Catálogo de publicaciones - revistas

<jats:title>A Volcanic Valley on Mars</jats:title> <jats:p> The formation of the Athabasca Valles, a valley network near the equator of Mars, has been debated for over a decade; both lava and water ice have been implicated. Some studies have attributed the polygonally patterned ground in this region to frost wedging in an ice-rich soil. Using images of the Athabasca region from the High Resolution Imaging Science Experiment onboard Mars Reconnaissance Orbiter, <jats:bold>Ryan and Christensen</jats:bold> (p. 449) detected numerous spirals on polygonally patterned ground. The spiral shapes are consistent with those of lava coils that form on the surface of Hawaiian lava flows, and cannot be explained by ice-related activities. </jats:p>

<jats:title>Space Organics</jats:title> <jats:p> The origin of the organic compounds found in meteorites and interplanetary dust particles is a matter of debate. Laboratory experiments suggest that these organics were inherited from the interstellar medium and predate the existence of the solar system. By using particle-tracking models, <jats:bold>Ciesla and Sandford</jats:bold> (p. 452, published online 29 March; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6080" page="424" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1219709">Nuth and Johnson</jats:related-article> </jats:bold> ) explored the possibility that the organics could have been produced within the outer reaches of the protoplanetary disk from which the solar system planets originate. Grains within a protoplanetary disk follow irregular paths; such orbital histories subjected model particles to ultraviolet radiation and temperatures that have been shown in laboratory experiments to generate organic compounds. </jats:p>

<jats:title>Getting Wetter Faster</jats:title> <jats:p> Theoretical projections, based on the relationship between temperature and the amount of water vapor the atmosphere can hold, suggest that global warming should intensify the strength of the atmospheric water cycle by about twice the rate as the thermodynamics and climate models predict. <jats:bold> Durack <jats:italic>et al.</jats:italic> </jats:bold> (p. 455) examined 50 years of observations of sea surface salinities and conclude that the patterns of change in the salinity data are consistent with the theoretical projections, rather than with those of the models. Thus, the global water cycle should intensify by 16 to 24% for a future increase of a global average temperature of 2 to 3°C. </jats:p>

<jats:title>Keep It Inside</jats:title> <jats:p> Some cyanobacteria form solid-phase calcium carbonate precipitates as a consequence of fixing CO <jats:sub>2</jats:sub> during photosynthesis. Usually, such carbonates form extracellularly near the surface of the cells, sometimes generating structures called stromatolites. In a biofilm growing on carbonate deposits in Lake Alchichica, Mexico, <jats:bold> Couradeau <jats:italic>et al.</jats:italic> </jats:bold> (p. 459; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6080" page="427" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1221055">Riding</jats:related-article> </jats:bold> ) discovered one species of cyanobacteria that also precipitates amorphous carbonate particles internally. Because the structure and chemical composition of these carbonates is distinct from those formed extracellularly, there may be cellular control over the mineralization process. These precipitates may influence physiological processes such as cell buoyancy and the sequestration of excess alkalinity generated during photosynthesis. </jats:p>

<jats:title>Acid Rate</jats:title> <jats:p> The low species richness of the acid mine drainage in the Richmond Mine (Iron Mountain, California, USA) provides an excellent opportunity to study microbial process on a genome scale in real time. <jats:bold>Denef and Banfield</jats:bold> (p. 462; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6080" page="422" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1221822">DeLong</jats:related-article> </jats:bold> ) report the further analysis of metagenomic data from 24 microbial biofilm samples collected over 8 years, at different sites. High-throughput, shotgun sequencing of the DNA was used to look for high-frequency, single-nucleotide polymorphisms in six genotypic variants of a chemolithoautotroph, <jats:italic>Leptospirillum</jats:italic> group II, the dominant primary producer and iron-oxidizer. Thirteen samples collected at one site over 5 years were analyzed to infer the accumulation of single-nucleotide substitutions for free-living populations over time. A series of very recent branching points in the phylogenetic tree that arose via homologous recombination were revealed. </jats:p>

<jats:title>Farmer Displaced European Hunters</jats:title> <jats:p> Our understanding of prehistoric demography and human evolution has been improved by analysis of ancient DNA. <jats:bold> Skoglund <jats:italic>et al.</jats:italic> </jats:bold> (p. 466) describe the retrieval and analysis of genomic DNA from ancient (~5000-year-old) northern European Neolithic individuals within modern Sweden. These include three hunter-gatherers from the Pitted Ware Culture horizon and one farmer ascribed to the Mid-Neolithic North-Central TRB culture. The hunter-gatherers displayed a distinct genetic signature, similar to that of extant northern Europeans, whereas the farmer's genetic signature more closely resembled southern Europeans, suggesting migration and admixture during the spread of farming. </jats:p>

<jats:title>Handling Nitrosative Stress</jats:title> <jats:p> S-nitrosylation of proteins is a principal mechanism of cellular signaling in eukaryotes but has not been reported in microbes. The transcription factor OxyR serves to protect bacterial cells from reactive oxygen species produced by cell metabolism in the presence of oxygen. <jats:bold> Seth <jats:italic>et al.</jats:italic> </jats:bold> (p. 470) found that when cells grew in the presence of nitrate, OxyR was modified by S-nitrosylation of the same cysteine residue that gets oxidized in cells grown aerobically. However, the nitrosylated OxyR activated a different set of genes—some of which appeared to protect the cell from excessive S-nitrosylation. </jats:p>

<jats:title>Acetylation and Autophagy</jats:title> <jats:p> Autophagy allows cells to digest their own components when necessary to survive stressful conditions. <jats:bold> Lin <jats:italic>et al.</jats:italic> </jats:bold> (p. 477) and <jats:bold> Yi <jats:italic>et al.</jats:italic> </jats:bold> (p. 474) describe signaling mechanisms in mammalian cells and yeast, respectively, by which autophagy is regulated by protein acetylation. In mammalian cells deprived of serum, the acetyltransferase TIP60 was activated by phosphorylation by the protein kinase GSK3 (glycogen synthase kinase 3). TIP60's target appeared to be a protein kinase central to autophagy regulation, ULK1. This activating pathway was required for autophagy in the absence of serum, but was not needed for autophagy in cells deprived of glucose. In the yeast <jats:italic>Saccharomyces cerevisiae</jats:italic> starved of nitrogen, another acetylation mechanism was uncovered. Starvation led to activation of the histone acetyltransferase Esa1, which acetylated the protein Atg3, a key component of the autophagy machinery, thus increasing its interaction with another autophagy protein, Atg8. </jats:p>

<jats:title>Acetylation and Autophagy</jats:title> <jats:p> Autophagy allows cells to digest their own components when necessary to survive stressful conditions. <jats:bold> Lin <jats:italic>et al.</jats:italic> </jats:bold> (p. 477) and <jats:bold> Yi <jats:italic>et al.</jats:italic> </jats:bold> (p. 474) describe signaling mechanisms in mammalian cells and yeast, respectively, by which autophagy is regulated by protein acetylation. In mammalian cells deprived of serum, the acetyltransferase TIP60 was activated by phosphorylation by the protein kinase GSK3 (glycogen synthase kinase 3). TIP60's target appeared to be a protein kinase central to autophagy regulation, ULK1. This activating pathway was required for autophagy in the absence of serum, but was not needed for autophagy in cells deprived of glucose. In the yeast <jats:italic>Saccharomyces cerevisiae</jats:italic> starved of nitrogen, another acetylation mechanism was uncovered. Starvation led to activation of the histone acetyltransferase Esa1, which acetylated the protein Atg3, a key component of the autophagy machinery, thus increasing its interaction with another autophagy protein, Atg8. </jats:p>

<jats:title>Generating Inflammasomes</jats:title> <jats:p> Inflammasomes are large, multiprotein complexes that assemble in response to infection that are also involved in the pathogenesis of a variety of other diseases, including type 2 diabetes and atherosclerosis. The assembly of the inflammasome triggers an inflammatory cascade that results in the activation of caspase-1 and production of the cytokines interleukin-1 and -18. Very little, however, is known about the specific signals that trigger inflammasome assembly. <jats:bold> Shenoy <jats:italic>et al.</jats:italic> </jats:bold> (p. 481, published online 29 March; see the Perspective by <jats:bold> <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" issue="6080" page="420" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1222362">Caffrey and Fitzgerald</jats:related-article> </jats:bold> ) now show that guanylate binding protein 5 (GBP5) promotes the assembly of the NLRP3-containing inflammasome in response to certain activation signals, such as pathogenic bacteria and adenosine triphosphate, but not others, like crystalline stimuli. Mice deficient in GBP5 exhibited impaired caspase-1 activation and production of cytokines. NLRP3 inflammasome–dependent responses to pathogenic bacteria and inflammatory stimuli were also impaired in mice lacking GBP5. </jats:p>