首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1163篇
  免费   51篇
  2021年   12篇
  2020年   12篇
  2019年   9篇
  2018年   30篇
  2017年   18篇
  2016年   22篇
  2015年   42篇
  2014年   44篇
  2013年   71篇
  2012年   80篇
  2011年   90篇
  2010年   45篇
  2009年   41篇
  2008年   68篇
  2007年   65篇
  2006年   59篇
  2005年   82篇
  2004年   79篇
  2003年   61篇
  2002年   58篇
  2001年   10篇
  2000年   8篇
  1999年   10篇
  1998年   6篇
  1996年   6篇
  1995年   8篇
  1994年   4篇
  1992年   5篇
  1990年   4篇
  1989年   4篇
  1986年   4篇
  1985年   10篇
  1984年   5篇
  1982年   9篇
  1980年   6篇
  1979年   8篇
  1973年   7篇
  1971年   5篇
  1969年   6篇
  1967年   4篇
  1966年   4篇
  1964年   6篇
  1963年   7篇
  1962年   8篇
  1961年   5篇
  1960年   7篇
  1959年   5篇
  1958年   4篇
  1957年   5篇
  1943年   3篇
排序方式: 共有1214条查询结果,搜索用时 171 毫秒
1.
2.
The membrane localization of the plasma membrane Ca2+-ATPase isoform 2 (PMCA2) in polarized cells is determined by alternative splicing; the PMCA2w/b splice variant shows apical localization, whereas the PMCA2z/b and PMCA2x/b variants are mostly basolateral. We previously reported that PMCA2b interacts with the PDZ protein Na+/H+ exchanger regulatory factor 2 (NHERF2), but the role of this interaction for the specific membrane localization of PMCA2 is not known. Here we show that co-expression of NHERF2 greatly enhanced the apical localization of GFP-tagged PMCA2w/b in polarized Madin-Darby canine kidney cells. GFP-PMCA2z/b was also redirected to the apical membrane by NHERF2, whereas GFP-PMCA2x/b remained exclusively basolateral. In the presence of NHERF2, GFP-PMCA2w/b co-localized with the actin-binding protein ezrin even after disruption of the actin cytoskeleton by cytochalasin D or latrunculin B. Surface biotinylation and fluorescence recovery after photobleaching experiments demonstrated that NHERF2-mediated anchorage to the actin cytoskeleton reduced internalization and lateral mobility of the pump. Our results show that the specific interaction with NHERF2 enhances the apical concentration of PMCA2w/b by anchoring the pump to the apical membrane cytoskeleton. The data also suggest that the x/b splice form of PMCA2 contains a dominant lateral targeting signal, whereas the targeting and localization of the z/b form are more flexible and not fully determined by intrinsic sequence features.  相似文献   
3.
We have isolated, purified and characterized six individual gamma-crystallin polypeptides present in the rat lens. Comparison of their amino acid compositions with the known structure of the six gamma-crystallin genes permits a one-to-one correspondence to be made between each protein synthesized and the encoding gene. This demonstrates that each of the six genes is actually expressed in vivo. Two classes of three gamma-crystallins each, which we have designated classes gamma ABC and gamma DEF, are known to exist, on the basis of internal sequence homology. We have measured the temperature-dependent phase-separation characteristics of solutions of the six purified gamma-crystallins, and find that the three members of the gamma DEF class (gamma 2-2, gamma 3-1 and gamma 4-1) are all cryo-proteins with relatively high phase-separation temperatures, whereas the three gamma ABC crystallins (gamma 1-1, gamma 1-2 and gamma 2-1) do not show phase separation above -7 degrees C. We have measured the spatial distribution in rat lens of each of the alpha-, beta- and gamma-crystallins as a function of age from 1 to 420 days, using size-exclusion and ion-exchange high-pressure liquid chromatography (HPLC). Our findings in the cortical layer permit us to establish the differential synthesis of each of the crystallins during lens development. Particular attention has been devoted to the spatial and temporal distribution of the six individual gamma-crystallins. Up to birth, synthesis of the three components of the gamma DEF class predominates, and in particular that of gamma 2-2. In subsequent development the three components of the gamma ABC class assume a greater proportion of monomeric crystallins synthesized, while beta s-crystallin synthesis predominates in late development. Our analysis of different layers within single lenses provides novel information on spatial gradients of the water-soluble and water-insoluble protein fractions as a function of age. We consider the consequences of these findings for lens transparency and opacity in both rat and mouse lens. We show that the high concentrations of gamma DEF-crystallins appear to be responsible for the opacity known to occur in young rat lenses. We conclude from these observations that close control of the differential synthesis of gamma-crystallins plays an important role in maintaining lens transparency during development.  相似文献   
4.
Low temperature induced opacification (cold cataract) of the nucleus of young mammalian lenses is associated with a phase separation of proteins in the lens cell cytoplasm. Calf lenses were treated with a variety of imido-esters and N-hydroxysuccinimide-esters, which react specifically with amino groups. Many potent inhibitors of phase separation cataract were identified which lower the opacification temperature by 6 degrees C or more. Lenses generally remain clear, colorless and soft. Furthermore, suppression of the cold cataract temperature is permanent upon removal of excess reagent.  相似文献   
5.
The laboratory conditions for reproducible routine determination of staphylococcal cell-surface hydrophobicity by the salt aggregation test were standardized. Fresh bacterial suspensions standardized to 5 times 109 cfu/ml gave the most reproducible results with both Staphylococcus aureus and coagulase-negative staphylococci. For relatively hydrophobic strains a 5-min reading time was necessary to detect bacterial aggregation in ammonium sulphate solutions ranging from 0.1 M to 1.5 M, pH 6.8. A × 10 hand lens facilitated reading aggregations. Overnight storage of bacterial suspensions at 20C reduced cell-surface hydrophobicity of all species, while storage at 4C reduced the hydrophobic nature of Staph. aureus strains. The hydrophobicity of coagulase-negative staphylococci rarely changed at 4C. A 10-fold dilution of fresh, standardized bacterial suspensions made it impossible to detect bacterial aggregation in ammonium sulphate solutions even with a hand lens. Under standardized conditions three types of staphylococcal cell aggregations were observed. The first looked like the slide agglutination for O antigens of Enterobacteriaceae, the second resembled H-agglutination, while the third had a filamentous appearance. These patterns indicated that more than one component might contribute to cell-surface hydrophobicity of both Staph. aureus and coagulase-negative staphylococci, or the same component might have different position on the cell surface.  相似文献   
6.
Summary We report here successful mutagenesis with Transposon Tn5 of three slow-growing strains of Rhizobium japonicum USDA 122, 61A76, USDA 74 and one fast-growing strain, USDA 191. Strains were chosen as representatives of different DNA homology and serogroups of this divergent species, which effectively nodulate North American soybean cultivars. The source of Tn5 was the suicide plasmid pGS9, which possesses broad host range N-type transfer genes in a narrow host range p15A replicon. The selection of Tn5 mutants was facilitated by the expression of the Tn5 encoded streptomycin gene in R. japonicum. Kanamycin and streptomycin resistant colonies appeared from interspecific crosses with E. coli at optimal frequencies of 10-6 for R. japonicum USDA 61A76 and USDA 191 and 5x10-7 for R. japonicum USDA 122 and USDA 74. Altogether, 6550 Tn5 mutants were isolated in USDA 122 and 61A76, and a small number from USDA 74 and USDA 191. Colony hybridization showed that all tested mutants of 61A76 and USDA 122 contained Tn5. Physical analysis of total DNAs from representative numbers of USDA 122, 61A76 and USDA 191 mutants revealed that each of them carried one copy of the transposon integrated randomly in the genome. This was also true for most USDA 74 mutants. Screening of mutants for auxotrophy showed frequencies of 0.2% for USDA 122 and 0.08% for 61A76. Several symbiotically defective mutants were identified on plants, Glycine soja and G. max.  相似文献   
7.
Summary A tumor model system of clones of myeloproliferative sarcoma virus (MPV)-transformed rat fibroblasts (NRK) with different growth properties and metastatic potential was studied. The relationship between metastatic behavior and composition of carbohydrate-binding proteins (lectins) was analyzed by affinity chromatography. The metastatic variant differs qualitatively from its parental clone in the presence of galactoside-binding proteins at apparent molecular weights of 80 kDa, 70 kDa, 22 kDa, 18 kDa and 16 kDa and of a fucose-binding protein at apparent molecular weight of 42 kDa. The -glucosyl-binding proteins at apparent molecular weights of 67 kDa and 53 kDa and a galactoside-binding protein of apparent molecular weight of 34 kDa, however, are not detectable in the metastatic variant in comparison to its parental clone. In this respect the parental clone shows closer resemblance to the clone 5–8#1 with different growth properties and low metastatic potential than to its own metastatic variant. Furthermore, only the parental clone has a melibiose- and a mannan-binding protein of an apparent molecular weight of 64 kDa and 14 kDa, respectively. Rosette formation as model system for intercellular interaction reveals differences in the inhibition pattern with sugar between the two clones 5–8#1 and 5–20#20, whereas the metastatic variant 5–20#20 (s) exhibits drastically reduced capability to form rosettes. Initial experiments demonstrate the feasibility of drug targeting to transformed fibroblasts via carbohydrate-binding proteins.  相似文献   
8.
Four histidine auxotrophs of Bradyrhizobium japonicum strain USDA 122 were isolated by random transposon Tn5 mutagenesis. These mutants arose from different, single transposition events as shown by the comparison of EcoRI and XhoI-generated Tn5 flanking sequences of genomic DNA. The mutants grew on minimal medium supplemented with l-histidine or l-histidinol but failed to grow with l-histidinol phosphate. While two of the muants were symbiotically defective and did not form nodules on Glycine max cvs. Lee and Peking and on Glycine soja, the other two mutants were symbiotically competent. Reversion to prototrophy occurred at a frequency of about 10-7 on growth medium without added antibiotics, but prototrophs could not be isolated from growth medium containing 200 g/ml kanamycin and streptomycin. The prototrophic revertants formed nodules on all the soybean cultivars examined. When histidine was supplied to the plant growth medium, both nodulation deficient mutants formed effective symbioses. On histidine unamended plants, nodules were observed infrequently. Three classes of bacterial colonies were isolated from such infrequent nodules: class 1 were kanamycin resistant-auxotrophs; class 2 were kanamycin sensitive-prototrophs; and class 3 were kanamycin-sensitive auxotrophs. Our results suggest that two Tn5 insertion mutations in B. japonicum leading to histidine auxotrophy, affect nodulation in some way. These mutations are in regions that show no homology to the Rhizobium meliloti common nodulation genes.  相似文献   
9.
Summary The percentage and absolute number of lymphocytes and Leu 7+ cells were significantly lower in HD even in active stages. There was no significant difference in the percentage of LGL between the three groups (control, active HD, inactive HD), however, because of differences in counts of lymphocytes the absolute number of LGL was significantly lower in HD even in the active group than that in healthy controls. The absolute count of LGL and Leu 7+ cells in patients in remission was significantly higher than that in active HD. Natural cytotoxicity against K-562 cells was also significantly lower in active patients in comparison with controls, while the percentage of cytotoxicity was slightly but not significantly higher in patients in remission than that in the active group. A positive correlation was observed between all the three examined parameters both in controls and in patients with active and inactive HD.  相似文献   
10.
B. Böddi  Katalin Kovács  F. Láng 《BBA》1983,722(2):320-326
Protochlorophyll (PChl) forms were performed in Triton X-100 detergent micelles. The concentration of Triton X-100 was 7·10?4 M (above the critical micellar concentration); the concentration of PChl varied between 1.6·10?5 and 1.8·10?4 M. Absorption, fluorescence and circular dichroism (CD) spectra were registered. The absorption spectra were resolved into Gaussian components by computer analysis. PChl forms with absorption bands at 632–634, 638, 652–654, 663–664, 668 and 676 nm and with fluorescence emission bands at 634–636, 640–644, 652–655, 677–678, 686 and 694–696 nm were observed in micellar solutions of different PChl concentrations. The CD spectra showed a strong dependence on the concentration of PChl: positive CD signals or positive Cotton effects were observed in the vicinity of 650 nm. The intensity of these signals increased in parallel with increasing concentration of PChl. No CD signals were found in the region of the longer wavelength absorption bands. These data show that the PChl exists in many different forms in this system, and the spectroscopic properties of these forms are determined by different molecular interactions viz., interactions of PChl with Triton X-100 or water molecules and/or by the aggregation of PChl.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号