全文获取类型
收费全文 | 828篇 |
免费 | 47篇 |
国内免费 | 1篇 |
出版年
2023年 | 3篇 |
2022年 | 11篇 |
2021年 | 11篇 |
2020年 | 9篇 |
2019年 | 6篇 |
2018年 | 11篇 |
2017年 | 15篇 |
2016年 | 26篇 |
2015年 | 36篇 |
2014年 | 42篇 |
2013年 | 46篇 |
2012年 | 60篇 |
2011年 | 60篇 |
2010年 | 38篇 |
2009年 | 44篇 |
2008年 | 42篇 |
2007年 | 60篇 |
2006年 | 63篇 |
2005年 | 50篇 |
2004年 | 38篇 |
2003年 | 36篇 |
2002年 | 51篇 |
2001年 | 11篇 |
2000年 | 6篇 |
1999年 | 8篇 |
1998年 | 10篇 |
1997年 | 15篇 |
1996年 | 6篇 |
1995年 | 6篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 9篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1980年 | 3篇 |
1978年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有876条查询结果,搜索用时 859 毫秒
131.
Amanda J. Cork Slobodan Jergic Sven Hammerschmidt Bostjan Kobe Vijay Pancholi Justin L. P. Benesch Carol V. Robinson Nicholas E. Dixon J. Andrew Aquilina Mark J. Walker 《The Journal of biological chemistry》2009,284(25):17129-17137
The flesh-eating bacterium group A Streptococcus (GAS) binds and activates human plasminogen, promoting invasive disease. Streptococcal surface enolase (SEN), a glycolytic pathway enzyme, is an identified plasminogen receptor of GAS. Here we used mass spectrometry (MS) to confirm that GAS SEN is octameric, thereby validating in silico modeling based on the crystal structure of Streptococcus pneumoniae α-enolase. Site-directed mutagenesis of surface-located lysine residues (SENK252 + 255A, SENK304A, SENK334A, SENK344E, SENK435L, and SENΔ434–435) was used to examine their roles in maintaining structural integrity, enzymatic function, and plasminogen binding. Structural integrity of the GAS SEN octamer was retained for all mutants except SENK344E, as determined by circular dichroism spectroscopy and MS. However, ion mobility MS revealed distinct differences in the stability of several mutant octamers in comparison with wild type. Enzymatic analysis indicated that SENK344E had lost α-enolase activity, which was also reduced in SENK334A and SENΔ434–435. Surface plasmon resonance demonstrated that the capacity to bind human plasminogen was abolished in SENK252 + 255A, SENK435L, and SENΔ434–435. The lysine residues at positions 252, 255, 434, and 435 therefore play a concerted role in plasminogen acquisition. This study demonstrates the ability of combining in silico structural modeling with ion mobility-MS validation for undertaking functional studies on complex protein structures.Streptococcus pyogenes (group A Streptococcus, GAS)8 is a common bacterial pathogen, causing over 700 million human disease episodes each year (1). These range from serious life-threatening invasive diseases including necrotizing fasciitis and streptococcal toxic shock-like syndrome to non-invasive infections like pharyngitis and pyoderma. Invasive disease, in combination with postinfection immune sequelae including rheumatic heart disease and acute poststreptococcal glomerulonephritis, account for over half a million deaths each year (1). Although a resurgence of GAS invasive infections has occurred in western countries since the mid-1980s, disease burden is much greater in developing countries and indigenous populations of developed nations, where GAS infections are endemic (2–4).GAS is able to bind human plasminogen and activate the captured zymogen to the serine protease plasmin (5–17). The capacity of GAS to do this plays a critical role in virulence and invasive disease initiation (3, 17–19). The plasminogen activation system in humans is an important and highly regulated process that is responsible for breakdown of extracellular matrix components, dissolution of blood clots, and cell migration (20, 21). Plasminogen is a 92-kDa zymogen that circulates in human plasma at a concentration of 2 μm (22). It consists of a binding region of five homologous triple loop kringle domains and an N-terminal serine protease domain that flank the Arg561–Val562 site (23), where it is cleaved by tissue plasminogen activator and urokinase plasminogen activator to yield the active protease plasmin (20, 23). GAS also has the ability to activate human plasminogen by secreting the virulence determinant streptokinase. Streptokinase forms stable complexes with plasminogen or plasmin, both of which exhibit plasmin activity (20, 24). Activation of plasminogen by the plasmin(ogen)-streptokinase complex circumvents regulation by the host plasminogen activation inhibitors, α2-antiplasmin and α2-macroglobulin (11, 20). GAS can bind the plasmin(ogen)-streptokinase complex and/or plasmin(ogen) directly via plasmin(ogen) receptors at the bacterial cell surface (6). These receptors include the plasminogen-binding group A streptococcal M-like protein (PAM) (25), the PAM-related protein (19), glyceraldehyde-3-phosphate dehydrogenase (GAPDH; also known as streptococcal plasmin receptor, Plr, or streptococcal surface dehydrogenase) (9, 26), and streptococcal surface enolase (SEN or α-enolase) (27). Interactions with these GAS receptors occurs via lysine-binding sites within the kringle domains of plasminogen (6).In addition to its ability to bind human plasminogen, SEN is primarily the glycolytic enzyme that converts 2-phosphoglycerate to phosphoenolpyruvate (27–29). SEN is abundantly expressed in the cytosol of most bacterial species but has also been identified as a surface-located protein in GAS and other bacteria including pneumococci, despite lacking classical cell surface protein motifs such as a signal sequence, membrane-spanning domain, or cell-wall anchor motif (27, 28, 30, 31). The interaction between SEN and plasminogen is reported to be facilitated by the two C-terminal lysine residues at positions 434 and 435 (27, 32). In contrast, an internal binding motif containing lysines at positions 252 and 255 in the closely related α-enolase of Streptococcus pneumoniae has been shown to play a pivotal role in the acquisition of plasminogen in this bacterial species (33). The octameric pneumococcal α-enolase structure consists of a tetramer of dimers. Hence, potential binding sites could be buried in the interface between subunits. In fact, the crystal structure of S. pneumoniae α-enolase revealed that the two C-terminal lysine residues are significantly less exposed than the internal plasminogen-binding motif (34).In this study, we constructed an in silico model of GAS SEN, based on the pneumococcal octameric α-enolase crystal structure, and validated this model using ion mobility (IM) mass spectrometry (MS). Site-directed mutagenesis followed by structural and functional analyses revealed that Lys344 plays a crucial role in structural integrity and enzymatic function. Furthermore, we demonstrate that the plasminogen-binding motif residues Lys252 and Lys255 and the C-terminal Lys434 and Lys435 residues are located adjacently in the GAS SEN structure and play a concerted role in the binding of human plasminogen. 相似文献
132.
Paavo Ahvenniemi Matthias Wolf Mari J. Lehtonen Paula Wilson Malgorzata German-Kinnari Jari P. T. Valkonen 《Journal of molecular evolution》2009,69(2):150-163
The rRNA cistron (18S–ITS1–5.8S–ITS2–28S) is used widely for phylogenetic analyses. Recent studies show that compensatory
base changes (CBC) in the secondary structure of ITS2 correlate with genetic incompatibility between organisms. Rhizoctonia solani consists of genetically incompatible strain groups (anastomosis groups, AG) distinguished by lack of anastomosis between
hyphae of strains. Phylogenetic analysis of internal transcribed spacer (ITS) sequences shows a strong correlation with AG
determination. In this study, ITS sequences were reannotated according to the flanking 5.8S and 28S regions which interact
during ribogenesis. One or two CBCs were detected between the ITS2 secondary structure of AG-3 potato strains as compared
to AG-3 tobacco strains, and between these two strains and all other AGs. When a binucleate Rhizoctonia species related to Ceratobasidiaceae was compared to the AGs of R. solani, which were multinucleate (3–21 nuclei per cell), 1–3 CBCs were detected. The CBCs in potato strains of AG-3 distinguish
them from AG-3 tobacco strains and other AGs yielding further evidence that the potato strains of AG-3 originally described
as R. solani are a species distinct from other AGs. The ITS1–5.8S–ITS2 sequences were analyzed by direct sequencing of PCR products from
497 strains of AG-3 isolated from potato. The same 10 and 4 positions in ITS1 and ITS2, respectively, contained variability
in 425 strains (86%). Nine different unambiguous ITS sequences (haplotypes) could be detected in a single strain by sequencing
cloned PCR products indicating that concerted evolution had not homogenized the rRNA cistrons in many AG-3 strains. Importantly,
the sequence variability did not affect the secondary structure of ITS2 and CBCs in AG-3.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
133.
Dorothe Kopp Julien Cucherousset Jari Syvranta Aurlia Martino Rgis Crghino Frdric Santoul 《Comptes rendus biologies》2009,332(8):741-746
During the last decades, non-native predatory fish species have been largely introduced in European lakes and rivers, calling for detailed information on the trophic ecology of co-existing native and non-native predators. The present study describes the trophic ecology of the introduced pikeperch (Sander lucioperca) in two southwestern French rivers, using stable isotope analysis. Pikeperch could be categorized as a top-predator, and had a significantly higher trophic position (TP, mean±SE=4.2±0.1) compared to other predatory fish such as the native pike (Esox lucius, TP=3.7±0.1) and the introduced European catfish (Silurus glanis, TP=3.8±0.1). Most studies of resource use in freshwaters consider predatory fish as ecologically equivalent; however, this study showed that the pikeperch occupied a higher trophic niche compared to other predatory species in the Lot and Tarn rivers (Garonne River basin). This apparent specialization may thus have consequences upon interspecific relationships within the predatory guild and upon the functional organization of biological communities. To cite this article: D. Kopp et al., C. R. Biologies 332 (2009). 相似文献
134.
Jari E. Heikkil? Markus J. V. V?h?-Koskela Janne J. Ruotsalainen Miika W. Martikainen Marianne M. Stanford J. Andrea McCart John C. Bell Ari E. Hinkkanen 《PloS one》2010,5(1)
Background
VA7 is a neurotropic alphavirus vector based on an attenuated strain of Semliki Forest virus. We have previously shown that VA7 exhibits oncolytic activity against human melanoma xenografts in immunodeficient mice. The purpose of this study was to determine if intravenously administered VA7 would be effective against human glioma.Methodology/Principal Findings
In vitro, U87, U251, and A172 human glioma cells were infected and killed by VA7-EGFP. In vivo, antiglioma activity of VA7 was tested in Balb/c nude mice using U87 cells stably expressing firefly luciferase in subcutaneous and orthotopic tumor models. Intravenously administered VA7-EGFP completely eradicated 100% of small and 50% of large subcutaneous U87Fluc tumors. A single intravenous injection of either VA7-EGFP or VA7 expressing Renilla luciferase (VA7-Rluc) into mice bearing orthotopic U87Fluc tumors caused a complete quenching of intracranial firefly bioluminescence and long-term survival in total 16 of 17 animals. In tumor-bearing mice injected with VA7-Rluc, transient intracranial and peripheral Renilla bioluminescence was observed. Virus was well tolerated and no damage to heart, liver, spleen, or brain was observed upon pathological assessment at three and ninety days post injection, despite detectable virus titers in these organs during the earlier time point.Conclusion
VA7 vector is apathogenic and can enter and destroy brain tumors in nude mice when administered systemically. This study warrants further elucidation of the mechanism of tumor destruction and attenuation of the VA7 virus. 相似文献135.
Host‐associated divergence in the activity of digestive enzymes in two populations of the gypsy moth Lymantria dispar (Lepidoptera: Erebidae) 下载免费PDF全文
Jelica Lazarević Milena Janković‐Tomanić Uroš Savković Mirko Đorđević Slobodan Milanović Biljana Stojković 《Entomological Science》2017,20(1):189-194
The gypsy moth is a generalist insect pest with an extremely wide host range. Adaptive responses of digestive enzymes are important for the successful utilization of plant hosts that differ in the contents and ratios of constituent nutrients and allelochemicals. In the present study, we examined the responses of α‐amylase, trypsin, and leucine aminopeptidase to two tree hosts (suitable oak, Quercus cerris, and unsuitable locust tree, Robinia pseudoacacia) in the fourth, fifth, and sixth instars of gypsy moth larvae originating from oak and locust tree forest populations (hereafter assigned as Quercus and Robinia populations, respectively). Gypsy moths from the Robinia forest had been adapting to this unsuitable host for more than 40 generations. To test for population‐level host plant specialization, we applied a two‐population × two‐host experimental design. We compared the levels, developmental patterns, and plasticities of the activities of enzymes. The locust tree diet increased enzyme activity in the fourth instar and reduced activity in advanced instars of the Quercus larvae in comparison to the oak diet. These larvae also exhibited opposite developmental trajectories on the two hosts, i.e. activity increased on the oak diet and decreased on the locust tree diet with the progress of instar. Larvae of the Robinia population were characterized by reduced plasticity of enzyme activity and its developmental trajectories. In addition, elevated trypsin activity in response to an unsuitable host was observed in all instar larvae of the Robinia population, which demonstrated that Robinia larvae had an improved digestive performance than did Quercus larvae. 相似文献
136.
Mayer L Bacić-Vrca V Sulentić P Sisić I Marić-Miholić V Romovski S Ljubić D 《Collegium antropologicum》2011,35(1):167-172
The aim of the study was to assess correlation of atherosclerosis severity as determined by two different methods of screening for atherosclerosis: (A) measurement of the cardio-ankle vascular index-CAVI by use of the VaseraVS-1500 vascular screening device, and (B) Framingham scale scoring. 52 subjects (28 male and 24 female) were enrolled in the study. Classification of study subjects into four quartiles based on theoretically calculated 10-year risk according to Framingham scale (medians: 1%, 3%, 4% and 15%) confirmed the risk increase to be associated with a statistically significant increase in CAVI, age and total cholesterol, and a statistically significant decrease in HDL-cholesterol (p < 0.001 all). Spearman correlation coefficients showed a statistically significant correlation of 10-year risk with CAVI (p = 0.0242; r = 0.4494). Study results suggested that simultaneous determination of CAVI and 10-year risk might prove justified. They are not contradictory, the more so, these two parameters showed a significant positive correlation. This test panel yields comprehensible, implying all the possible consequences and highly motivating information that may stimulate the person for lifestyle modification. 相似文献
137.
Plant viruses use sieve elements in phloem as the route of long-distance movement and systemic infection in plants. Plants, in turn, deploy RNA silencing, R-gene mediated defence and other mechanisms to prevent phloem transport of viruses. Cell-to-cell movement of viruses from an initially infected leaf to stem and other parts of the plant could be another possibility for systemic invasion, but it is considered to be too slow. This idea is supported by observations made on viruses that are deficient in phloem loading. The leaf abscission zone forming at the base of the petiole may constitute a barrier that prevents viral cell-to-cell movement. The abscission zone and protective layer are difficult to localize in the petiole until the leaf reaches an advanced stage of senescence. Viruses tagged with the green fluorescent protein are helpful for localization and study of the developing abscission zone. 相似文献
138.
Tanner NA Tolun G Loparo JJ Jergic S Griffith JD Dixon NE van Oijen AM 《The EMBO journal》2011,30(9):1830-1840
During DNA replication, repetitive synthesis of discrete Okazaki fragments requires mechanisms that guarantee DNA polymerase, clamp, and primase proteins are present for every cycle. In Escherichia coli, this process proceeds through transfer of the lagging-strand polymerase from the β sliding clamp left at a completed Okazaki fragment to a clamp assembled on a new RNA primer. These lagging-strand clamps are thought to be bound by the replisome from solution and loaded a new for every fragment. Here, we discuss a surprising, alternative lagging-strand synthesis mechanism: efficient replication in the absence of any clamps other than those assembled with the replisome. Using single-molecule experiments, we show that replication complexes pre-assembled on DNA support synthesis of multiple Okazaki fragments in the absence of excess β clamps. The processivity of these replisomes, but not the number of synthesized Okazaki fragments, is dependent on the frequency of RNA-primer synthesis. These results broaden our understanding of lagging-strand synthesis and emphasize the stability of the replisome to continue synthesis without new clamps. 相似文献
139.
Botuyan MV Nominé Y Yu X Juranic N Macura S Chen J Mer G 《Structure (London, England : 1993)》2004,12(7):1137-1146
BRCT tandem domains, found in many proteins involved in DNA damage checkpoint and DNA repair pathways, were recently shown to be phosphopeptide binding motifs. Using solution nuclear magnetic resonance (NMR) spectroscopy and mutational analysis, we have characterized the interaction of BRCA1-BRCT domains with a phosphoserine-containing peptide derived from the DNA repair helicase BACH1. We show that a phenylalanine in the +3 position from the phosphoserine of BACH1 is bound to a conserved hydrophobic pocket formed between the two BRCT domains and that recognition of the phosphate group is mediated by lysine and serine side chains from the amino-terminal BRCT domain. Mutations that prevent phosphopeptide binding abolish BRCA1 function in DNA damage-induced checkpoint control. Our NMR data also reveal a dynamic interaction between BRCA1-BRCT and BACH1, where the bound phosphopeptide exists as an equilibrium of two conformations and where BRCA1-BRCT undergoes a transition to a more rigid conformation upon peptide binding. 相似文献
140.
Cellularization of the early Drosophila embryo is a modified form of cytokinesis that gives rise to the blastoderm epithelium through polarized membrane growth. The gene slow-as-molasses encodes a novel protein essential for the formation of a plasma membrane domain that initiates membrane growth during cellularization. 相似文献