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321.
322.
Siddiqa A Sims-Mourtada JC Guzman-Rojas L Rangel R Guret C Madrid-Marina V Sun Y Martinez-Valdez H 《Nature》2001,410(6826):383-387
323.
Understanding the relative timings of climate events in the Northern and Southern hemispheres is a prerequisite for determining the causes of abrupt climate changes. But climate records from the Patagonian Andes and New Zealand for the period of transition from glacial to interglacial conditions--about 14.6-10 kyr before present, as determined by radiocarbon dating--show varying degrees of correlation with similar records from the Northern Hemisphere. It is necessary to resolve these apparent discrepancies in order to be able to assess the relative roles of Northern Hemisphere ice sheets and oceanic, atmospheric and astronomical influences in initiating climate change in the late-glacial period. Here we report pollen records from three sites in the Lake District of southern Chile (41 degrees S) from which we infer conditions similar to modern climate between about 13 and 12.2 14C kyr before present (BP), followed by cooling events at about 12.2 and 11.4 14C kyr BP, and then by a warming at about 9.8 14C kyr BP. These events were nearly synchronous with important palaeoclimate changes recorded in the North Atlantic region, supporting the idea that interhemispheric linkage through the atmosphere was the primary control on climate during the last deglaciation. In other regions of the Southern Hemisphere, where climate events are not in phase with those in the Northern Hemisphere, local oceanic influences may have counteracted the effects that propagated through the atmosphere. 相似文献
324.
325.
Montgomery KT Lee E Miller A Lau S Shim C Decker J Chiu D Emerling S Sekhon M Kim R Lenz J Han J Ioshikhes I Renault B Marondel I Yoon SJ Song K Murty VV Scherer S Yonescu R Kirsch IR Ried T McPherson J Gibbs R Kucherlapati R 《Nature》2001,409(6822):945-946
Our sequence-tagged site-content map of chromosome 12 is now integrated with the whole-genome fingerprinting effort. It provides accurate and nearly complete bacterial clone coverage of chromosome 12. We propose that this integrated mapping protocol serves as a model for constructing physical maps for entire genomes. 相似文献
326.
Evidence for ubiquitous strong electron-phonon coupling in high-temperature superconductors 总被引:3,自引:0,他引:3
Lanzara A Bogdanov PV Zhou XJ Kellar SA Feng DL Lu ED Yoshida T Eisaki H Fujimori A Kishio K Shimoyama JI Noda T Uchida S Hussain Z Shen ZX 《Nature》2001,412(6846):510-514
Coupling between electrons and phonons (lattice vibrations) drives the formation of the electron pairs responsible for conventional superconductivity. The lack of direct evidence for electron-phonon coupling in the electron dynamics of the high-transition-temperature superconductors has driven an intensive search for an alternative mechanism. A coupling of an electron with a phonon would result in an abrupt change of its velocity and scattering rate near the phonon energy. Here we use angle-resolved photoemission spectroscopy to probe electron dynamics-velocity and scattering rate-for three different families of copper oxide superconductors. We see in all of these materials an abrupt change of electron velocity at 50-80 meV, which we cannot explain by any known process other than to invoke coupling with the phonons associated with the movement of the oxygen atoms. This suggests that electron-phonon coupling strongly influences the electron dynamics in the high-temperature superconductors, and must therefore be included in any microscopic theory of superconductivity. 相似文献
327.
Magnetic carbon 总被引:9,自引:0,他引:9
Makarova TL Sundqvist B Höhne R Esquinazi P Kopelevich Y Scharff P Davydov VA Kashevarova LS Rakhmanina AV 《Nature》2001,413(6857):716-718
The discovery of nanostructured forms of molecular carbon has led to renewed interest in the varied properties of this element. Both graphite and C60 can be electron-doped by alkali metals to become superconducting; transition temperatures of up to 52 K have been attained by field-induced hole-doping of C60 (ref. 2). Recent experiments and theoretical studies have suggested that electronic instabilities in pure graphite may give rise to superconducting and ferromagnetic properties, even at room temperature. Here we report the serendipitous discovery of strong magnetic signals in rhombohedral C60. Our intention was to search for superconductivity in polymerized C60; however, it appears that our high-pressure, high-temperature polymerization process results in a magnetically ordered state. The material exhibits features typical of ferromagnets: saturation magnetization, large hysteresis and attachment to a magnet at room temperature. The temperature dependences of the saturation and remanent magnetization indicate a Curie temperature near 500 K. 相似文献
328.
Sphingolipids in mammalian cell signalling 总被引:12,自引:0,他引:12
Sphingolipids and their metabolites, ceramide, sphingosine and sphingosine-1-phosphate, are involved in a variety of cellular
processes including differentiation, cellular senescence, apoptosis and proliferation. Ceramide is the main second messenger,
and is produced by sphingomyelinase-induced hydrolysis of sphingomyelin and by de novo synthesis. Many stimuli, e.g. growth
factors, cytokines, G protein-coupled receptor agonists and stress (UV irradiation) increase cellular ceramide levels. Sphingomyelin
in the plasma membrane is located primarily in the outer (extracellular) leaflet of the bilayer, whilst sphingomyelinases
are found at the inner (cytosolic) face and within lysosomes/endosomes. Such cellular compartmentalisation restricts the site
of ceramide production and subsequent interaction with target proteins. Glycosphingolipids and sphingomyelin together with
cholesterol are major components of specialised membrane microdomains known as lipid rafts, which are involved in receptor
aggregation and immune responses. Many signalling molecules, for example Src family tyrosine kinases and glycosylinositolphosphate-anchored
proteins, are associated with rafts, and disruption of these domains affects cellular responses such as apoptosis. Sphingosine
and sphingosine-1-phosphate derived from ceramide are also signalling molecules. In particular, sphingosine-1-phosphate is
involved in proliferation, differentiation and apoptosis. Sphingosine-1-phosphate can act both extracellularly through endothelial-differentiating
gene (EDG) family G protein-coupled receptors and intracellularly through direct interactions with target proteins. The importance
of sphingolipid signalling in cardiovascular development has been reinforced by recent reports implicating EDG receptors in
the regulation of embryonic cardiac and vascular morphogenesis.
Received 16 May 2001; received after revision 29 June 2001; accepted 3 July 2001 相似文献
329.
Arnaiz-Villena A Guillén J Ruiz-del-Valle V Lowy E Zamora J Varela P Stefani D Allende LM 《Cellular and molecular life sciences : CMLS》2001,58(8):1159-1166
Mitochondrial cytochrome b (cyt b) from 24 Carduelini species including crossbills, bullfinches, grosbeaks, rosefinches, and other related, but not conclusively classified species, was sequenced. These sequences were also compared with all the available sequences from the genera Carduelis, Serinus, and Passer. Phylogenetic analyses consistently gave the same groups of finches and the calculated divergence times suggest that speciation of the studied species occurred between 14 and 3 million years ago (Miocene-Pliocene), appearing before the Passer, Carduelis, and Serinus genera. Pleistocene glaciations may have been important in sub-speciation. Crossbills are integrated within the genus Carduelis, and within redpolls; the common crossbill shows subspeciation with Loxia japonica in the Pleistocene epoch. Pinicola enucleator groups together with bullfinches and is probably the ancestor of the group. Hawfinch is only distantly related to the studied groups, and might either represent an isolated genus or be related to the New World genus Hesperiphona. The grosbeak genera Eophona and Mycerobas are clearly sister groups, and species belonging to the former might have given rise to Mycerobas species. The isolated (in classification) Uragus sibiricus and Haematospiza sipahi are included within the genus Carpodacus (rosefinches); Carpodacus nipalensis is outside the genus Carpodacus in the molecular analyses and might be an isolated species or related to the genus Montifringilla. 相似文献
330.
Sinnarajah S Dessauer CW Srikumar D Chen J Yuen J Yilma S Dennis JC Morrison EE Vodyanoy V Kehrl JH 《Nature》2001,409(6823):1051-1055
The heterotrimeric G-protein Gs couples cell-surface receptors to the activation of adenylyl cyclases and cyclic AMP production (reviewed in refs 1, 2). RGS proteins, which act as GTPase-activating proteins (GAPs) for the G-protein alpha-subunits alpha(i) and alpha(q), lack such activity for alpha(s) (refs 3-6). But several RGS proteins inhibit cAMP production by Gs-linked receptors. Here we report that RGS2 reduces cAMP production by odorant-stimulated olfactory epithelium membranes, in which the alpha(s) family member alpha(olf) links odorant receptors to adenylyl cyclase activation. Unexpectedly, RGS2 reduces odorant-elicited cAMP production, not by acting on alpha(olf) but by inhibiting the activity of adenylyl cyclase type III, the predominant adenylyl cyclase isoform in olfactory neurons. Furthermore, whole-cell voltage clamp recordings of odorant-stimulated olfactory neurons indicate that endogenous RGS2 negatively regulates odorant-evoked intracellular signalling. These results reveal a mechanism for controlling the activities of adenylyl cyclases, which probably contributes to the ability of olfactory neurons to discriminate odours. 相似文献