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111.
Recent mutagenesis experiments have confirmed our hypothesis that a segment between S5 and S6 forms the ion selective portion of voltage-gated ion channels. Based on these and other new data, we have revised previous models of the general folding pattern of voltage-gated channel proteins and have developed atomic scale models of the entire transmembrane region of the Shaker A K+ channel. In these models, the ion selective region is a beta-barrel that spans the outer half of the membrane. The inner half of the pore is larger. The voltage-dependent conformational changes of activation gating are modeled to occur by the "helical screw" mechanism, in which the four S4 segments move along and rotate about their axes. These changes are followed by a voltage-independent conformational change, in which the segments linking S4 to S5 move from blocking the intracellular entrance of the pore to forming part of the lining of the large inner portion of the pore. The NH2-terminal of the protein was modeled as an alpha-helix that plugs the intracellular half of the pore to inactivate the channel.  相似文献   
112.
A new combination,Glandularia x hybrida, and a lectotype are proposed for the common Garden Vervain (Verbena xhybrida). The plant is of horticultural origin, but occurs widely in North America and Mexico as an adventive.  相似文献   
113.
Ionactis caelestis Leary & Nesom is a new species known from a single population that occurs on the Aztec Sandstone near Bridge Mountain in the Spring Mountains of Clark County, Nevada. It is placed in the genus Ionactis (=Aster subg. Ianthe) on the basis of its crowded, multicipital crown, lack of persistent basal leaves and presence of densely arranged cauline ones, strongly carinate phyllaries, blue rays, disc style branches with linear-lanceolate appendages, asymmetric carpopodia, double pappus, and chromosome number of 2n = 9 II. A key to the four species of the genus emphasizes the distinction of the new species in its taproot, the abundant, large, glandular trichomes on its stems and leaves, and disc flowers with sterile ovaries. Ionactis is more similar to the goldenaster (Heterotheca) lineage than to Aster, with which it has been allied formerly. The core of the goldenaster genera differ from Ionactis primarily in their yellow-rayed heads, the crystal complement within cells of their disc corollas, and their primarily multinerved achenes.  相似文献   
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Sucrose phosphate synthase and sucrose accumulation at low temperature   总被引:26,自引:4,他引:22  
Guy CL  Huber JL  Huber SC 《Plant physiology》1992,100(1):502-508
The influence of growth temperature on the free sugar and sucrose phosphate synthase content and activity of spinach (Spinacia oleracea) leaf tissue was studied. When plants were grown at 25°C for 3 weeks and then transferred to a constant 5°C, sucrose, glucose, and fructose accumulated to high levels during a 14-d period. Predawn sugar levels increased from 14- to 20-fold over the levels present at the outset of the low-temperature treatment. Sucrose was the most abundant free sugar before, during, and after exposure to 5°C. Leaf sucrose phosphate synthase activity was significantly increased by the low-temperature treatment, whereas sucrose synthase and invertases were not. Synthesis of the sucrose phosphate synthase subunit was increased during and after low-temperature exposure and paralleled an increase in the steady-state level of the subunit. The increases in sucrose and its primary biosynthetic enzyme, sucrose phosphate synthase, are discussed in relation to adjustment of metabolism to low nonfreezing temperature and freezing stress tolerance.  相似文献   
119.
The major ecdysteroids in large worker pupae of the leaf-cutting ant Acromyrmex octospinosus were characterized at the peak ecdysteroid concentration by using high-performance liquid chromatography, enzyme immunoassay, and mass spectrometry. In decreasing amounts, they were determined to be makisterone A, an unidentified C28 ecdysteroid bearing a molecular weight of 494, 20-hydroxyecdysone (ratio of 1 to 6 as compared to makisterone A), and putative but negligible ecdysone. The presence of both C28 and C27 ecdysteroids is discussed in relation to the content of 4-desmethylsterols determined by gas chromatography and mass spectrometry to be ergosta-5,7,24 (28)-trien-3β-ol, ergosterol, ergosta-5,7-dien-3β-ol and ergosta-7,24(28)-dien-3β-ol for the main sterols, and with a small amount of cholesterol.  相似文献   
120.
Two genes encoding 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase were localized in human and rat chromosomes. PFKFB1 (previously PFRX), which encodes the liver and muscle isozymes, was assigned to Xq22-q31 in the rat and to Xq27–q28 in the human by in situ hybridization using probes generated by the polymerase chain reaction. PFKFB2, which encodes the heart isozyme of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, was assigned to chromosome 13 in the rat and to chromosome 1 in the human by hybridization of DNA from somatic cell hybrids. By in situ hybridization, this gene was localized to the regions 13q24–25 in the rat and 1q31 in the human.  相似文献   
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