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排序方式: 共有233条查询结果,搜索用时 15 毫秒
81.
Capak PL Riechers D Scoville NZ Carilli C Cox P Neri R Robertson B Salvato M Schinnerer E Yan L Wilson GW Yun M Civano F Elvis M Karim A Mobasher B Staguhn JG 《Nature》2011,470(7333):233-235
Massive clusters of galaxies have been found that date from as early as 3.9 billion years (3.9 Gyr; z = 1.62) after the Big Bang, containing stars that formed at even earlier epochs. Cosmological simulations using the current cold dark matter model predict that these systems should descend from 'protoclusters'-early overdensities of massive galaxies that merge hierarchically to form a cluster. These protocluster regions themselves are built up hierarchically and so are expected to contain extremely massive galaxies that can be observed as luminous quasars and starbursts. Observational evidence for this picture, however, is sparse because high-redshift protoclusters are rare and difficult to observe. Here we report a protocluster region that dates from 1 Gyr (z = 5.3) after the Big Bang. This cluster of massive galaxies extends over more than 13 megaparsecs and contains a luminous quasar as well as a system rich in molecular gas. These massive galaxies place a lower limit of more than 4 × 10(11) solar masses of dark and luminous matter in this region, consistent with that expected from cosmological simulations for the earliest galaxy clusters. 相似文献
82.
Lorenzen ED Nogués-Bravo D Orlando L Weinstock J Binladen J Marske KA Ugan A Borregaard MK Gilbert MT Nielsen R Ho SY Goebel T Graf KE Byers D Stenderup JT Rasmussen M Campos PF Leonard JA Koepfli KP Froese D Zazula G Stafford TW Aaris-Sørensen K Batra P Haywood AM Singarayer JS Valdes PJ Boeskorov G Burns JA Davydov SP Haile J Jenkins DL Kosintsev P Kuznetsova T Lai X Martin LD McDonald HG Mol D Meldgaard M Munch K Stephan E Sablin M Sommer RS Sipko T Scott E Suchard MA Tikhonov A Willerslev R 《Nature》2011,479(7373):359-364
Despite decades of research, the roles of climate and humans in driving the dramatic extinctions of large-bodied mammals during the Late Quaternary period remain contentious. Here we use ancient DNA, species distribution models and the human fossil record to elucidate how climate and humans shaped the demographic history of woolly rhinoceros, woolly mammoth, wild horse, reindeer, bison and musk ox. We show that climate has been a major driver of population change over the past 50,000 years. However, each species responds differently to the effects of climatic shifts, habitat redistribution and human encroachment. Although climate change alone can explain the extinction of some species, such as Eurasian musk ox and woolly rhinoceros, a combination of climatic and anthropogenic effects appears to be responsible for the extinction of others, including Eurasian steppe bison and wild horse. We find no genetic signature or any distinctive range dynamics distinguishing extinct from surviving species, emphasizing the challenges associated with predicting future responses of extant mammals to climate and human-mediated habitat change. 相似文献
83.
Conservation of the D-mannose-adhesion protein among type 1 fimbriated members of the family Enterobacteriaceae 总被引:21,自引:0,他引:21
A variety of genera and species of the family Enterobacteriaceae bear surface fimbriae that enable them to bind to D-mannose residues on eukaryotic cells. Until recently, it was thought that the D-mannose binding site was located in the major structural subunit (FimA), of relative molecular mass (Mr) 17,000 (17 K), of these organelles in Escherichia coli. New evidence indicates that this binding site resides instead in a minor protein Mr 28-31 K (FimH) located at the tips and at long intervals along the length of the fimbriae, and is reminiscent of the minor tip adhesion proteins of pyelonephritis-associated pili (Pap) and S fimbriae. In contrast to the antigenic heterogeneity of the major FimA subunit, the antigenic structure of FimH is conserved among different strains of E. coli. Here, we report an even broader conservation of this minor adhesion protein extending to other genera and species of type 1 fimbriated Enterobacteriaceae. Our results may have implications for the development of broadly protective vaccines against Gram-negative bacillary infections in animals and perhaps in man. 相似文献
84.
Lipoxygenase metabolites of arachidonic acid as second messengers for presynaptic inhibition of Aplysia sensory cells 总被引:20,自引:0,他引:20
D Piomelli A Volterra N Dale S A Siegelbaum E R Kandel J H Schwartz F Belardetti 《Nature》1987,328(6125):38-43
Biochemical and biophysical studies on Aplysia sensory neurons indicate that inhibitory responses to the molluscan peptide FMRFamide are mediated by lipoxygenase metabolites of arachidonic acid. These compounds are a new class of second messengers in neurons. 相似文献
85.
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88.
Mustard JF Murchie SL Pelkey SM Ehlmann BL Milliken RE Grant JA Bibring JP Poulet F Bishop J Dobrea EN Roach L Seelos F Arvidson RE Wiseman S Green R Hash C Humm D Malaret E McGovern JA Seelos K Clancy T Clark R Marais DD Izenberg N Knudson A Langevin Y Martin T McGuire P Morris R Robinson M Roush T Smith M Swayze G Taylor H Titus T Wolff M 《Nature》2008,454(7202):305-309
Phyllosilicates, a class of hydrous mineral first definitively identified on Mars by the OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et l'Activitié) instrument, preserve a record of the interaction of water with rocks on Mars. Global mapping showed that phyllosilicates are widespread but are apparently restricted to ancient terrains and a relatively narrow range of mineralogy (Fe/Mg and Al smectite clays). This was interpreted to indicate that phyllosilicate formation occurred during the Noachian (the earliest geological era of Mars), and that the conditions necessary for phyllosilicate formation (moderate to high pH and high water activity) were specific to surface environments during the earliest era of Mars's history. Here we report results from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) of phyllosilicate-rich regions. We expand the diversity of phyllosilicate mineralogy with the identification of kaolinite, chlorite and illite or muscovite, and a new class of hydrated silicate (hydrated silica). We observe diverse Fe/Mg-OH phyllosilicates and find that smectites such as nontronite and saponite are the most common, but chlorites are also present in some locations. Stratigraphic relationships in the Nili Fossae region show olivine-rich materials overlying phyllosilicate-bearing units, indicating the cessation of aqueous alteration before emplacement of the olivine-bearing unit. Hundreds of detections of Fe/Mg phyllosilicate in rims, ejecta and central peaks of craters in the southern highland Noachian cratered terrain indicate excavation of altered crust from depth. We also find phyllosilicate in sedimentary deposits clearly laid by water. These results point to a rich diversity of Noachian environments conducive to habitability. 相似文献
89.
The neonatal Fc receptor (FcRn) transports maternal IgG across epithelial barriers, thereby providing the fetus or newborn with humoral immunity before its immune system is fully functional. In newborn rats, FcRn transfers IgG from milk to blood by apical-to-basolateral transcytosis across intestinal epithelial cells. The pH difference between the apical (pH 6.0-6.5) and basolateral (pH 7.4) sides of intestinal epithelial cells facilitates the efficient unidirectional transport of IgG, because FcRn binds IgG at pH 6.0-6.5 but not at pH 7 or more. As milk passes through the neonatal intestine, maternal IgG is removed by FcRn-expressing cells in the proximal small intestine (duodenum and jejunum); remaining proteins are absorbed and degraded by FcRn-negative cells in the distal small intestine (ileum). Here we use electron tomography to make jejunal transcytosis visible directly in space and time, developing new labelling and detection methods to map individual nanogold-labelled Fc within transport vesicles and simultaneously to characterize these vesicles by immunolabelling. Combining electron tomography with a non-perturbing endocytic label allowed us to conclusively identify receptor-bound ligands, resolve interconnecting vesicles, determine whether a vesicle was microtubule-associated, and accurately trace FcRn-mediated transport of IgG. Our results present a complex picture in which Fc moves through networks of entangled tubular and irregular vesicles, only some of which are microtubule-associated, as it migrates to the basolateral surface. New features of transcytosis are elucidated, including transport involving multivesicular body inner vesicles/tubules and exocytosis through clathrin-coated pits. Markers for early, late and recycling endosomes each labelled vesicles in different and overlapping morphological classes, revealing spatial complexity in endo-lysosomal trafficking. 相似文献
90.
Hakonarson H Grant SF Bradfield JP Marchand L Kim CE Glessner JT Grabs R Casalunovo T Taback SP Frackelton EC Lawson ML Robinson LJ Skraban R Lu Y Chiavacci RM Stanley CA Kirsch SE Rappaport EF Orange JS Monos DS Devoto M Qu HQ Polychronakos C 《Nature》2007,448(7153):591-594
Type 1 diabetes (T1D) in children results from autoimmune destruction of pancreatic beta cells, leading to insufficient production of insulin. A number of genetic determinants of T1D have already been established through candidate gene studies, primarily within the major histocompatibility complex but also within other loci. To identify new genetic factors that increase the risk of T1D, we performed a genome-wide association study in a large paediatric cohort of European descent. In addition to confirming previously identified loci, we found that T1D was significantly associated with variation within a 233-kb linkage disequilibrium block on chromosome 16p13. This region contains KIAA0350, the gene product of which is predicted to be a sugar-binding, C-type lectin. Three common non-coding variants of the gene (rs2903692, rs725613 and rs17673553) in strong linkage disequilibrium reached genome-wide significance for association with T1D. A subsequent transmission disequilibrium test replication study in an independent cohort confirmed the association. These results indicate that KIAA0350 might be involved in the pathogenesis of T1D and demonstrate the utility of the genome-wide association approach in the identification of previously unsuspected genetic determinants of complex traits. 相似文献