全文获取类型
收费全文 | 3892篇 |
免费 | 330篇 |
国内免费 | 1篇 |
专业分类
4223篇 |
出版年
2021年 | 39篇 |
2020年 | 21篇 |
2019年 | 30篇 |
2018年 | 34篇 |
2017年 | 46篇 |
2016年 | 70篇 |
2015年 | 117篇 |
2014年 | 148篇 |
2013年 | 184篇 |
2012年 | 204篇 |
2011年 | 195篇 |
2010年 | 150篇 |
2009年 | 149篇 |
2008年 | 209篇 |
2007年 | 227篇 |
2006年 | 208篇 |
2005年 | 211篇 |
2004年 | 227篇 |
2003年 | 221篇 |
2002年 | 239篇 |
2001年 | 57篇 |
2000年 | 41篇 |
1999年 | 48篇 |
1998年 | 82篇 |
1997年 | 58篇 |
1996年 | 47篇 |
1995年 | 67篇 |
1994年 | 50篇 |
1993年 | 52篇 |
1992年 | 50篇 |
1991年 | 47篇 |
1990年 | 27篇 |
1989年 | 48篇 |
1988年 | 35篇 |
1987年 | 33篇 |
1986年 | 23篇 |
1985年 | 43篇 |
1984年 | 44篇 |
1983年 | 25篇 |
1982年 | 38篇 |
1981年 | 35篇 |
1980年 | 19篇 |
1978年 | 20篇 |
1977年 | 20篇 |
1976年 | 33篇 |
1975年 | 28篇 |
1974年 | 23篇 |
1973年 | 20篇 |
1971年 | 21篇 |
1968年 | 19篇 |
排序方式: 共有4223条查询结果,搜索用时 15 毫秒
81.
A Bacillus megaterium Plasmid System for the Production, Export, and One-Step Purification of Affinity-Tagged Heterologous Levansucrase from Growth Medium 总被引:1,自引:0,他引:1 下载免费PDF全文
Marco Malten Rebekka Biedendieck Martin Gamer Ann-Christin Drews Simon Stammen Klaus Buchholz Lubbert Dijkhuizen Dieter Jahn 《Applied microbiology》2006,72(2):1677-1679
A multiple vector system for the production and export of recombinant affinity-tagged proteins in Bacillus megaterium was developed. Up to 1 mg/liter of a His6-tagged or Strep-tagged Lactobacillus reuteri levansucrase was directed into the growth medium, using the B. megaterium esterase LipA signal peptide, and recovered by one-step affinity chromatography. 相似文献
82.
Monomethylamine methyltransferase of the archaebacterium Methanosarcina barkeri contains a novel amino acid, pyrrolysine, encoded by the termination codon UAG. Initial studies suggest that pyrrolysine may be co-translationally inserted during protein synthesis, probably by a mechanism analogous to that operating during selenocysteine incorporation. 相似文献
83.
84.
Tumor angiogenesis plays an important role in tumor growth, aggression and metastasis. Many molecules have been demonstrated as positive regulators of angiogenesis, including vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), epidermal growth factor (EGF), and others. In recent years, significant progress has been made in the research on anti-angiogenic strategies for tumor therapies. In this review, anti-angiogenic active immunotherapies for tumors based on vaccination with xenogeneic homologous molecules and non-xenogeneic homologous molecules are discussed. 相似文献
85.
Subsequent addition of 1,2-benzenedithiol (S2-H2) and nBuLi to a solution of [Ru(NO)Cl3 · xMeOH] in THF afforded exclusively the monomeric species NBu4[RuII(NO)(S2)2] (1). Formation of dimeric (NBu4)2[RuII(NO)(S2)2]2 (2) has been confirmed when the deprotonated ligand S2-Li2 was added to [Ru(NO)Cl3 · xMeOH] and allowed to stir for 30 h. The monomer 1 undergoes aerial oxidation to give (NBu4)2[RuIV(S2)3] (3). The reaction between RuCl3 · xH2O and S2-H2 in the presence of NaOMe, afforded the dinulear RuIII species (NMe4)2[RuIII(S2)2]2 (4). A modified method for the preparation of 1 is being employed to synthesize the osmium analogue NBu4[Os(NO)(S2)2] (5) effectively. The solid state structures of 1, 2 and 3 were determined by X-ray crystal structure analysis. A comparison of relevant bond distance data suggests that 1,2-benzenedithiolate acts as an “innocent” ligand. 相似文献
86.
In Pseudomonas aeruginosa, the CbrA/CbrB two-component system is instrumental in the maintenance of the carbon-nitrogen balance and for growth on carbon sources that are energetically less favorable than the preferred dicarboxylate substrates. The CbrA/CbrB system drives the expression of the small RNA CrcZ, which antagonizes the repressing effects of the catabolite repression control protein Crc, an RNA-binding protein. Dicarboxylates appear to cause carbon catabolite repression by inhibiting the activity of the CbrA/CbrB system, resulting in reduced crcZ expression. Here we have identified a conserved palindromic nucleotide sequence that is present in upstream activating sequences (UASs) of promoters under positive control by CbrB and σ(54) RNA polymerase, especially in the UAS of the crcZ promoter. Evidence for recognition of this palindromic sequence by CbrB was obtained in vivo from mutational analysis of the crcZ promoter and in vitro from electrophoretic mobility shift assays using crcZ promoter fragments and purified CbrB protein truncated at the N terminus. Integration host factor (IHF) was required for crcZ expression. CbrB also activated the lipA (lipase) promoter, albeit less effectively, apparently by interacting with a similar but less conserved palindromic sequence in the UAS of lipA. As expected, succinate caused CbrB-dependent catabolite repression of the lipA promoter. Based on these results and previously published data, a consensus CbrB recognition sequence is proposed. This sequence has similarity to the consensus NtrC recognition sequence, which is relevant for nitrogen control. 相似文献
87.
Katharina Zacher Michael Y. Roleda Angela Wulff Dieter Hanelt Christian Wiencke 《Phycological Research》2009,57(3):186-193
In Antarctica ozone depletion is highest during spring, coinciding with the reproduction of many seaweed species. Propagules are the life-stage of an alga most susceptible to environmental perturbations. Therefore, fertile thalli of Iridaea cordata (Turner) Bory (Rhodophyta) were collected in the eulittoral of King George Island (Antarctica) to examine spore susceptibility to ultraviolet radiation (UVR). In the laboratory, freshly released tetraspores were exposed to photosynthetically active radiation (PAR) (400–700 nm), PAR+UV-A (320–700 nm) or PAR+UV-A+UV-B (280–700 nm). Photosynthetic efficiency was measured during 1–8 h of exposure and after 48 h of recovery. Additionally, mycosporine-like amino acids (MAAs) and DNA damage were determined. Saturating irradiance of photosynthesis of freshly released tetraspores was 57 µmol photons m−2 s−1 . Exposure to increasing fluence of PAR reduced photosynthetic efficiency. UVR further decreased the photosynthetic efficiencies of the tetraspores but spores were able to recover completely after UVR exposure and 2 days post-cultivation under low PAR. DNA damage was minimal and lesions were effectively repaired under photoreactivating light. Concentrations of the MAAs shinorine and palythine were higher in tetraspores treated with UVR than in spores only exposed to PAR. Generally, the tetraspores show a good UV tolerance. This flexible response of the tetraspores of this species to changing radiation conditions enables the alga to grow along a considerable depth gradient from the sublittoral to the eulittoral where they can be exposed to enhanced UVBR under conditions of stratospheric ozone depletion. 相似文献
88.
Dieter C. Wasshausen 《Brittonia》2002,54(4):286-297
Four new species,Justicia oldemanii, J. mcdowellii, J. wilhelminensis, andJ. prevostiae are described, illustrated, and compared with their closest relatives. A key to species ofJusticia from the Guianas is also provided. 相似文献
89.
Saturation of the cell's protein folding capacity and accumulation of inactive incompletely folded protein often accompanying the overexpression of membrane proteins (MPs) presents an obstacle to their efficient purification in a functional form for structural studies. We present a novel strategy for optimization of functional MP expression in Saccharomyces cerevisiae. This approach exploits the unfolded protein response (UPR) pathway, a stress signaling mechanism that senses the accumulation of unfolded proteins in the endoplasmic reticulum. We demonstrate that a high level of UPR induction upon expression of a MP reflects impaired functional expression of that protein. Tuning the expression level of the protein so as to avoid or minimize UPR induction results in its increased functional expression. UPR status can therefore serve as a proxy variable for the extent of impaired expression of a MP that may even be applicable in the absence of knowledge of the protein's biological function. 相似文献
90.
Unterreitmeier S Fuchs A Schäffler T Heym RG Frishman D Langosch D 《Journal of molecular biology》2007,374(3):705-718
Interactions of transmembrane helices play a crucial role in the folding and oligomerisation of integral membrane proteins. In order to uncover novel sequence motifs mediating these interactions, we randomised one face of a transmembrane helix with a set of non-polar or moderately polar amino acids. Those sequences capable of self-interaction upon integration into bacterial inner membranes were selected by means of the ToxR/POSSYCCAT system. A comparison between low/medium-affinity and high-affinity sequences reveals that high-affinity sequences are strongly enriched in phenylalanine residues that are frequently observed at the − 3 position of GxxxG motifs, thus yielding FxxGxxxG motifs. Mutation of Phe or GxxxG in selected sequences significantly reduces self-interaction of the transmembrane domains without affecting their efficiency of membrane integration. Conversely, grafting FxxGxxxG onto unrelated transmembrane domains strongly enhances their interaction. Further, we find that FxxGxxxG is significantly over-represented in transmembrane domains of bitopic membrane proteins. The same motif contributes to self-interaction of the vesicular stomatitis virus G protein transmembrane domain. We conclude that Phe stabilises membrane-spanning GxxxG motifs. This is one example of how the role of certain side-chains in helix-helix interfaces is modulated by sequence context. 相似文献