Planetary science. Learning to think like Martians     

Zuber MT 《Science (New York, N.Y.)》2003,302(5651):1694-1695

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Retention of xenon in quartz and Earth's missing xenon     

Sanloup C  Schmidt BC  Chamorro Perez EM  Jambon A  Gregoryanz E  Mezouar M 《Science (New York, N.Y.)》2005,310(5751):1174-1177

The reactivity of xenon with terrestrial oxides was investigated by in situ synchrotron x-ray diffraction. At high temperature (T > 500 kelvin), some silicon was reduced, and the pressure stability of quartz was expanded, attesting to the substitution of some xenon for silicon. When the quartz was quenched, xenon diffused out and only a few weight percent remained trapped in samples. These results show that xenon can be covalently bonded to oxygen in quartz in the lower continental crust, providing an answer to the missing xenon problem; synthesis paths of rare gas compounds are also opened.  相似文献   

Extensive gene traffic on the mammalian X chromosome     

Emerson JJ  Kaessmann H  Betrán E  Long M 《Science (New York, N.Y.)》2004,303(5657):537-540

Mammalian sex chromosomes have undergone profound changes since evolving from ancestral autosomes. By examining retroposed genes in the human and mouse genomes, we demonstrate that, during evolution, the mammalian X chromosome has generated and recruited a disproportionately high number of functional retroposed genes, whereas the autosomes experienced lower gene turnover. Most autosomal copies originating from X-linked genes exhibited testis-biased expression. Such export is incompatible with mutational bias and is likely driven by natural selection to attain male germline function. However, the excess recruitment is consistent with a combination of both natural selection and mutational bias.  相似文献   

Melanopsin is required for non-image-forming photic responses in blind mice     

Panda S  Provencio I  Tu DC  Pires SS  Rollag MD  Castrucci AM  Pletcher MT  Sato TK  Wiltshire T  Andahazy M  Kay SA  Van Gelder RN  Hogenesch JB 《Science (New York, N.Y.)》2003,301(5632):525-527

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Deciphering the reaction dynamics underlying optimal control laser fields     

Daniel C  Full J  González L  Lupulescu C  Manz J  Merli A  Vajda S  Wöste L 《Science (New York, N.Y.)》2003,299(5606):536-539

Femtosecond high-resolution pump-probe experiments have been used together with theoretical ab initio quantum calculations and wave packet dynamics simulations to decode an optimal femtosecond pulse that is generated from adaptive learning algorithms. This pulse is designed to maximize the yield of the organometallic ion CpMn(CO)3 while hindering the competing fragmentation. The sequential excitation and ionization of the target ion are accomplished by an optimized field consisting of two dominant subpulses with optimal frequencies and time delays.  相似文献   

A bacterial toxin that controls cell cycle progression as a deoxyribonuclease I-like protein     

Lara-Tejero M  Galán JE 《Science (New York, N.Y.)》2000,290(5490):354-357

Many bacterial pathogens encode a multisubunit toxin, termed cytolethal distending toxin (CDT), that induces cell cycle arrest, cytoplasm distention, and, eventually, chromatin fragmentation and cell death. In one such pathogen, Campylobacter jejuni, one of the subunits of this toxin, CdtB, was shown to exhibit features of type I deoxyribonucleases. Transient expression of this subunit in cultured cells caused marked chromatin disruption. Microinjection of low amounts of CdtB induced cytoplasmic distention and cell cycle arrest. CdtB mutants with substitutions in residues equivalent to those required for catalysis or magnesium binding in type I deoxyribonucleases did not cause chromatin disruption. CDT holotoxin containing these mutant forms of CdtB did not induce morphological changes or cell cycle arrest.  相似文献   

A structural mechanism for MscS gating in lipid bilayers     

Vásquez V  Sotomayor M  Cordero-Morales J  Schulten K  Perozo E 《Science (New York, N.Y.)》2008,321(5893):1210-1214

The mechanosensitive channel of small conductance (MscS) is a key determinant in the prokaryotic response to osmotic challenges. We determined the structural rearrangements associated with MscS activation in membranes, using functorial measurements, electron paramagnetic resonance spectroscopy, and computational analyses. MscS was trapped in its open conformation after the transbilayer pressure profile was modified through the asymmetric incorporation of lysophospholipids. The transition from the closed to the open state is accompanied by the downward tilting of the transmembrane TM1-TM2 hairpin and by the expansion, tilt, and rotation of the TM3 helices. These movements expand the permeation pathway, leading to an increase in accessibility to water around TM3. Our open MscS model is compatible with single-channel conductance measurements and supports the notion that helix tilting is associated with efficient pore widening in mechanosensitive channels.  相似文献   

Subcellular calcium transients visualized by confocal microscopy in a voltage-clamped vertebrate neuron   总被引：25，自引：0，他引：25  

A Hernández-Cruz  F Sala  P R Adams 《Science (New York, N.Y.)》1990,247(4944):858-862

Confocal laser-scanned microscopy and long-wavelength calcium (Ca2+) indicators were combined to monitor both sustained and rapidly dissipating Ca2+ gradients in voltage-clamped sympathetic neurons isolated from the bullfrog. After a brief activation of voltage-dependent Ca2+ channels, Ca2+ spreads inwardly, and reaches the center of these spherical cells in about 300 milliseconds. Although the Ca2+ redistribution in the bulk of the cytosol could be accounted for with a radial diffusion model, local nonlinearities, suggesting either nonuniform Ca2+ entry or spatial buffering, could be seen. After electrical stimulation, Ca2+ signals in the nucleus were consistently larger and decayed more slowly than those in the cytosol. A similar behavior was observed when release of intracellular Ca2+ was induced by caffeine, suggesting that in both cases large responses originate from Ca2+ release sites near or within the nucleus. These results are consistent with an amplification mechanism involving Ca2(+)-induced Ca2+ release, which could be relevant to activity-dependent, Ca2(+)-regulated nuclear events.  相似文献   

Carbon tetrachloride at hepatotoxic levels blocks reversibly gap junctions between rat hepatocytes   总被引：2，自引：0，他引：2  

J C Sáez  M V Bennett  D C Spray 《Science (New York, N.Y.)》1987,236(4804):967-969

Electrical coupling and dye coupling between pairs of rat hepatocytes were reversibly reduced by brief exposure to halogenated methanes (CBrCl3, CCl4, and CHCl3). The potency of different halomethanes in uncoupling hepatocytes was comparable to their hepatotoxicity in vivo, and the rank order was the same as that of their tendency to form free radicals. The effect of carbon tetrachloride (CCl4) on hepatocytes was substantially reduced by prior treatment with SKF 525A, an inhibitor of cytochrome P-450, and by exposure to the reducing reagent beta-mercaptoethanol. Halomethane uncoupling occurred with or without extracellular calcium and did not change intracellular concentrations of calcium and hydrogen ions or the phosphorylation state of the main gap-junctional protein. Thus the uncoupling appears to depend on cytochrome P-450 oxidative metabolism in which free radicals are generated and may result from oxidation of the gap-junctional protein or of a regulatory molecule that leads to closure of gap-junctional channels. Decreases in junctional conductance may be a rapid cellular response to injury that protects healthy cells by uncoupling them from unhealthy ones.  相似文献   

Western hegemony over african agriculture in Southern Rhodesia and its continuing threat to food security in independent zimbabwe     

Sam L. J. Page  Helán E. Page 《Agriculture and Human Values》1991,8(4):3-18

Zimbabwe's communal farmers are now less food secure than they were two generations ago. The roots of this decline lie not only in the confinement of Africans to marginal land but also in the historic forced replacement of their sustainable, indigenous farming system with one whose productivity now relies on the use of large amounts of expensive chemical inputs. Environmentally-friendly, traditional farming practices such as pyro-culture, minimum tillage, mixed cropping, and bush fallowing were completely wiped out and replaced with a highly technical western or ganic farming system based on plough cultivation and continuous monocultures of commodity crops, that were supposed to be sustained by liberal amounts of green and animal manures. This gave rise to an effective agricultural hegemony that was due, mainly, to the zealous dedication of an American Mormon missionary whose motives were evangelical rather than scientific.  相似文献