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   402篇
  免费   25篇
  2023年   1篇
  2022年   9篇
  2021年   9篇
  2020年   9篇
  2019年   2篇
  2018年   15篇
  2017年   9篇
  2016年   21篇
  2015年   9篇
  2014年   27篇
  2013年   46篇
  2012年   25篇
  2011年   47篇
  2010年   21篇
  2009年   21篇
  2008年   27篇
  2007年   37篇
  2006年   22篇
  2005年   19篇
  2004年   16篇
  2003年   14篇
  2002年   7篇
  2001年   3篇
  2000年   2篇
  1999年   3篇
  1998年   1篇
  1997年   1篇
  1996年   1篇
  1995年   1篇
  1994年   1篇
  1990年   1篇
排序方式: 共有427条查询结果,搜索用时 17 毫秒
181.
Innate immune system is a primary line of defense in fish that protects it from the invading pathogens. Antimicrobial peptides (AMPs) are widely distributed in nature and are essential components of innate immunity. These molecules enable the host’s innate immune system to fight against a variety of infectious agents. One such AMP, hepcidin, is a cysteine rich amphipathic peptide. We have amplified, cloned and characterized hepcidin like AMP from Schizothorax richardsonii that inhabits one of the most difficult aquatic ecosystems in the Indian Himalayas. The cDNA encoding hepcidin like peptide was amplified as a 371 bp fragment with an open reading frame (ORF) of 279 nucleotides flanked by 5′ and 3′ UTRs of 70 and 22 bases respectively. This ORF encodes a peptide of 93 amino acids with a signal peptide of 24 amino acids and a mature peptide of 25 amino acids. The mature hepcidin like peptide of S. richardsonii has eight cystine residues that participate in the formation of four disulfide bonds, a unique feature of hepcidin like AMPs. A 3D model of hepcidin like mature peptide was generated using Modeller 9.10 which was validated using PROCHECK and ERRAT. Phylogenetic analysis of hepcidin like AMP from S. richardsonii revealed that it was closely related to hepcidin from olive barb (Puntius sarana).  相似文献   
182.
L-asparaginase (L-ASP) is a therapeutic enzyme used clinically for the treatment of childhood acute lymphoblastic leukemia. L-ASP’s anticancer activity is believed to be associated primarily with depletion of asparagine, but secondary glutaminase activity has also been implicated in its anticancer mechanism of action. To investigate the effects of L-ASP on amino acid metabolism, we have developed an LC–MS/MS metabolomics platform for high-throughput quantitation of 29 metabolites, including all 20 proteinogenic amino acids, 6 metabolically related amino acid derivatives (ornithine, citrulline, sarcosine, taurine, hypotaurine, and cystine), and 3 polyamines (putrescince, spermidine, and spermine) in adherent cultured cells. When we examined the response of OVCAR-8 ovarian cancer cells in culture to L-ASP, asparagine was depleted from the medium within seconds. Interestingly, intracellular asparagine was also depleted rapidly, and the mechanism was suggested to involve rapid export of intracellular asparagine followed by rapid conversion to aspartic acid by L-ASP. We also found that L-ASP-induced cell death was more closely associated with glutamine concentration than with asparagine concentration. Time-course analysis revealed the dynamics of amino acid metabolism after feeding cells with fresh medium. Overall, this study provides new insight into L-ASP’s mechanism of action, and the optimized analytical method can be extended, with only slight modification, to other metabolically active amino acids, related compounds, and a range of cultured cell types.  相似文献   
183.
Little is known about the regulation of eicosanoid synthesis proximal to the activation of cytosolic phospholipase A2α (cPLA2α), the initial rate-limiting step. The current view is that cPLA2α associates with intracellular/phosphatidylcholine-rich membranes strictly via hydrophobic interactions in response to an increase of intracellular calcium. In opposition to this accepted mechanism of two decades, ceramide 1-phosphate (C1P) has been shown to increase the membrane association of cPLA2α in vitro via a novel site in the cationic β-groove of the C2 domain (Stahelin, R. V., Subramanian, P., Vora, M., Cho, W., and Chalfant, C. E. (2007) J. Biol. Chem. 282, 20467–204741). In this study we demonstrate that C1P is a proximal and required bioactive lipid for the translocation of cPLA2α to intracellular membranes in response to inflammatory agonists (e.g. calcium ionophore and ATP). Last, the absolute requirement of the C1P/cPLA2α interaction was demonstrated for the production of eicosanoids using murine embryonic fibroblasts (cPLA2α−/−) coupled to “rescue” studies. Therefore, this study provides a paradigm shift in how cPLA2α is activated during inflammation.Eicosanoids are a class of bioactive lipids derived from the 20-carbon fatty acid, arachidonic acid (AA),2 including prostaglandins, prostacyclins, thromboxanes, and leukotrienes. The production of AA is the initial rate-limiting step in the production of eicosanoids, and the major phospholipase that regulates eicosanoids synthesis in response to agonists is group IVA cytosolic phospholipase A2 (cPLA2α) (2, 3). Activation of cPLA2 in cells requires the association of the enzyme with intracellular membranes in a Ca2+-dependent manner. This translocation of cPLA2α from the cytosol to intracellular membranes is mediated by a Ca2+-dependent lipid binding domain (CaLB domain) located at the N terminus of the enzyme (47). The CaLB domain is ∼60 amino acids and binds phosphatidylcholine (PC) in a Ca2+-dependent manner (3, 810). However, it is not known if physiologic calcium is sufficient to activate and translocate cPLA2α to membranes in cells or if activation also requires the generation of other activating lipids, such as the focus of this study, ceramide 1-phosphate (C1P).One possible activating lipid, phosphatidylinositol 4,5-diphosphate, was ruled out by Balboa and co-workers (11) as a lipid co-factor required for the translocation of the enzyme. This group showed that the interaction with this lipid (via its catalytic domain) was required for full activity of cPLA2α after the enzyme translocated to the membrane (11). Another recent report by Leslie and co-workers (12) confirmed these findings, and a recent study by our laboratory corroborated these findings utilizing biophysical approaches (1). Specifically, we showed that C1P induced a dramatic increase of cPLA2α activity strictly by increasing the residence time of cPLA2α to membranes, whereas phosphatidylinositol 4,5-diphosphate enhanced the enzymes catalytic activity and membrane penetration (13, 14).Recent studies from our laboratory have also demonstrated that C1P enhances the association of cPLA2α with membranes in vitro via a novel interactions site adjacent to the calcium binding region II of the C2 domain. Mutations of specific amino acids of this region significantly reduced the affinity for C1P (>65%) without an effect on basal enzyme activity, calcium-dependent PC affinity (supplemental Table 1), and phosphatidylinositol 4,5-diphosphate activation/affinity (1, 14). The identification and characterization of the C1P interaction site in cPLA2α allowed our laboratory to determine whether C1P played a role in regulating cPLA2α translocation and, thus, eicosanoid synthesis in response to inflammatory agonists.  相似文献   
184.
DNA polymerases are essential enzymes in all domains of life for both DNA replication and repair. The primary DNA replication polymerase from Sulfolobus solfataricus (SsoDpo1) has been shown previously to provide the necessary polymerization speed and exonuclease activity to replicate the genome accurately. We find that this polymerase is able to physically associate with itself to form a trimer and that this complex is stabilized in the presence of DNA. Analytical gel filtration and electrophoretic mobility shift assays establish that initially a single DNA polymerase binds to DNA followed by the cooperative binding of two additional molecules of the polymerase at higher concentrations of the enzyme. Protein chemical crosslinking experiments show that these are specific polymerase–polymerase interactions and not just separate binding events along DNA. Isothermal titration calorimetry and fluorescence anisotropy experiments corroborate these findings and show a stoichiometry where three polymerases are bound to a single DNA substrate. The trimeric polymerase complex significantly increases both the DNA synthesis rate and the processivity of SsoDpo1. Taken together, these results suggest the presence of a trimeric DNA polymerase complex that is able to synthesize long DNA strands more efficiently than the monomeric form.  相似文献   
185.
Srivastava P  Nath N  Deb JK 《Plasmid》2006,56(1):24-34
Plasmid pCR1 is a cryptic plasmid harboured by Corynebacterium renale. It is the smallest corynebacterial plasmid known to date. Although its natural host is animal corynebacteria, it can replicate in several strains of soil corynebacteria. It can also replicate in Escherichia coli, in which it is stably maintained. The copy number of pCR1 in this host is higher than that of pUC19, with which it shows unidirectional incompatibility. It is also incompatible with pBK2, a plasmid bearing the common corynebacterial replicon pBL1. Its size is 1488bp, as revealed by DNA sequencing. A total of eight open reading frames (ORF) were detected in this plasmid, the largest of which codes for a putative Rep protein of predicted molecular mass of 21kDa. The plasmid pCR1 can be mobilized by the plasmid R6K from E. coli to other corynebacteria. Sequence analysis revealed the presence of an oriT homologous to that of R64. An E. coli plasmid pKL1 shows more than 90% identity with pCR1. Like many coryenbacterial plasmids, pCR1 also replicates by rolling circle mode.  相似文献   
186.
Biosynthesis of beta-lactam antibiotics by fungi and actinomycetes is markedly affected by compounds containing nitrogen. The different processes employed by the spectrum of microbes capable of making these valuable compounds are affected differently by particular compounds. Ammonium ions, except at very low concentrations, exert negative effects via nitrogen metabolite repression, sometimes involving the nitrogen regulatory gene nre. Certain amino acids are precursors or inducers, whereas others are involved in repression and, in certain cases, as inhibitors of biosynthetic enzymes and of enzymes supplying precursors. The most important amino acids from the viewpoint of regulation are lysine, methionine, glutamate and valine. Surprisingly, diamines such as diaminopropane, putrescine and cadaverine induce cephamycin production by actinomycetes. In addition to penicillins and cephalosporins made by fungi and cephamycins made by actinomycetes, other beta-lactams are made by actinomycetes and unicellular bacteria. These include clavams (e.g., clavulanic acid), carbapenems (e.g., thienamycin), nocardicins and monobactams. Here also, amino acids are precursors and inhibitors, but only little is known about regulation. In the case of the simplest carbapenem made by unicellular bacteria, i.e., 1-carba-2-em-3-carboxylic acid, quorum sensors containing homoserine lactone are inducers.  相似文献   
187.
Primary ovarian pregnancy is a rare form of ectopic pregnancy that must be demonstrated with use of 4 Spiegelberg criteria. It is usually diagnosed at laparotomy or laparoscopy, although it may resemble a hemorrhagic corpus luteum. Successful conservative management of ovarian pregnancy with methotrexate has been reported only occasionally. This may be partly because of the rarity of this condition and partly because when medical treatment is successful, the patient does not need to undergo laparotomy or laparoscopy, and an occasional ovarian pregnancy may have been diagnosed as a tubal pregnancy. We present a case of ovarian pregnancy (diagnosed at laparotomy) for which initial medical management with methotrexate failed despite favorable prognostic factors. Whether the unusual location (ovary) could have contributed toward treatment failure is unknown.  相似文献   
188.
In bacteria, ribosomes stalled on nonstop mRNAs are rescued by tmRNA in a unique process called trans-translation. The two known tmRNA functions in trans-translation are (1) a tRNA-like function, which transfers the partially synthesized protein fragment to itself; and (2) an mRNA-like function, which enables ribosomes to resume and terminate translation on tmRNA as a surrogate template. We present evidence to demonstrate that tmRNA performs a third function, namely, facilitating the degradation of the causative defective mRNA. Our investigations have revealed the identity of key sequence determinants that promote the degradation of the nonstop mRNA. These sequence determinants are located in the distal part of the tmRNA open reading frame, encoding the ultimate, penultimate, and anti-penultimate amino acids of the peptide tag. We show that mutation of these tmRNA sequence elements has an adverse affect on the disposal of the nonstop mRNA, while leaving the tRNA and mRNA functions entirely unaffected. More significantly, specific mutations that change the nucleotide sequence of the peptide-reading frame without altering the nature or identity of the encoded amino acids still exhibit the characteristic defect in nonstop mRNA decay. In contrast, mutations in codons 3, 4, 5, and 6 of the tmRNA open reading frame do not have an adverse affect on degradation of defective mRNAs. Based on these results, we propose that tmRNA plays an important role in promoting the decay of nonstop mRNAs and that sequence elements in the distal segment of the peptide-reading frame make sequence-specific contributions that are crucial for this activity.  相似文献   
189.
A series of substituted piperazine derivatives have been synthesized and tested for antimicrobial activity. The antibacterial activity was tested against Staphylococcus aureus (MTCCB 737), Pseudomonas aeruginosa (MTCCB 741), Streptomyces epidermidis (MTCCB 1824) and Escherichia coli (MTCCB 1652), and antifungal activity against Aspergillus fumigatus, Aspergillus flavus and Aspergillus niger. All synthesized compounds showed significant activity against bacterial strains but were found to be less active against tested fungi. In vitro toxicity tests demonstrated that compounds 4d and 6a showed very less toxicity against human erythrocytes.  相似文献   
190.
Enterococci are opportunistic pathogens and among the leading causes of nosocomial infections. Enterococcus faecalis, the dominant species among infection-derived isolates, has recently been recognized as capable of forming biofilms on abiotic surfaces in vitro as well as on indwelling medical devices. A few bacterial factors known to contribute to biofilm formation in E. faecalis have been characterized. To identify additional factors which may be important to this process, we utilized a Tn917-based insertional mutagenesis strategy to generate a mutant bank in a high-biofilm-forming E. faecalis strain, E99. The resulting mutant bank was screened for mutants exhibiting a significantly reduced ability to form biofilms. One mutant, P101D12, which showed greater than 70% reduction in its ability to form biofilms compared to the wild-type parent, was further characterized. The single Tn917 insertion in P101D12 was mapped to a gene, bee-2, encoding a probable cell wall-anchored protein. Sequence information for the region flanking bee-2 revealed that this gene was a member of a locus (termed the bee locus for biofilm enhancer in enterococcus) comprised of five genes encoding three putative cell wall-anchored proteins and two probable sortases. Contour-clamped homogeneous electric field gel and Southern hybridization analyses suggested that the bee locus is likely harbored on a large conjugative plasmid. Filter mating assays using wild-type E99 or mutant P101D12 as a donor confirmed that the bee locus could transfer conjugally at high frequency to recipient E. faecalis strains. This represents the first instance of the identification of a mobile genetic element conferring biofilm-forming property in E. faecalis.  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