全文获取类型
收费全文 | 91篇 |
免费 | 10篇 |
出版年
2020年 | 2篇 |
2018年 | 2篇 |
2017年 | 6篇 |
2016年 | 4篇 |
2015年 | 4篇 |
2014年 | 5篇 |
2013年 | 6篇 |
2012年 | 4篇 |
2011年 | 6篇 |
2010年 | 2篇 |
2009年 | 8篇 |
2008年 | 2篇 |
2007年 | 3篇 |
2006年 | 4篇 |
2005年 | 3篇 |
2004年 | 2篇 |
2003年 | 3篇 |
2002年 | 2篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1977年 | 2篇 |
1975年 | 4篇 |
1973年 | 1篇 |
1935年 | 1篇 |
排序方式: 共有101条查询结果,搜索用时 15 毫秒
1.
WhileEscherichia coli is common as a commensal organism in the distal ileum and colon, the presence of colonization factors (CF) on pathogenic strains ofE. coli facilitates attachment of the organism to intestinal receptor molecules in a species- and tissue-specific fashion. After the initial adherence, colonization occurs, and the involvement of additional virulence determinants leads to illness. EnterotoxigenicE. coli (ETEC) is the most extensively studied of the five categories ofE. coli that cause diarrheal disease, and has the greatest impact on health worldwide. ETEC can be isolated from domestic animals and humans. The biochemistry, genetics, epidemiology, antigenic characteristics, and cell and receptor binding properties of ETEC have been extensively described. Another major category, enteropathogenicE. coli (EPEC), has virulence mechanisms, primarily effacement and cytoskeletal rearrangement of intestinal brush borders, that are distinct from ETEC. An EPEC CF receptor has been purified and characterized as a sialidated transmembrane glycoprotein complex directly attached to actin, thereby associating CF-binding with host-cell response. Three, additional categories ofE. coli diarrheal disease, their colonization factors and their host cell receptors are discussed. It appears that biofilms exist in the intestine in a manner similar to oral bacterial biofilms, and thatE. coli is part of these biofilms as both commensals and pathogens.Abbreviations CF
colonization factor
- CFA
Colonization Factor Antigen
- CS
coli-surface-associated antigen
- EAggEC
enteroaggregativeE. coli
- ECDD
E. coli diarrheal disease
- EHEC
enterohemorrhagicE. coli
- EIEC
enteroinvasiveE. coli
- EPEC
enteropathogenicE. coli
- ETEC
enterotoxigenicE. coli
- Gal
galactose
- GalNAc
N-acetyl galactosamine
- LT
heat-labile toxin
- NeuAc
N-acetyl neuraminic acid
- PCF
Putative colonization factor
- RBC
red blood cells
- SLT
Shiga-like toxin
- ST
heat-stable toxin 相似文献
2.
DNA sequences were determined for three to five alleles of the bride-of-
sevenless (boss) gene in each of four species of Drosophila. The product of
boss is a transmembrane receptor for a ligand coded by the sevenless gene
that triggers differentiation of the R7 photoreceptor cell in the compound
eye. Population parameters affecting the rate and pattern of molecular
evolution of boss were estimated from the multinomial configurations of
nucleotide polymorphisms of synonymous codons. The time of divergence
between D. melanogaster and D. simulans was estimated as approximately 1
Myr, that between D. teissieri and D. yakuba as approximately 0.75 Myr, and
that between the two pairs of sibling species as approximately 2 Myr. (The
boss genes themselves have estimated divergence times approximately 50%
greater than the species divergence times.) The effective size of the
species was estimated as approximately 5 x 10(6), and the average mutation
rate was estimated as 1-2 x 10(-9)/nucleotide/generation. The ratio of
amino acid polymorphisms within species to fixed differences between
species suggests that approximately 25% of all possible single-step amino
acid replacements in the boss gene product may be selectively neutral or
nearly neutral. The data also imply that random genetic drift has been
responsible for virtually all of the observed differences in the portion of
the boss gene analyzed among the four species.
相似文献
3.
Evolutionary origin of human and primate malarias: evidence from the circumsporozoite protein gene 总被引:8,自引:1,他引:7
We have analyzed the conserved regions of the gene coding for the
circumsporozoite protein (CSP) in 12 species of Plasmodium, the malaria
parasite. The closest evolutionary relative of P. falciparum, the agent of
malignant human malaria, is P. reichenowi, a chimpanzee parasite. This is
consistent with the hypothesis that P. falciparum is an ancient human
parasite, associated with humans since the divergence of the hominids from
their closest hominoid relatives. Three other human Plasmodium species are
each genetically indistinguishable from species parasitic to nonhuman
primates; that is, for the DNA sequences included in our analysis, the
differences between species are not greater than the differences between
strains of the human species. The human P. malariae is indistinguishable
from P. brasilianum, and P. vivax is indistinguishable from P. simium; P.
brasilianum and P. simium are parasitic to New World monkeys. The human P.
vivax-like is indistinguishable from P. simiovale, a parasite of Old World
macaques. We conjecture that P. malariae, P. vivax, and P. vivax-like are
evolutionarily recent human parasites, the first two at least acquired only
within the last several thousand years, and perhaps within the last few
hundred years, after the expansion of human populations in South America
following the European colonizations. We estimate the rate of evolution of
the conserved regions of the CSP gene as 2.46 x 10(-9) per site per year.
The divergence between the P. falciparum and P. reichenowi lineages is
accordingly dated 8.9 Myr ago. The divergence between the three lineages
leading to the human parasites is very ancient, about 100 Myr old between
P. malariae and P. vivax (and P. vivax-like) and about 165 Myr old between
P. falciparum and the other two.
相似文献
4.
Oligomeric forms of the membrane-bound acetylcholine receptor disclosed upon extraction of the M(r) 43,000 nonreceptor peptide 下载免费PDF全文
FJ Barrantes 《The Journal of cell biology》1982,92(1):60-68
Oligomeric forms of the acetylcholine receptor are directly visualized by electron microscopy in receptor-rich membranes from torpedo marmorata. The receptor structures are quantitatively correlated with the molecular species so far identified only after detergent solubilization, and further related to the polypeptide composition of the membranes and changes thereof. The structural identification is made possibly by the increased fragility of the membranes after extraction of nonreceptor peptides and their subsequent disruption upon drying onto hydrophilic carbon supports. Receptor particles in native membranes depleted of nonreceptor peptides appear as single units of 7-8 nm, and double and multiple aggregates thereof. Particle doublets having a main-axis diameter of 19 +/- 3 nm predominate in these membranes. Linear aggregates of particles similar to those observed in rotary replicas of quick-frozen fresh electrolytes (Heuser, J.E. and S. R. Salpeter. 1979, J. Cell Biol. 82: 150-173) are also present in the alkaline-extracted membranes. Chemical modifications of the thiol groups shift the distribution of structural species. Dithiothreitol reduction, which renders almost exclusively the 9S, monomeric receptor form, results in the observation of the 7-8 nm particle in isolated form. The proportion of doublets increases in membranes alkylated with N-ethylmaleimide. Treatment with 5,5’-dithiobis-(nitrobenzoic acid) increases the proportion of higher oligomeric species, and particle aggregates (n=oligo) predominate. The nonreceptor v-peptide (doublet of M(r) 43,000) appears to play a role in the receptor monomer-polymer equilibria. Receptor protein and v-peptide co-aggregate upon reduction and reoxidation of native membranes. In membranes protected ab initio with N- ethylmaleimide, only the receptor appears to self-aggregate. The v-peptide cannot be extracted from these alkylated membranes, though it is easily released from normal, subsequently alkylated or reduced membranes. A stabilization of the dimeric species by the nonreceptor v-peptide is suggested by these experiments. Monospecific antibodies against the v-peptide are used in conjunction with rhodamine- labeled anti-bodies in an indirect immunoflourescence assay to map the vectorial exposure of the v-peptide. When intact membranes, v-peptide depleted and “holey” native membranes (treated with 0.3 percent saponin) are compared, maximal labeling is obtained with the latter type of membranes, suggesting a predominantly cytoplasmic exposure of the antigenic determinants of the v-peptide in the membrane. The influence of the v-peptide in the thiol-dependent interconversions of the receptor protein and the putative topography of the peptide are analyzed in the light of the present results. 相似文献
5.
C W Carlson G A Tejwani R C Baxter E H Ulm B M Pogell 《The Journal of biological chemistry》1975,250(13):4996-5002
Dialyzed rabbit liver cytosol was specifically freed of endogenous fructose-1,6-diphosphatase by immunoadsorption on a column of Sepharose-immobilized anti-fructose-1,6-diphosphatase. This material increased the specific activity of homogeneous enzyme to the maximal rate observed with EDTA and shifted the pH optimum from 8.4 to 7.4. With oleate or other fatty acids as activators, the hydrolysis of fructose-1,6-diphosphatase by enzyme, at neutral pH, showed nonlinear initial rates dropping to lower linear rates. Cytosol activator acted synergistically with oleate both to increase neutral enzyme activity and to maintain the high initial catalytic rates. After sucrose density centrifugation or gel filtration, the cytosol had no effect by itself, but still potentiated oleate activation. The factor was destroyed by treatment with subtilisin or trypsin, but all attempts to identify a unique protein component in cytosol were unsuccessful. The presence of Na dodecyl-SOJ, deoxycholate, or urea did not improve the resolution of the factor, but these compounds did lower the K50 for activation by cytosol. Since fatty acids are the only unique compounds which have been isolated from cytosol which activated fructose-1,6-diphosphatase, it appears that soluble proteins can act as natural carriers for the fatty acids. This was supported by the fact that both dialyzed rabbit alpha-globulins and muscle phosphofructokinase also acted synergistically with oleate in a manner similar to cytosol. Phosphatidic acid and phosphatidylserine activated fructose-1,6-diphosphatase, and their action was synergistic with oleate. Glutathione (1 mM) activated the enzyme 5-fold at pH 7.3 and its effects were additive with oleate and cytosol or alpha-globulins. 相似文献
6.
7.
Phosphorylation and Stabilization of Arabidopsis MAP Kinase Phosphatase 1 in Response to UV-B Stress
MAP kinase phosphatases (MKPs) are important regulators of the activation levels and kinetics of MAP kinases. This is crucial for a large number of physiological processes during development and growth, as well as interactions with the environment, including the response to ultraviolet-B (UV-B) stress. Arabidopsis MKP1 is a key regulator of MAP kinases MPK3 and MPK6 in response to UV-B stress. However, virtually nothing is presently known about the post-translational regulation of plant MKPs in vivo. Here, we provide evidence that MKP1 is a phosphoprotein in vivo and that MKP1 accumulates in response to UV-B stress. Moreover, proteasome inhibitor experiments suggest that MKP1 is constantly turned-over under non-stress conditions and that MKP1 is stabilized upon stress treatment. Stress-responsive phosphorylation and stabilization of MKP1 demonstrate the post-translational regulation of a plant MKP in vivo, adding an additional regulatory layer to MAP kinase signaling in plants. 相似文献
8.
A systematic approach was used to examine the induction of dicentric chromosomes in human lymphocytes irradiated in vitro with monochromatized synchrotron radiation produced with photon energies in the range from 1.83 keV to 17.4 keV. To avoid potential confounding factors that could influence the outcome of the experiments, only blood from one individual was used. Since for the irradiation experiments with these low photon energies the local dose variations would become unacceptable, monolayers of lymphocytes attached within 3 h PHA stimulation have been used. The culture conditions ensured that the chromosome analysis could be performed exclusively in metaphases of the first cell cycle in vitro. There is a strong indication for a systematic change of the coefficient of the linear quadratic dose-response relationship from 1.83 keV (=1.26±0.28) with increasing energy up to 6.9 keV (=8.24±0.41) and a decrease with further increase of energy up to 17.4 keV (=3.83±1.72). A tendency for a systematic change of the coefficient seems to be present at energies of 6.9 keV (=8.04±0.40) and 4.8 keV (=9.48±1.57) as well as at energies of 3.10 keV (=2.99±0.51) and 1.83 keV (=0.40±0.25). These results agree in essence with the previously published large data set from Sasakis laboratory. 相似文献
9.
Distinct regulation of salinity and genotoxic stress responses by Arabidopsis MAP kinase phosphatase 1 下载免费PDF全文
Ulm R Ichimura K Mizoguchi T Peck SC Zhu T Wang X Shinozaki K Paszkowski J 《The EMBO journal》2002,21(23):6483-6493
The Arabidopsis genome contains 20 genes encoding mitogen-activated protein kinases (MAPKs), which drastically outnumbers genes for their negative regulators, MAP kinase phosphatases (MKPs) (five at most). This contrasts sharply with genomes of other eukaryotes where the number of MAPKs and MKPs is approximately equal. MKPs may therefore play an important role in signal integration in plants, through concerted regulation of several MAPKs. Our previous studies identified Arabidopsis MKP1 and showed that its deficiency in the mkp1 mutant results in plant hypersensitivity to genotoxic stress. Here, we identify a set of MAPKs that interact with MKP1, and show that the activity level of one of these, MPK6, is regulated by MKP1 in vivo. Moreover, using expression profiling, we identified a specific group of genes that probably represent targets of MKP1 regulation. Surprisingly, the identity of these genes and interacting MAPKs suggested involvement of MKP1 in salt stress responses. Indeed, mkp1 plants have increased resistance to salinity. Thus MKP1 apparently plays a pivotal role in the integration and fine-tuning of plant responses to various environmental challenges. 相似文献
10.
At the ultrastructural observation scale of fully mineralized tissues (l=1-10 mum), transmission electron micrographs (TEM) reveal that hydroxyapatite (HA) is situated both within the fibrils and extrafibrillarly, and that the majority of HA lies outside the fibrils. The extrafibrillar amount of HA varies from tissue to tissue. By means of mathematical modeling, we here provide strong indications that there exists a physical quantity that is the same inside and outside the fibrils, for all different fully mineralized tissues. This quantity is the average mineral concentration in the non-collagenous space. This space is the sum of the extrafibrillar volume and of the volume of the fibrils that is not occupied by collagen molecules. Two independent sets of experimental observations covering a large range of tissue mass densities establish the relevance of our proposition: (i) mass density measurements and diffraction spacing measurements, re-analyzed through a dimensionally consistent packing model; (ii) optical density measurements of TEMs. The aforementioned average uniform HA-concentration in the extracollagenous space of the ultrastructure may emphasize the putative role played by a number of non-collagenous organic molecules in providing the chemical boundary conditions for mineralization of HA in the extracollagenous space. The probable existence of an average uniform extracollagenous HA concentration has far-reaching consequences for the mechanical behavior of mineralized tissues. 相似文献