Catálogo de publicaciones - revistas

<jats:title>Unseen Planet</jats:title> <jats:p> The orbits of planetary bodies can be affected by gravitational interactions with other planets in the same system. If the planets can be seen to pass in front of their star (or to transit), the gravitational perturbations translate into variations in transit duration. <jats:bold> Nesvorný <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="1133" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1221141">1133</jats:related-article> , published online 10 May; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6085" page="1121" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1222590">Murray</jats:related-article> </jats:bold> ) inferred the presence of a far distant, previously unknown planet from the transit timing variations of a planet previously detected by the Kepler space telescope. The new planet is in a 57-day orbit and does not transit its host star. The analysis also revealed a third possible planet with a mass 1.7 times that of the Earth in a 6.8-day orbit. The radially spaced, roughly coplanar, and nearly circular orbits of the two confirmed planets mirror planetary orbits in our own solar system. </jats:p>

<jats:title>Dissecting Cooper Pairs</jats:title> <jats:p> Angle-resolved photoemission spectroscopy (ARPES) is used in the study of the electronic structure of complex materials. Recently, time-resolved ARPES has become possible, where the state of the system is excited by a short “pump” pulse, and ARPES is performed using a second “probe” pulse applied after varying times. <jats:bold> Smallwood <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="1137" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1217423">1137</jats:related-article> ) used this technique to study the recombination of Cooper pairs—the fundamental charge carriers in superconductors—in a cuprate high-temperature superconductor. </jats:p>

<jats:title>Updating the Triode with Graphene</jats:title> <jats:p> In early electronics, the triode—a vacuum device that combined a diode and an electrical grid—was used to control and amplify signals, but was replaced in most applications by solid-state silicon electronics. One characteristic of silicon-metal interfaces is that the Schottky barrier created—which acts as a diode—does not change with the work function of the metal—the Fermi level is pinned by the presence of surface states. <jats:bold> Yang <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="1140" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1220527">1140</jats:related-article> , published online 17 May) now show that for a graphene-silicon interface, Fermi-level pinning can be overcome and a triode-type device with a variable barrier, a “barristor,” can be made and used to create devices such as inverters. </jats:p>

<jats:title>Going Up Against the Grain Boundaries</jats:title> <jats:p> Exfoliated graphene sheets are single crystals that exhibit excellent electronic properties, but their fabrication is too slow for large-scale device fabrication. Growth methods such as chemical vapor deposition are faster, but create polycrystalline graphene sheets that contain grain boundaries that can scatter charge carriers and decrease performance. <jats:bold> Tsen <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="1143" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1218948">1143</jats:related-article> ) found that the presence of overlapping domains within polycrystalline graphene samples could increase conductivity of samples by an order of magnitude, allowing them to rival exfoliated samples. </jats:p>

<jats:title>Untangling a Spectral Thicket</jats:title> <jats:p> Atoms and molecules absorb characteristic frequencies of ligh; however, assigning the spectra of certain molecules remains a challenge. For example, the loosely bound complex of CO with H2 produces a dense thicket of measured spectral lines that has resisted elucidation for over a decade. Now, <jats:bold> Jankowski <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="1147" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1221000">1147</jats:related-article> ) provide detailed theoretical calculations that match the observed pattern and thereby elucidate the vibrational and rotational states in play. The result may ultimately help to tease out the collisional dynamics of H <jats:sub>2</jats:sub> and CO in the interstellar medium. </jats:p>

<jats:title>The Real McCoy</jats:title> <jats:p> Some secreted proteins are phosphorylated, the most prominent example being the milk protein casein, but the enzymes that catalyze such phosphorylation have not been identified. (The proteins known as “casein kinases” are in fact cytosolic proteins and do not mediate physiological phosphorylation of casein.) <jats:bold> Tagliabracci <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="1150" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1217817">1150</jats:related-article> , published online 10 May) searched for a human protein with the characteristics expected of a secretory protein kinase and identified Fam20C. Mutations in the gene encoding Fam20C cause defects in bone formation. Furthermore, the consensus sequence for Fam20C phosphorylation was found in several secreted proteins that function in biomineralization. Thus, Fam20C appears to be the “real” casein kinase and to function in bone physiology. </jats:p>

<jats:title>Variation on a Theme</jats:title> <jats:p> Given the incredible diversity and complexity in social behavior and ecology that exists across animal taxa, revealing the evolution of neural mechanisms for behavior is a great challenge. <jats:bold>O'Connell and Hofmann</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1154" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1218889">1154</jats:related-article> ) examined the expression profiles of several genes involved in the social behavior network and the mesolimbic reward system in 88 species across five vertebrate lineages. A remarkable level of conservation was observed in brain regions linked to social behavior and decision-making, but flexibility seems to have been maintained through variability in neuroendocrine ligand expression across the brain. </jats:p>

<jats:title>Managing Trade-Offs</jats:title> <jats:p> Most organisms experience selection on a host of traits to determine their likelihood to succeed evolutionarily. However, specific traits may experience trade-offs in determining an organism's optimal phenotype. <jats:bold> Shoval <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="1157" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1217405">1157</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="6085" page="1114" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1223193">Noor and Milo</jats:related-article> </jats:bold> ) relate physical traits to the task that they are optimizing using a Pareto curve, a power law probability distribution, to show that a single set of trait values optimizes performance at a given task and that performance decreases as an organism's phenotype moves away from this set of trait values. The results suggest how selection makes the best trade-offs for an arbitrary number of tasks and traits and may explain examples of evolutionary variation. </jats:p>

<jats:title>Dissecting Chitin Binding</jats:title> <jats:p> The chitin in fungal cells walls serves as a trigger to initiate plant defenses against pathogenic fungi. <jats:italic>Arabidopsis</jats:italic> detects these signals through a cell surface chitin receptor whose intracellular kinase domain initiates a signaling cascade in response to chitin that activates the plant's response to infection. <jats:bold> Liu <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="1160" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1218867">1160</jats:related-article> ) have now solved the crystal structure of the <jats:italic>Arabidopsis</jats:italic> chitin receptor AtCERK1. The results show how chitin binds to the receptor and suggest that the biological response requires dimerisation of the receptor when it binds a chitin oligomer at least seven or eight subunits long. </jats:p>

<jats:title>See How They Grow</jats:title> <jats:p> Controlled assembly and disassembly of the actin cytoskeleton is essential for processes such as cell motility, cytokinesis, and tumor metastasis. The formation of new actin filaments appears to involve the protein formin paired with another actin assembly-promoting factor. <jats:bold> Breitsprecher <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="1164" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1218062">1164</jats:related-article> ) used triplecolor single-molecule fluorescence microscopy to visualize actin assembly promoted by the formin, mDia1, and the tumor-suppressor, adenomatous polyposis coli (APC). The two assembly factors interacted directly to initiate filament assembly, after which mDia1 moved with the growing barbed ends while APC remained at the site of nucleation. </jats:p>