Catálogo de publicaciones - revistas

<jats:title>Movement in a Tight Squeeze</jats:title> <jats:p> The motion of flexible polymer chains in a dense melt or concentrated solution is described by reptation theory, in which a single chain is considered to snake back and forth inside a virtual confining tube formed by all its neighboring chains. A number of theories have been proposed for stiffer molecules, but it has been hard to obtain experimental data to determine the thermal motion of stiff filaments. <jats:bold> Fakhri <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="1804" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1197321">1804</jats:related-article> ) visualized carbon nanotubes directly as a model system for stiff polymers diffusing in a gel, and found that even a slight increase in flexibility significantly sped up diffusion of stiff filaments under confinement. The rotational diffusion constant grew linearly with the flexibility and, counterintuitively, did not depend on the degree of crowding. </jats:p>

<jats:title>Tuning Semiconductor Dopants</jats:title> <jats:p> Dopants in semiconductors can alter their conductivity or introduce spin centers that alter their magnetic properties. Generally, the charge state of a dopant and field it creates are fixed. <jats:bold>Lee and Gupta</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1807" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1197434">1807</jats:related-article> , published online 9 December) studied Mn dopants in GaAs with a low-temperature scanning tunneling microscope (STM). Using the STM to position As vacancies at different distances from the Mn dopants revealed that the As vacancy tuned the local electrostatic field of the dopant. </jats:p>

<jats:title>Moving Walls</jats:title> <jats:p> The current-induced movement of magnetic domain walls in magnetic nanowires is a candidate for a new architecture in logic processing and memory. Controlling the motion and position of the domain walls as they move along the wires in excess of 100 meters per second requires an understanding of the processes involved. <jats:bold> Thomas <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="1810" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1197468">1810</jats:related-article> ) investigated the dynamics of magnetic domain wall motion, looking at the acceleration, constant motion, and deceleration processes in detail. The whole process could be described in terms of the inertia of the domain wall. The distance traveled was simply proportional to the length of the current pulse used to move the wall, which should simplify implementation in a circuit or network architecture. </jats:p>

<jats:title>Extraterrestrial Atmosphere</jats:title> <jats:p> The detection of oxygen in the atmospheres of Jupiter's icy moons, Europa and Ganymede, and the presence of this gas as the main constituent of the atmosphere that surrounds Saturn's rings, has suggested the possibility of oxygen atmospheres around the icy moons that orbit inside Saturn's magnetosphere. Using the Ion Neutral Mass Spectrometer onboard the Cassini spacecraft, <jats:bold> Teolis <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="1813" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1198366">1813</jats:related-article> , published online 25 November; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6012" page="1755" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1200473">Cruikshank</jats:related-article> </jats:bold> ) report the detection of a very tenuous oxygen and carbon dioxide atmosphere around Saturn's icy moon Rhea. As with other icy satellites, this atmosphere is maintained through the dissociation of surface molecules and ejection into the atmosphere as a result of Saturn's magnetospheric radiation. </jats:p>

<jats:title>A Safety Catch on Immune Response</jats:title> <jats:p> The complement system is an integral part of the innate immune system. When triggered, it initiates a cascade that marks intruders for elimination and stimulates immune responses. The key amplification step is cleavage of a complex comprising the complement fragment C3b and factor B (C3bB) by factor D (FD). <jats:bold> Forneris <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="1816" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1195821">1816</jats:related-article> ) now describe the crystal structure of C3bB and its complex with FD. The structures support a mechanism in which membrane-bound C3b stabilizes an open form of factor B (FB) that has its scissile bond accessible. FD binds through a site distant from its catalytic center to the open form of FB, which activates FD. The two conformational equilibria represent a double safety-catch that would allow tight regulation of this immune response pathway. </jats:p>

<jats:title>Exploiting Variation</jats:title> <jats:p> Molecular chaperones help newly synthesized proteins fold, protecting the macromolecular machinery of the cell from various stresses; for example, the highly conserved heat shock proteins (hsp) protect against elevated temperature. Hsp90 has also been suggested both to buffer against and to potentiate existing genetic variation in a population. To investigate the generality of these claims, <jats:bold>Jarosz and Lindquist</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1820" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1195487">1820</jats:related-article> ) screened 96 <jats:italic>Saccharomyces cerevisiae</jats:italic> strains from various ecological niches—soil, fruit, sake, beer, and infected humans—as well as assessed their adaptive value under different growth conditions. Hsp90 determined the adaptive value of ∼20% of the genetic variation in baker's yeast, with half of the traits being buffered, and half potentiated by hsp90. </jats:p>

<jats:title>Curing Insect Brain Tumors</jats:title> <jats:p> Mammalian tumors often show ectopic expression of genes that normally function only in the germ line. The possible contribution of cancer germline (CG) genes to malignancy is unknown. <jats:bold> Janic <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="1824" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1195481">1824</jats:related-article> ; see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6012" page="1761" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1200772"> <jats:bold>Wu and Ruvkun</jats:bold> </jats:related-article> ) found that CG genes are also expressed in <jats:italic>Drosophila</jats:italic> brain tumors caused by mutants in the gene <jats:italic>lethal (3) malignant brain tumor</jats:italic> . Moreover, inactivation of some of these genes suppressed fly tumor growth. Because some <jats:italic>Drosophila</jats:italic> CG genes are orthologs of human CG genes, it is possible that inactivation of germline genes may show human tumor suppressor activity. </jats:p>

<jats:title>Reds Versus Greens</jats:title> <jats:p> Self-incompatibility (SI) allows plants to prevent inbreeding. Crosses with distant relatives (outbreeding) can also be problematic and is prevented by unilateral interspecific incompatability (UI). In the nightshade family, SI functions within green-fruited species, whereas crosses between green-fruited and red-fruited species (which includes tomato) results in UI. <jats:bold>Li and Chetelat</jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="1827" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1197908">1827</jats:related-article> ) found a gene, related to known SI genes within this family, that differs in transcript length and function between individuals that are red-fruited and those that are green-fruited. A survey of species shows that the green-fruited species have a functional allele of this gene, whereas the transcript of this gene in red-fruited species, which are self-compatible, produce a putatively nonfunctional protein. These findings suggest that cultivated tomato may have lost the ability to pollinate other species within the same family, owing to the loss of this protein. </jats:p>

<jats:title>Mind Reading</jats:title> <jats:p> One core component of social cognition, especially of the kind practiced by humans, is the capacity to formulate a representation of what someone else believes to be true, even if that belief is not anchored in reality. Holding two such beliefs in mind—one false and one true—is no simple feat, and up until a few years ago, it was generally accepted that such a capacity did not arise until children were 3 to 4 years old. Since then, a flurry of studies, using a variety of interrogation measures, has suggested that much-younger humans might, in fact, possess this capacity, commonly referred to as a theory of mind. <jats:bold> Kovács <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="1830" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1190792">1830</jats:related-article> ) devised an ingenious behavioral paradigm and applied it both to adults and to infants, which suggests that the representations of others' beliefs are indeed formed in the same way in adults and in infants. </jats:p>

<jats:title>Exiting the Birthplace</jats:title> <jats:p> In the developing mammalian brain, new neurons are not always born where they are needed. In order for immature neurons of the mouse cerebellum to leave their birthplace in the germinal zone and find their functional niche in the brain, the neurons need to migrate. <jats:bold> Famulski <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="1834" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1198480">1834</jats:related-article> , published online 25 November; see the Perspective by <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6012" page="1754" related-article-type="in-this-issue" vol="330" xlink:href="10.1126/science.1200475"> <jats:bold>Métin and Luccardini</jats:bold> </jats:related-article> ) now show that ubiquitin-mediated protein degradation regulates development of specific cell adhesions that the neurons need in order to exit their birthplace en route to their final functional location. </jats:p>