Catálogo de publicaciones - revistas

<jats:title>PTIP in Immunoglobulin Switching</jats:title> <jats:p> One of the hallmarks of humoral immunity is the ability of immunoglobulins (Ig) to undergo class switch recombination (CSR). Through genetic recombining of the Ig heavy chain, Igs maintain their antigen specificity but gain the ability to interact with different cell surface receptors required for successful pathogen clearance. CSR requires transcription at the Ig heavy chain locus to initiate genetic rearrangement. Changes in chromatin accessibility are thought to promote CSR-associated transcription. <jats:bold> Daniel <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="917" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1187942">917</jats:related-article> , published online 29 July; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5994" page="914" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1194316">Singh and Demarco</jats:related-article> </jats:bold> ) now show that trimethylation of histone 3 at lysine 4 (H3K4me3) controls the accessibility of the Ig heavy chain locus to CSR and that PTIP (Pax interaction with transcription-activation domain protein-1), a component of the histone methylase complex, is required for this modification. Mouse PTIP-deficient B cells exhibited impaired CSR. PTIP was required both for the recruitment RNA Polymerase II and for subsequent chromatin remodeling, including histone acetylation, which occurs during CSR. Largely independent of its function in transcription initiation in CSR, PTIP also associated with double-stranded DNA breaks during CSR and promoted genome stability. These dual functions of PTIP may be important for the precise coordination of chromatin accessibility and recombination required during CSR. </jats:p>

<jats:title>Through a Lens Darkly</jats:title> <jats:p> According to recent measurements, 72% of the energy content in the universe is in the form of dark energy, a gravitationally repulsive constituent that is powering the accelerating expansion of the universe but whose nature is unknown. Now, <jats:bold> Jullo <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="924" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1185759">924</jats:related-article> ) show how observations of systems of multiple images produced by the strong gravitational lensing effect of a single mass distribution can be used to constrain the properties of dark energy. Applied to the cluster Abel 1689—a galaxy cluster that is known for its lensing properties—and combined with the results of other techniques, this method brings down the overall error in the equation of state parameter of dark energy by 30%. </jats:p>

<jats:title>When Black Holes Collide</jats:title> <jats:p> When galaxies merge, their central black holes interact, initially coming together in a binary system and eventually coalescing into one single black hole. The dynamics of orbiting binary black holes in vacuum is well understood; however, when black holes merge, their accretion disks are expected to combine into a circumbinary disk anchoring a magnetic field. Numerical simulations from <jats:bold> Palenzuela <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="927" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1191766">927</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="5994" page="908" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1194182">Yunes</jats:related-article> </jats:bold> ) now show that when this surrounding environment is taken into account, the black holes effectively stir the plasma that surrounds them, generating collimated beams of electromagnetic radiation that then transition to a single jet as the black holes merge due to the emission of gravitational waves. </jats:p>

<jats:title>Directed Emission</jats:title> <jats:p> The atomlike features of quantum dots—the discrete energy levels and subsequent emission of light at discrete wavelengths—make them key building blocks in optoelectronic circuits and optical communication. However, the emitted light tends to be omnidirectional, which limits applications that require accurate transmission between sender and receiver. <jats:bold> Curto <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="930" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1191922">930</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="5994" page="910" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1194352">Giessen and Lippitz</jats:related-article> </jats:bold> ) have designed an optical antenna, a shrunk-down version of the Yagi-Uda design used in microwave and radio communication, and show that coupling the quantum dot to the antenna provides control over the direction of the emitted light. </jats:p>

<jats:title>Keeping Nanoparticles Small</jats:title> <jats:p> Heterogeneous catalysts that consist of small metal nanoparticles absorbed on oxide supports can deactivate over time through a process called sintering. Elevated temperatures increase the rate of diffusion of metal atoms over the support, and larger, less reactive particles grow at the expense of smaller ones. Some supports that contain reducible metals, such as cerium oxide, tend to resist sintering better than oxides such as alumina. <jats:bold>Farmer and Campbell</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="933" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1191778">933</jats:related-article> ) present an analysis of previous calorimetry data for silver nanoparticles on magnesium oxides and cerium oxide surfaces and show that nanoparticles smaller than 1000 atoms are bound much more strongly to reduced cerium oxide. The energetic driving force for creating larger particles is quite low on these surfaces and increases the lifetime of smaller particles. </jats:p>

<jats:title>Lunar Lobate Scarps Revealed</jats:title> <jats:p> Lunar lobate scarps are relatively small-scale landforms that are thought to be formed by tectonic thrust faulting. Previously, lunar lobate scarps could only be identified clearly in high-resolution Apollo Panoramic Camera images confined to the lunar equatorial zone. Now, an analysis by <jats:bold> Watters <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="936" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1189590">936</jats:related-article> ) of images returned by the Lunar Reconnaissance Orbiter Camera reveals 14 previously unknown lobate scarps and shows that lunar lobate scarps may be globally distributed. Their appearance suggests that lunar scarps are relatively young landforms (less than 1 Ga), possibly formed during a recent episode of global lunar radial contraction. </jats:p>

<jats:title>Reversing the Trend</jats:title> <jats:p> Terrestrial net primary productivity (NPP, the amount of atmospheric carbon fixed by plants and accumulated as biomass) increased from 1982 through 1999, which has been attributed to factors such as nitrogen deposition, CO <jats:sub>2</jats:sub> fertilization, forest regrowth, and climatic changes. <jats:bold>Zhao and Running</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="940" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1192666">940</jats:related-article> ) used satellite data to estimate global terrestrial NPP over the past decade and found that the earlier trend has been reversed and that NPP has been decreasing. Combining this result with climate change data suggests that large-scale droughts are responsible for the decline. Future widespread droughts caused by global warming may thus further weaken the terrestrial carbon sink. </jats:p>

<jats:title>The Pitfalls of Re-Replication</jats:title> <jats:p> The cell has several layers of regulation to ensure that the genome is replicated once and only once during cell division, presumably to avoid the formation of duplicated sequences, which have the potential to recombine and compromise genome stability. In agreement with this idea, <jats:bold> Green <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="943" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1190966">943</jats:related-article> ; see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="5994" page="911" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1194261"> <jats:bold> Kaochar <jats:italic>et al.</jats:italic> </jats:bold> </jats:related-article> ) now show that re-replication of a marked yeast replication origin causes a significant increase in copy number variation in the region encompassing the origin. The observed duplications range in size from 135 to 470 kilobases and are almost all tandemly arrayed in a head-to-tail orientation. The duplicated regions are bounded by Ty repetitive elements and arise from nonallelic homologous recombination between the re-replicated repetitive elements. </jats:p>

<jats:title> <jats:italic>Legionella</jats:italic> Hijacks Rab </jats:title> <jats:p> <jats:italic>Legionella pneumophila</jats:italic> can infect eukaryotic cells and takes up residence within intracellular vacuoles, where it multiplies. In order to produce and maintain this intracellular niche, the pathogen must manipulate membrane trafficking within the host cell. Now, <jats:bold> Müller <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="946" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1192276">946</jats:related-article> , published online 22 July) describe the ability of <jats:italic>Legionella pneumophila</jats:italic> to manipulate vesicular trafficking by the covalent modification of the small guanosine triphosphatase (GTPase) Rab1, which normally regulates the transport of endoplasmic reticulum–derived vesicles in eukaryotic cells. The <jats:italic>Legionella</jats:italic> protein DrrA is released into the cytosol of infected cells, where it specifically AMPylates a tyrosine residue of one of the regulating regions of Rab1. The modification renders the Rab protein inaccessible to GTPase-activating proteins and thus locks it in its active guanosine triphosphate–bound state. </jats:p>

<jats:title>Chloroplast Division Machinery</jats:title> <jats:p> The machinery for photosynthesis, which captures the Sun's energy to generate carbohydrates, generally resides in subcellular chloroplasts of plant cells. Chloroplasts must divide as the plant cell divides, but to do so requires their own plastid dividing machinery. <jats:bold> Yoshida <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="949" related-article-type="in-this-issue" vol="329" xlink:href="10.1126/science.1190791">949</jats:related-article> : see the cover) have now analyzed the plastid dividing machinery of the single-celled alga <jats:italic>Cyanidioschyzon merolae</jats:italic> , whose cells each contain a single chloroplast. The plastid dividing machinery is made up of polysaccharide chains and the proteins that make them, which together generate a ring that constricts to physically divide the chloroplast. </jats:p>