全文获取类型
收费全文 | 2705篇 |
免费 | 281篇 |
国内免费 | 2篇 |
出版年
2023年 | 13篇 |
2022年 | 39篇 |
2021年 | 64篇 |
2020年 | 43篇 |
2019年 | 49篇 |
2018年 | 50篇 |
2017年 | 47篇 |
2016年 | 84篇 |
2015年 | 158篇 |
2014年 | 196篇 |
2013年 | 176篇 |
2012年 | 285篇 |
2011年 | 282篇 |
2010年 | 166篇 |
2009年 | 134篇 |
2008年 | 199篇 |
2007年 | 165篇 |
2006年 | 160篇 |
2005年 | 158篇 |
2004年 | 136篇 |
2003年 | 104篇 |
2002年 | 103篇 |
2001年 | 16篇 |
2000年 | 5篇 |
1999年 | 14篇 |
1998年 | 29篇 |
1997年 | 11篇 |
1996年 | 9篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 11篇 |
1992年 | 5篇 |
1991年 | 4篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1980年 | 2篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1948年 | 2篇 |
1941年 | 1篇 |
1937年 | 1篇 |
1936年 | 2篇 |
1934年 | 1篇 |
排序方式: 共有2988条查询结果,搜索用时 31 毫秒
11.
Thomas Hiltonen Jan Karlsson Kristin Palmqvist Adrian K. Clarke Göran Samuelsson 《Planta》1995,195(3):345-351
An intracellular carbonic anhydrase (CA; EC 4.2.1.1) was purified and characterised from the unicellular green alga Coccomyxa sp. Initial studies showed that cultured Coccomyxa cells contain an intracellular CA activity around 100 times higher than that measured in high-CO2-grown cells of Chlamydomonas reinhardtii CW 92. Purification of a protein extract containing the CA activity was carried out using ammonium-sulphate precipitation followed by anion-exchange chromatography. Proteins were then separated by native (non-dissociating) polyacrylamide gel electrophoresis, with each individual protein band excised and assayed for CA activity. Measurements revealed CA activity associated with two discrete protein bands with similar molecular masses of 80 +5 kDa. Dissociation by denaturing polyacrylamide gel electrophoresis showed that both proteins contained a single polypeptide of 26 kDa, suggesting that each 80-kDa native protein was a homogeneous trimer. Isoelectric focusing of the 80-kDa proteins also produced a single protein band at a pH of 6.5. Inhibition studies on the purified CA extract showed that 50% inhibition of CA activity was obtained using 1 M azetazolamide. Polyclonal antibodies against the 26-kDa CA were produced and shown to have a high specific binding to a single polypeptide in soluble protein extracts from Coccomyxa cells. The same antiserum, however, failed to cross-react with soluble proteins isolated from two different species of green algae, Chlamydomonas reinhardtii and Chlorella vulgaris. Correspondingly, antisera directed against pea chloroplastic CA, extracellular CA from C. reinhardtii and human CAII, showed no cross-hybridisation to the 26-kDa polypeptide in Coccomyxa. The 26-kDa protein was confirmed as being a CA by N-terminal sequencing of two internal polypeptide fragments and alignment of these sequences with that of previously identified CA proteins from several different species.Abbreviations CA
carbonic anhydrase
- CCM
CO2-concentrating mechanism
- IEF
isoelectric focusing
- Rubisco
ribulose-l,5-bisphosphate carboxylase/oxygenase
We would like to thank Drs. Cecilia Forsman, Inga-Maj Johansson and Nalle Jonsson for their valuable advice concerning the isolation of CA. This work was supported by the Swedish Natural Research Council and Seth M. Kempes Memorial foundation. 相似文献
12.
13.
The Staphylococcus aureus ileS gene, encoding isoleucyl-tRNA synthetase, is a member of the T-box family.
下载免费PDF全文
![点击此处可从《Journal of bacteriology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
F J Grundy M T Haldeman G M Hornblow J M Ward A F Chalker T M Henkin 《Journal of bacteriology》1997,179(11):3767-3772
The Staphylococcus aureus ileS gene, encoding isoleucyl-tRNA synthetase (IleRS), contains a long mRNA leader region. This region exhibits many of the features of the gram-positive synthetase gene family, including the T box and leader region terminator and antiterminator. The terminator was shown to be functional in vivo, and readthrough increased during growth in the presence of mupirocin, an inhibitor of IleRS activity. The S. aureus ileS leader structure includes several critical differences from the other members of the T-box family, suggesting that regulation of this gene in S. aureus may exhibit unique features. 相似文献
14.
A family of modular type mannuronan C-5-epimerase genes controls alginate structure in Azotobacter vinelandii 总被引:3,自引:1,他引:2
Helga Ertesvåg Hilde Kristin Høidal Ingrid Kathrin Hals Anne Rian Berit Doseth Svein Valla 《Molecular microbiology》1995,16(4):719-731
The ToxR protein is a transmembrane protein that regulates the expression of several virulence factors of Vibrio cholerae. Previous analysis of fusion proteins between ToxR and alkaline phosphatase (ToxR-PhoA) suggested that ToxR was active as a dimer. In order to determine whether dimerization of the ToxR periplasmic domain was essential for activity, this domain was replaced by monomeric and dimeric protein domains. Surprisingly, PhoA (dimeric), β-lactamase (monomeric, ToxR–Bla), or the leucine zipper of GCN4 (dimeric, ToxR-GCN4-M) could substitute functionally for the ToxR periplasmic domain. ToxR-GCN4 fusion proteins, in which the ToxR trans-membrane domain was eliminated (ToxR-GCN4-C), were inactive, but an additional fusion protein that contained a heterologous membrane-spanning domain retained activity. Strains containing each of these ToxR fusion proteins were analysed for in vivo colonization properties and response to in vitro growth conditions that are known to affect expression of the ToxR regulon. Strains containing ToxR-GCN4-M and ToxR-Bla responded like wild-type strains to in vitro growth conditions. In the infant-mouse colonization model, strains containing ToxR fusion proteins were all deficient in colonization relative to strains containing wild-type ToxR, and strains containing monomeric ToxR-Bla were most severely outcompeted. These results suggest that, under in vitro conditions, ToxR does not require a dimerized periplasmic domain, but that, under in vivo conditions, the correct conformation of the ToxR periplasmic domain may be more important for function. 相似文献
15.
Experimental support for the use of fluid aqueous organic solvent systems and subzero temperatures in mechanistic studies of -galactosidase is presented. The enzyme was stable and retained catalytic activity and structural integrity in 50% aqueous dimethyl sulfoxide and 60% aqueous methanol at 0°C; at lower temperatures higher concentrations of cosolvent may be successfully used. The effects of dimethyl sulfoxide on the catalytic and structural properties of the enzyme were investigated in detail. For the -galactoside-catalyzed h ydrolysis ofo-nitrophenyl--D-galactoside the value ofk
cat decreased in a linear manner with increasing cosolvent concentration, whereasK
m increased exponentially. The decrease ink
cat paralleled the decrease in water concentration, consistent with rate-limiting hydrolysis of a galactosylenzyme intermediate. The increase inK
m is attributed to less favorable partitioning of the substrate to the active site in the cryosolvent compared to aqueous solution. ThepH*-rate profile for this reaction at 0°C in 50% dimethyl sulfoxide was similar to that in aqueous solution, withpK*1=5.8 andpK*2=8.0. Linear Arrhenius plots, with energies of activation of 13.9 and 16.0 kcal mol–1, respectively, were obtained for the -galactosidase-catalyzed hydrolysis ofo-nitrophenyl- andp-nitrophenyl--D-galactosides in 50% dimethyl sulfoxide at temperatures to –57°C. Examination of the intrinsic fluorescence and ultraviolet spectra of the enzyme as a function of increasing cosolvent concentration showed no evidence for structural perturbation up to and including 50% dimethyl sulfoxide at 0°C. We conclude that these cryosolvent systems are suitable for mechanistic investigations of -galactosidase, in particular for trapping intermediates at subzero temperatures. 相似文献
16.
Twelve of 21 human, hemolytic, fecal isolates ofEscherichia coli produced type 1 hemolysin (HLY1), an extracellular, heat-labile molecule (alpha-hemolysin). Although no common plasmid species was apparent, 11 of 12 HLY1 strains possessed a plasmid60 megadaltons (Mdal); 5 of 9 strains with other hemolysins possessed a plasmid of comparable molecular mass (Fisher's exact probability=0.0805). One derivative of an HLY1+strain, which contained a 125 Mdal plasmid, no longer expressed HLY1 and contained a single 102 Mdal plasmid. The presence of large plasmids of varying size and an apparent deletion mutation in HLY1 strains suggest that HLY1 determinants are located on a small, unstable genetic element. In an initial survey of 224 human fecal isolates ofE. coli, the predominant hemolytic serotype was 06:H-, and conversely most (85%) 06:H-isolates were HLY1+. Serotype appears to play an important role in HLY1 expression. 相似文献
17.
18.
19.
Erika C. Freeman Irena F. Creed Blake Jones Ann‐Kristin Bergstrm 《Global Change Biology》2020,26(9):4966-4987
The interacting effects of global changes—including increased temperature, altered precipitation, reduced acidification and increased dissolved organic matter loads to lakes—are anticipated to create favourable environmental conditions for cyanobacteria in northern lakes. However, responses of cyanobacteria to these global changes are complex, if not contradictory. We hypothesized that absolute and relative biovolumes of cyanobacteria (both total and specific genera) are increasing in Swedish nutrient‐poor lakes and that these increases are associated with global changes. We tested these hypotheses using data from 28 nutrient‐poor Swedish lakes over 16 years (1998–2013). Increases in cyanobacteria relative biovolume were identified in 21% of the study sites, primarily in the southeastern region of Sweden, and were composed mostly of increases from three specific genera: Merismopedia, Chroococcus and Dolichospermum. Taxon‐specific changes were related to different environmental stressors; that is, increased surface water temperature favoured higher Merismopedia relative biovolume in low pH lakes with high nitrogen to phosphorus ratios, whereas acidification recovery was statistically related to increased relative biovolumes of Chroococcus and Dolichospermum. In addition, enhanced dissolved organic matter loads were identified as potential determinants of Chroococcus suppression and Dolichospermum promotion. Our findings highlight that specific genera of cyanobacteria benefit from different environmental changes. Our ability to predict the risk of cyanobacteria prevalence requires consideration of the environmental condition of a lake and the sensitivities of the cyanobacteria genera within the lake. Regional patterns may emerge due to spatial autocorrelations within and among lake history, rates and direction of environmental change and the niche space occupied by specific cyanobacteria. 相似文献
20.
Josep Lloret Alba Serrat Gudmundur Thordarson Kristin Helle Angelique Jadaud Isabel Bruno Francesc Ordines Paolo Sartor Pierluigi Carbonara Hans-Joachim Rätz 《Journal of fish biology》2021,98(6):1572-1584
Many deep-water fish populations, being K-selected species, have little resilience to overexploitation and may be at serious risk of depletion as a consequence. Sea warming represents an additional threat. In this study, the condition, or health, of several populations of common ling (Molva molva), blue ling (Molva dypterygia) and Mediterranean or Spanish ling (Molva macrophthalma) inhabiting different areas in the North Atlantic and the Mediterranean was evaluated, to shed light on the challenges these deep-water species are facing in the context of fishing activity and a warming climate. The data on the condition of Molva populations which are analysed here have been complemented with data on abundance and, for the southernmost species (Mediterranean ling), with two other health indicators (parasitism and hepato-somatic index). Despite some exceptions (e.g., common ling in Icelandic waters), this study shows that the condition of many populations of Molva species in the northeastern Atlantic and the Mediterranean Sea has worsened, a trend which, in recent decades, has usually been found to be accompanied by a decline in their abundance. In addition, the poor health status of most populations of common ling, blue ling and Mediterranean ling considered in this analysis points to a lower sustainability of these populations in the future. Overall, the health status and abundance of Molva populations in the northeastern Atlantic and the Mediterranean suggest that only some populations located in the North Atlantic may be able to rebuild, whereas the populations in southern North Atlantic and the Mediterranean, which are probably most at risk from sea warming, are facing serious difficulties in doing so. In the context of fisheries and global warming, this study's results strongly indicate that management bodies need to consider the health status of many of the populations of Molva species, particularly in southern European waters, before implementing their decisions. 相似文献