全文获取类型
收费全文 | 2207篇 |
免费 | 64篇 |
国内免费 | 92篇 |
出版年
2023年 | 11篇 |
2022年 | 30篇 |
2021年 | 32篇 |
2020年 | 23篇 |
2019年 | 28篇 |
2018年 | 41篇 |
2017年 | 27篇 |
2016年 | 35篇 |
2015年 | 38篇 |
2014年 | 79篇 |
2013年 | 116篇 |
2012年 | 50篇 |
2011年 | 100篇 |
2010年 | 69篇 |
2009年 | 76篇 |
2008年 | 88篇 |
2007年 | 87篇 |
2006年 | 97篇 |
2005年 | 79篇 |
2004年 | 70篇 |
2003年 | 59篇 |
2002年 | 59篇 |
2001年 | 51篇 |
2000年 | 47篇 |
1999年 | 47篇 |
1998年 | 51篇 |
1997年 | 55篇 |
1996年 | 57篇 |
1995年 | 49篇 |
1994年 | 63篇 |
1993年 | 41篇 |
1992年 | 47篇 |
1991年 | 35篇 |
1990年 | 33篇 |
1989年 | 50篇 |
1988年 | 52篇 |
1987年 | 39篇 |
1986年 | 33篇 |
1985年 | 50篇 |
1984年 | 51篇 |
1983年 | 29篇 |
1982年 | 45篇 |
1981年 | 42篇 |
1980年 | 30篇 |
1979年 | 14篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1976年 | 8篇 |
1975年 | 7篇 |
1973年 | 10篇 |
排序方式: 共有2363条查询结果,搜索用时 15 毫秒
101.
102.
The symbiotic root nodule, an organ formed on leguminous plants, is a product of successful interactions between the host plant and the soil bacteria, Rhizobium spp. Plant hormones play an important role in the genesis of this organ. The hormonal balance appears to be modulated by the signals produced by bacteria. Many host genes induced during nodule organogenesis and the symbiotic state have been identified and characterized from several legumes. These genes encode nodule-specific proteins (nodulins) which perform diverse functions in root nodule development and metabolism. Formation of a subcellular compartment housing the bacteria is essential to sustain the symbiotic state, and several nodulins are involved in maintaining the integrity and function of this compartment. The bacteroid enclosed in the perbacteroid membrane behaves as an 'organelle,'completely dependent on the host for all its requirements for carbon, nitrogen and other essential elements. Thus it seems likely that the nodulins in the peribacteroid membrane perform specific transport functions. While the function of a few other nodulins is known (e.g. nodulin-100, nodulin-35), a group of uncharacterized nodulins exists in soybean root nodules. These nodulins share structural similarities and seem to have been derived from a common ancestor. Induction of nodulin genes occurs prior to and independent of nitrogen fixation, and thus is a prelude to symbiosis. Although some of the early nodulin genes are induced prior to or during infection, induction of late nodulins requires endocytotic release of bacteria. 相似文献
103.
Immunocytolocalization of glutamine synthetase in mesophyll and phloem of leaves ofSolanum tuberosum L. 总被引:1,自引:0,他引:1
Summary Localization of glutamine synthetase inSolanum tuberosum leaves was investigated by techniques of Western tissue printing and immunogold electron microscopy. Anti-GS antibodies used in immunolocalization recognize two peptides (45 kDa and 42 kDa) on Western blots. Antibody stained tissue prints on nitrocellulose membranes allowed low resolution localization of GS. Immunostaining was most evident in the adaxial phloem of the leaf midribs and petiole veins. High-resolution localization of glutamine synthetase by immunogold electron microscopy revealed that this enzyme occurs in both the chloroplasts and the cytosol ofS. tuberosum leaf cells. However, GS was specifically associated with the chloroplasts of mesophyll cells and with the cytoplasm of phloem companion cells. The evidence for cell-specific localization of chloroplast and cytosolic GS presented here agrees with the recently reported cell-specific pattern of expression of GUS reporter gene, directed by promoters for chloroplast and cytosolic GS form in tobacco transgenic plants. These data provide additional clues to the interpretation of the functional role of these different isoenzymes and its relationship with their specific localization.Abbreviations BSA
bovine serum albumin
- EM
electron microscope
- GOGAT
glutamate synthase
- GS
glutamine synthetase
- GUS
-glucuronidase
- IgG
immunoglobulin
- PBS
phosphate buffer saline
- SDS-PAGE
sodium dodecyl sulphate-polyacrylamide gel electrophoresis 相似文献
104.
Thomas W. Becker Michel Caboche Elisa Carrayol Bertrand Hirel 《Plant molecular biology》1992,19(3):367-379
105.
Regulation of amidase formation in mutants from Pseudomonas aeruginosa PAO lacking glutamine synthetase activity 总被引:1,自引:0,他引:1
Dick B. Janssen Patricia M. Herst Han M. L. J. Joosten Chris van der Drift 《Archives of microbiology》1982,131(4):344-346
The formation of amidase was studied in mutants from Pseudomonas aeruginosa PAO lacking glutamine synthetase activity. It appeared that catabolite repression of amidase synthesis by succinate was partially relieved when cellular growth was limited by glutamine. Under these conditions, a correlation between amidase and urease formation was observed. The results suggest that amidase formation in strain PAO is subject to nitrogen control and that glutamine or some compound derived from it mediates the nitrogen repression of amidase. 相似文献
106.
We have studied the changes in the activities of both nitrogenase (switch off) and glutamine synthetase in Rhodospirillum rubrum upon addition of ammonium ions or glutamine to nitrogen fixing cultures. Both activities decrease drastically and return in a parallel manner when added ammonia is metabolized. The decrease in glutamine synthetase activity does not seem to be primarily due to adenylylation of the enzyme. Addition of glutamine to cells starved for nitrogen results in inactivation of glutamine synthetase but nitrogenase is only partially switched off.Abbreviations CeMe3NBr
Cetyltrimethylammonium bromide
- Hepes
N-2-hydroxyethyl-piperazine-N-2 sulfonic acid
- MSO
methionine-D,L-sulfoximine
- Tea-Dmg
triethanol amine-3,3-dimethylglutaric acid 相似文献
107.
Levels of ammonia-assimilating enzymes (glutamate dehydrogenase, glutamine synthetase, glutamate synthase) were determined in extracts of Sporotrichum pulverulentum grown under different conditions with respect to both nitrogen source and concentration. Evolution of 14CO2 from 14C-synthetic lignin by fungal cultures grown under parallel conditions was also determined as a measure of lignin decomposition and the suppressive effect of nitrogen on ligninolysis confirmed. Under low nitrogen conditions, fungal extracts exhibited relatively high levels of NADP-dependent glutamate dehydrogenase and glutamine synthetase dehydrogenase. Conversely, in high nitrogen extracts, lower levels of NADP-dependent glutamate dehydrogenase and glutamine synthetase activity, and higher levels of NAD-dependent glutamate dehydrogenase, were recorded. Possible effects of enzyme activities on intracellular pool concentrations of glutamate/glutamine, and the implications for the regulation of lignin metabolism, are discussed.A preliminary report was presented at The Ekman Days 1981, International Symposium on Wood and Pulping Chemistry, Stockholm, Sweden, June 9–12, 1981. 相似文献
108.
Glutamine synthetase (GS) of Rhodopseudomonas sphaeroides is regulated by adenylylation and deadenylylation. The extent of adenylylation/deadenylylation of the enzyme in cell free extracts was influenced by inorganic phosphate (P
i), -ketoglutarate, ATP and other nucleotides. While P
i and -ketoglutarate stimulated deadenylylation, ATP and other nucleotides enhanced adenylylation of the GS. By using proper combinations of the effectors and incubation conditions, any desired adenylylation state of GS could be adjusted in vitro. The enzyme was purified to electrophoretic homogenity by three steps including affinity chromatography on 5-AMP-Sepharose. Adenylylated and deadenylylated enzyme showed different UV-spectra and isoelectric points. The native enzyme had a molecular weight of 600,000, deadenylylated subunits of 50,000±1,000. Electron microscopic investigations revealed a dodecameric arrangement of subunits in two hexameric planes. 相似文献
109.
Decapitation or red light irradiation (R) inhibited growth and Golgi-localized glucan synthetase (GS I) activity in the mesocotyl of intact maize (Zea mays L.) seedlings. Applied auxin (indole-3-acetic acid) prevented the effects of R and of decapitation on both growth and GS I. Auxin applied several hours after irradiation prevented any further decline in GS I but did not restore it. Mesocotyl segments incubated in solution elongated in response to auxin but lost GS I with time regardless of the presence of exogenous auxin. An attached seed was necessary for maintenance of GS I in the dark-grown mesocotyl.Abbreviations GS
glucan synthetase
- IAA
indole-3-acetic acid
- R
red light 相似文献
110.
Glutamine synthetase was found to be increased in C-6 glioma cells as a result of increasing culture passage and N-6,2'-O-dibutyryl cyclic AMP (dbcAMP) treatment. At low passage dbcAMP produced a 2.5-fold increase in glutamine synthetase activity per unit of cellular protein. At high passage control glutamine synthetase was approximately double that seen at low passage, but dbcAMP produced an additional 65% increase. Lactate dehydrogenase activity was also increased by dbcAMP treatment at both low and high passage, but culture passage produced no change in the lactate dehydrogenase. With increasing culture passage, the ratio of cellular protein to DNA doubled. Therefore, expression of data per unit of protein tended to minimize the apparent changes in activity. The maximum increase in glutamine synthetase activity produced by both dbcAMP and increasing culture passage and expressed on a DNA basis was 5.6-fold. The increase in glutamine synthetase activity was generally linear during the first 20 h of drug treatment, after which enzyme activity remained nearly constant up to 72 h. Ninety percent or more of the dbcAMP remained in the medium at the end of 48-h exposure of cells to dbcAMP. 8-br-Cyclic AMP also increased glutamine synthetase activity of C-6-cels, but n-butyrate did not. Isoproterenol, which increases cyclic AMP in C-6-cells, increased glutamine synthetase activity. The effect of isoproterenol on glutamine synthetase was inhibited by the beta-adrenergic blocking agent sotalol. Cycloheximide (10 micrograms/ml) inhibited the dbcAMP effect on glutamine synthetase activity and also decreased the control enzyme activity by 60%. 相似文献