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San Carlos olivine crystals under laboratory conditions of 26 gigapascals and 973 to 1473 kelvin (conditions typical of subducted slabs at a depth of 720 kilometers) for periods of a few minutes to 19 hours transformed to the phase assemblage of perovskite and magnesiowustite in two stages: (i) the oxygen sublattice transformed into a cubic close-packed lattice, forming a metastable spinelloid, and (ii) at higher temperatures or longer run durations, this spinelloid broke down to perovskite and magnesiowustite by redistributing silicon and magnesium while maintaining the general oxygen framework. The breakdown was characterized by a blocking temperature of 1000 kelvin, below which olivine remained metastable, and by rapid kinetics once the reaction was activated. 相似文献
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66.
The design and catalytic properties of a simplified ribonuclease P RNA 总被引:21,自引:0,他引:21
Ribonuclease P (RNase P) RNA is the catalytic moiety of the ribonucleoprotein enzyme that removes precursor sequences from the 5' ends of pre-transfer RNAs in eubacteria. Phylogenetic variation according to recently proposed secondary structure models was used to identify structural elements of the RNase P RNA that are dispensable for catalysis. A simplified RNase P RNA that consists only of evolutionarily conserved features was designed, synthesized, and characterized. Although the simplified RNA (Min 1 RNA) is only 263 nucleotides in length, in contrast to the 354 to 417 nucleotides of naturally occurring RNase P RNAs, its specificity of pre-tRNA cleavage is identical to that of the native enzymes. Moreover, the catalytic efficiencies of the Min 1 RNA and the native RNA enzymes are similar. These results focus the search for the catalytic elements of RNase P RNAs to their conserved structure. 相似文献
67.
Specificity for aminoacylation of an RNA helix: an unpaired, exocyclic amino group in the minor groove 总被引:14,自引:0,他引:14
K Musier-Forsyth N Usman S Scaringe J Doudna R Green P Schimmel 《Science (New York, N.Y.)》1991,253(5021):784-786
An acceptor stem G3.U70 base pair is a major determinant of the identity of an alanine transfer RNA. Hairpin helices and RNA duplexes consisting of complementary single strands are aminoacylated with alanine if they contain G3.U70. Chemical synthesis of RNA duplexes enabled the introduction of base analogs that tested the role of specific functional groups in the major and minor grooves of the RNA helix. The results of these experiments indicate that an unpaired guanine 2-amino group at a specific position in the minor groove of an RNA helix marks a molecule for aminoacylation with alanine. 相似文献
68.
Mixed leukocyte stimulation of normal peripheral leukocytes by autologous lymphoblastoid cells 总被引:9,自引:0,他引:9
Lymphocyte culture lines can be established by using buffy coat cells from normal or diseased individuals. Established lymphocytoblasts from a normal donor were used in the one-way mixed leukocyte culture test and were found to stimulate the peripheral lymphocytes of that donor. These lymphocytoblasts were also capable of stimulating allogeneic lymphocyte populations. 相似文献
69.
Acetabularia chloroplast DNA: electron microscopic visualization 总被引:2,自引:0,他引:2
Isolated chloroplasts of Acetabularia mediterranea, lysed by osmotic shock in a protein monolayer, released large amounts of supercoiled DNA. In some cases, the DNA radiated in large loops from centers of organization. The amount of DNA per chloroplast may be similar to that of a bacterium. 相似文献
70.
Energized configurations of heart mitochondria in situ 总被引:3,自引:0,他引:3
R A Harris C H Williams M Caldwell D E Green E Valdivia 《Science (New York, N.Y.)》1969,165(894):700-703
Changes in the configurational state of the cristal membranes of rat heart mitochondria within the living cell can be induced by imposing energizing conditions. An exact correlation has been established between the configurational states of the cristal membrane and the energy states of the mitochondrion. The configurational changes observed in mitochondria in situ are comparable to those established for beef heart mitochondria in vitro and are consistent with the postulate of the conformational basis of energy transductions in membrane systems. The formation of paracrystalline arrays is one of the noteworthy features of configurational changes of mitochondria in situ. 相似文献