermore, the electrolyte-electrode system operates with much faster kinetics than carbon electrodes. </jats:p>

<jats:title>Particles Without Borders</jats:title> <jats:p> Aerosols have important and often adverse impacts on atmospheric composition, air quality, and climate. However, aerosols can be transported long distances, limiting the efficacy of local regulations. <jats:bold> Yu <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="566" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1217576">566</jats:related-article> ) used satellite data to estimate how much of the aerosol load in the atmosphere above North America originates overseas. Approximately half of the dust and pollution over North America comes from Asia, Europe, Africa, and the Middle East. Asian dust is the largest fraction of this total. Furthermore, potential increases in dust emissions in response to climate change might overwhelm any reductions in pollution from emerging Asian economies. </jats:p>

<jats:title>A Picture of Disappearing Ice</jats:title> <jats:p> Global warming is accelerating the loss of ice sheet mass by melting, sublimation, and erosion of their margins. In order to provide a better context for understanding contemporary losses, a longer record of the recent past is needed. <jats:bold> Kjær <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="569" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1220614">569</jats:related-article> ) extend the record of thinning along the northwest margin of the Greenland Ice Sheet back to the mid-1980s, by using archived aerial photographs in conjunction with a digital elevation model and comparing their results to more recent data. Northwestern Greenland has experienced two dynamic ice loss events in the past three decades. Local ice loss appears to be caused by a combination of predictable surface processes that operate over decadal time scales and ones that involve the rapid movement of ice over periods of 3 to 5 years that exhibit strong regional differences. </jats:p>

<jats:title>Secondary Endogenous Small and Interfering</jats:title> <jats:p> In many eukaryotes, Piwi proteins bind small noncoding Piwi-interacting RNAs (piRNAs) that function to silence transposons in the germ line and protect the germ line from transposable element–driven recombination and mutation. <jats:bold> Bagijn <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="574" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1220952">574</jats:related-article> , published online 14 June; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6094" page="529" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1227095"> <jats:bold>Xiol and Pillai</jats:bold> </jats:related-article> </jats:bold> ) show that in the nematode, <jats:italic>Caenorhabditis elegans</jats:italic> , a messenger RNA (mRNA) that contains a piRNA target sequence gives rise to a second, downstream class of small RNAs known as secondary endogenous small interfering RNAs, or endo siRNAs. These endo siRNAs map to the vicinity of the piRNA complementary sequence in the mRNA target and depend on both Piwi and on factors involved in the related RNA interference pathway for their genesis, but not on the Piwi slicer activity. Mapping the endo siRNAs reveals that piRNAs can target imperfectly matched targets and that piRNAs target a subset of both transposons and endogenous genes for silencing. </jats:p>

<jats:title>Learning to Recognize a Cuckoo</jats:title> <jats:p> Species that are parasitized by cuckoos have evolved several strategies for trying to avoid having their nests hijacked—one of the most obvious being outright attacking, or mobbing, of cuckoos that enter the area. However, cuckoos are not without evolved defenses—most common cuckoo females look remarkably similar to a small hawk, and this mimicry deters mobbing. <jats:bold>Thorogood and Davies</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="578" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1220759">578</jats:related-article> ; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6094" page="532" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1225997">Mappes and Lindström</jats:related-article> </jats:bold> ) show that social learning in parasitized birds can thwart this protective mimicry. When hosts observe mimics being mobbed, they are more likely to mob them, themselves, later. However, the hosts will only mob the color morph that they observed being mobbed. This specificity may have allowed for the evolution and maintenance of two female morphs within common cuckoos. </jats:p>

<jats:title>Successful Invaders</jats:title> <jats:p> Invasive species have been integrated into ecosystems worldwide, and in many cases can cause significant ecological and economic damage. Not all non-native species, however, become invasive species; thus there has been much effort put toward understanding what makes a non-native “colonizer” an invader. It has been thought that, in general, successful invaders tend to be those that produce a large amount of offspring over a very short period of time, however, this pattern is absent in many successful invaders. <jats:bold> Sol <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="580" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1221523">580</jats:related-article> ) looked at over 2700 invasions by bird species across the world and found no relationship between population growth rate and invasion success, although rapidly reproducing species could have an advantage when the new environment resembled their native environment. Furthermore, in many cases, those species that could prioritize survival, and delay reproduction, were much more successful than those forced to reproduce regardless of environmental conditions. </jats:p>

<jats:title>Is Winter Past?</jats:title> <jats:p> Some plants are adapted to wait through the winter before their genetic programs for flowering are initiated. But the duration of winter varies with location, and thus, to avoid flowering too late into the summer or exposing blossoms to frost, the adaptation needs to hold the flowers off for just the right amount of time. Working with <jats:italic>Arabidopsis thaliana</jats:italic> , <jats:bold> Coustham <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="584" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1221881">584</jats:related-article> , published online 12 July) identified a quantitative mechanism that “counts” off the duration of winter. FLC, a repressor of flowering, carries sequence polymorphisms that direct epigenetic events. Plants with fewer of these polymorphisms are adapted to short winters, and plants with all four polymorphisms are adapted to long winters. </jats:p>

<jats:title>Initiating Mitochondrial Repair</jats:title> <jats:p> The mitochondrial unfolded protein response (UPRmt) mediates the up-regulation of nuclear encoded mitochondrial chaperone genes in response to mitochondrial dysfunction. How mitochondrial dysfunction is communicated to the nucleus is unclear, but requires the transcription factor, ATFS-1. <jats:bold> Nargund <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="587" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1223560">587</jats:related-article> , published online 14 June) found that the key point of regulation in UPRmt signaling is mitochondrial protein import efficiency of ATFS-1. In addition to a nuclear localization sequence (NLS), ATFS-1 has a mitochondrial targeting sequence (MTS) that is necessary for UPRmt repression. ATFS-1 is normally imported efficiently into mitochondria and degraded by the Lon protease. However, in the presence of stress, some ATFS-1 fails to be imported into mitochondria and is trafficked to the nucleus. The juxtaposition of a C-terminal NLS to an N-terminal MTS in a transcriptional activator thus couples unfolded protein load in the mitochondrial matrix to a rectifying transcriptional response in the nucleus. </jats:p>

<jats:title>Clamping Down</jats:title> <jats:p> Crystal structures of RNA polymerase show that a “clamp” region which surrounds the DNA binding site can adopt conformations ranging from a closed to an open state. <jats:bold> Chakraborty <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="591" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1218716">591</jats:related-article> ) used single-molecule fluorescence energy transfer experiments to detect the clamp's conformational changes in solution during the transcription cycle. The results support a model in which a clamp opening allows DNA to be loaded into the active-center cleft and unwound. Direct interactions with DNA likely trigger clamp closure upon formation of a catalytically competent transcription initiation complex. </jats:p>

<jats:title>The Song of the Elephant</jats:title> <jats:p> In mammals, vocal sound production generally occurs in one of two ways, either through muscular control—as when a cat purrs or, more commonly, by air passing through the vocal folds—which occurs in humans and facilitates production of extremely high frequency bat calls. Over the past 20 years, it has been recognized that elephants can communicate through extremely low frequency infrasonic sounds. Taking advantage of a natural death of an elephant in a zoo, <jats:bold> Herbst <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="595" related-article-type="in-this-issue" vol="337" xlink:href="10.1126/science.1219712">595</jats:related-article> ) examined the biomechanics of elephant sound production in an excised elephant larynx. Self-sustained vocal-fold vibrations, without the presence of any neural control, were used to produce infrasonic elephant sounds, using the same mechanism as singing in humans and echolocation in bats. </jats:p>