Catálogo de publicaciones - revistas

<jats:title>Bring Out the β-Barrel</jats:title> <jats:p> The assembly of β-barrel membrane proteins, which are found in the outer membrane of Gram-negative bacteria and in the mitochondria and chloroplasts of eukaryotes, is poorly understood. Now <jats:bold> Hagan <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="890" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1188919">890</jats:related-article> , published online 8 April; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5980" page="831" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1190507"> <jats:bold> Stroud <jats:italic>et al.</jats:italic> </jats:bold> </jats:related-article> </jats:bold> ) describe the development of a reconstituted system that recapitulates the process of assembly of β-barrel membrane proteins by the <jats:italic>Escherichia coli</jats:italic> Bam complex. The assembly of a protein substrate required the purified five-protein Bam complex and several subcomplexes and a chaperone, but did not require an external input of energy. </jats:p>

<jats:title>Pedigree Weevils</jats:title> <jats:p> The phenotype of an organism is related to its additive genetic phenotype, the small but numerous genetic differences between individuals affecting their phenotype, and its mutational load, which is the number of mutations an individual carries. How these two factors affect an individual's ability to leave offspring in the next generation is a major component of evolutionary theory, but evidence for a direct relationship has been lacking. By estimating the contribution of partially recessive mutations to additive genetic variation in seed-feeding cow-pea weevils over a seven-generation pedigree breeding program involving variation in levels of inbreeding, <jats:bold> Tomkins <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="892" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1188013">892</jats:related-article> ) demonstrate an association between genetic quality and the mutational load carried, especially for males. </jats:p>

<jats:title>Demise of the Lizards</jats:title> <jats:p> Despite pessimistic forecasts from recent studies examining the effects of global climate change on species, and observed extinctions in local geographic areas, there is little evidence so far of global-scale extinctions. <jats:bold> Sinervo <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="894" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1184695">894</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="5980" page="832" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1190374"> <jats:bold> Huey <jats:italic>et al.</jats:italic> </jats:bold> </jats:related-article> </jats:bold> ) find that extinctions resulting from climate change are currently reducing global lizard diversity. Climate records during the past century were synthesized with detailed surveys of Mexican species at 200 sites over the past 30 years. Temperature change has been so rapid in this region that rates of adaptation have not kept pace with climate change. The models were then extended to all families of lizards at &gt;1000 sites across the globe, and suggest that climate change-induced extinctions are currently affecting worldwide lizard assemblages. </jats:p>

<jats:title>Nitrate for Me, Ammonium for You</jats:title> <jats:p> The interdependence of plant nitrogen uptake and plant responses to carbon dioxide is well established, but the influence of inorganic nitrogen form—i.e., whether nitrate or ammonium—has been largely ignored. <jats:bold> Bloom <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="899" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1186440">899</jats:related-article> ) present evidence from five independent methods in both a monocot and dicot species that carbon dioxide inhibition of nitrate assimilation is a major determinant of plant responses to rising atmospheric concentrations of carbon dioxide. This finding explains several phenomena, including carbon dioxide acclimation and decline in food quality. The large variation in these phenomena among species, locations, or years derives from the large variation in the relative dependence of plants on nitrate and ammonium as nitrogen sources among species, locations, or years. The relative importance of ammonium and nitrate for plant N nutrition in future cropping systems will be critical for quantity and quality of food. </jats:p>

<jats:title>Population Meltdown</jats:title> <jats:p> Populations of wild animals, including deer and moose, are often actively managed by hunting. Following such harvesting, populations of some exploited animal species collapse, whereas others are able to withstand exploitation. To understand the reasons for these varied responses, <jats:bold> Fryxell <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="903" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1185802">903</jats:related-article> ) developed a mathematical model which predicts that weak regulation causes damped population cycles with period lengths on the order of decades. The model was tested using time-series data for hunted populations of moose and deer in three ecosystems in Norway and Canada. </jats:p>

<jats:title> Counting Na <jats:sup>+</jats:sup> Channels One by One </jats:title> <jats:p> Understanding how nerve cells integrate their synaptic inputs and generate their output signals requires the identification of voltage-dependent ion channels on the axo-somato-dendritic surface of central neurons. Using improved ultrastructural immunocytochemistry techniques, <jats:bold>Lorincz and Nusser</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="906" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1187958">906</jats:related-article> ) found that a newly described voltage-gated sodium channel, Nav1.6, was present not only at nodes of Ranvier and axon initial segments but also at much lower, but functionally significant levels, in dendrites of CA1 pyramidal cells. However, other brain Na <jats:sup>+</jats:sup> channels were not present in these dendrites, suggesting that dendritic sodium spikes result from somatic activation of this particular type of sodium channel. </jats:p>

<jats:title>It's a Knockout</jats:title> <jats:p> The malaria parasite is one of the most important pathogens of humans. Increasing drug-resistance is an imminent public health disaster, and we urgently need to find new drugs. The recently acquired malarial genomes provide a plethora of targets. However, due to the genetic intractability of the parasite, it has been difficult to identify essential genes in the clinically relevant blood-stage of the parasite. <jats:bold> Dvorin <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="910" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1188191">910</jats:related-article> ) investigated the function of a <jats:italic>Plasmodium falciparum</jats:italic> plant-like calcium-dependent protein kinase, PfCDPK5, which is expressed in the invasive blood-stage forms of the parasite. A system for conditional protein expression allowed the production of a functional knockout in the bloodstream stage of the parasite. PfCDPK5 was required for parasite egress from the human host erythrocyte, an essential step in the parasite life cycle. </jats:p>

<jats:title>siRNA Movement in Plant Tissues</jats:title> <jats:p> Long-distance movement of RNA interference (RNAi)–derived signals in plants plays an important role in development and in defense against viral attack. The nature of the signals that spread from cell to cell is not known, although evidence suggests that they are nucleic acids of some sort (see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5980" page="834" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1190510">Martienssen</jats:related-article> </jats:bold> ). <jats:bold> Molnar <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="872" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1187959">872</jats:related-article> , published online 22 April) and <jats:bold> Dunoyer <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="912" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1185880">912</jats:related-article> , published online 22 April) now show that in <jats:italic>Arabidopsis</jats:italic> , both exogenous and endogenous small interfering RNAs (siRNAs), rather than their long double-stranded precursor RNAs, are the molecules that transfer information between plant cells. A viral protein that counters RNAi though sequestering siRNAs blocked spreading of a transgene RNAi silencing signal. Furthermore, siRNA-processing enzymes were required in the source, and not the recipient, cells for spreading, and bombardment of plants with double-stranded siRNAs directly showed siRNA spread between cells. Endogenous siRNAs also spread between tissues and were capable of directing DNA methylation of target sequences in distant tissues. </jats:p>

<jats:title>Epigenetic Maps</jats:title> <jats:p> Methylation of genomic DNA on cytosine bases provides critical epigenetic regulation of gene expression and is involved in silencing transposable elements (TEs) and repeated sequences, as well as regulating imprinted gene expression. <jats:bold> Zemach <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="916" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1186366">916</jats:related-article> , published online 15 April; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5980" page="837" related-article-type="in-this-issue" vol="328" xlink:href="10.1126/science.1190738">Jeltsch</jats:related-article> </jats:bold> ) analyzed DNA methylation in the genomes of five plants, five fungi, and seven animals by bisulfite sequencing. The data suggest that land plants and vertebrates, which have extensive DNA methylation, are under strong selective pressure to repress TEs, because of their sexual mode of reproduction. Unicellular animals and fungi that reproduce asexually are more likely to lose TE methylation. Although gene body methylation is evolutionarily ancient, it is also mutagenic, and so loss of this pathway has been relatively common and occurred early in fungal evolution and later in several plant and animal lineages. </jats:p>

<jats:p>Mass spectrometry is clearly having its moment in the sun. Once restricted to a handful of well-equipped research laboratories, mass spectrometers have now become almost ubiquitous. Even modestly funded scientists can usually get their samples analyzed in a core mass spectrometry facility, and user-friendly spectrometers are becoming a fixture of forensic and clinical labs worldwide.</jats:p>