全文获取类型
收费全文 | 20716篇 |
免费 | 1893篇 |
国内免费 | 9篇 |
出版年
2023年 | 70篇 |
2022年 | 57篇 |
2021年 | 299篇 |
2020年 | 211篇 |
2019年 | 281篇 |
2018年 | 304篇 |
2017年 | 302篇 |
2016年 | 503篇 |
2015年 | 836篇 |
2014年 | 915篇 |
2013年 | 1136篇 |
2012年 | 1598篇 |
2011年 | 1643篇 |
2010年 | 1012篇 |
2009年 | 884篇 |
2008年 | 1304篇 |
2007年 | 1365篇 |
2006年 | 1264篇 |
2005年 | 1252篇 |
2004年 | 1224篇 |
2003年 | 1135篇 |
2002年 | 1063篇 |
2001年 | 256篇 |
2000年 | 197篇 |
1999年 | 219篇 |
1998年 | 265篇 |
1997年 | 166篇 |
1996年 | 142篇 |
1995年 | 200篇 |
1994年 | 192篇 |
1993年 | 165篇 |
1992年 | 143篇 |
1991年 | 153篇 |
1990年 | 136篇 |
1989年 | 122篇 |
1988年 | 120篇 |
1987年 | 130篇 |
1986年 | 118篇 |
1985年 | 114篇 |
1984年 | 110篇 |
1983年 | 136篇 |
1982年 | 121篇 |
1981年 | 126篇 |
1980年 | 98篇 |
1979年 | 65篇 |
1978年 | 63篇 |
1977年 | 63篇 |
1976年 | 72篇 |
1975年 | 46篇 |
1973年 | 33篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
1.
2.
3.
4.
Identification of 3-hydroxypalmitic acid methyl ester as a novel autoregulator controlling virulence in Ralstonia solanacearum 总被引:3,自引:2,他引:1
Albert B. Flavier Steven J. Clough Mark A. Schell & Timothy P. Denny 《Molecular microbiology》1997,26(2):251-259
Expression of virulence genes in Ralstonia solanacearum , a phytopathogenic bacterium, is controlled by a complex regulatory network that integrates multiple signal inputs. Production of several virulence determinants is co-ordinately reduced by inactivation of phcB , but is restored by growth in the presence of a volatile extracellular factor (VEF) produced by wild-type strains of R. solanacearum . The VEF was purified from spent culture broth by distillation, solvent extraction, and liquid chromatography. Gas chromatography and mass spectroscopy identified 3-hydroxypalmitic acid methyl ester (3-OH PAME) as the major component in the single peak of VEF activity. Authentic 3-OH PAME and the purified VEF were active at ≤1 nM, and had nearly equivalent specific activities for stimulating the expression of eps (the biosynthetic locus for extracellular polysaccharide) in a phcB mutant. Authentic 3-OH PAME also increased the production of three virulence factors by a phcB mutant over 20-fold to wild-type levels, restored normal cell density-associated expression of eps and increased expression of eps when delivered via the vapour phase. Reanalysis of the PhcB amino acid sequence suggested that it is a small-molecule S -adenosylmethionine-dependent methyltransferase, which might catalyse synthesis of 3-OH PAME from a naturally occurring fatty acid. Biologically active concentrations of extracellular 3-OH PAME were detected before the onset of eps expression, suggesting that it is an intercellular signal that autoregulates virulence gene expression in wild-type R. solanacearum . Other than acyl-homoserine lactones, 3-OH PAME is the only endogenous fatty acid derivative shown to be an autoregulator and may be the first example of a new family of compounds that can mediate long-distance intercellular communication. 相似文献
5.
Nicking by transesterification: the reaction catalysed by a relaxase 总被引:17,自引:5,他引:12
DNA relaxases play an essential role in the initiation and termination of conjugative DNA transfer. Purification and characterization of relaxases from several plasmids has revealed the reaction mechanism: relaxases nick duplex DNA in a site- and strand-specific manner by catalysing a transesterification. The product of the reaction is a nicked double-stranded DNA molecule with a sequestered 3'-OH and the relaxase covalently bound to the 5' end of the cleaved strand via a phosphotyrosyl linkage. The relaxase-catalysed transesterification is isoenergetic and reversible; a second transesterification ligates the nicked DNA. However, the covalent nucleoprotein complex is relatively long-lived, a property that is likely to be essential for its role as an intermediate in the process of conjugative DNA transfer. Subsequent unwinding of the nicked DNA intermediate is required to produce the single strand of DNA transferred to the recipient cell. This reaction is catalysed by a DNA helicase, an activity intrinsic to the relaxase protein in some, but not all, plasmid systems. The first relaxase-catalysed transesterification is essential for initiation of conjugative strand transfer, whereas the second is presumably required for termination of the process. The relaxase, in conjunction with several auxiliary proteins, forms the relaxation complex or relaxosome first described nearly 30 years ago as being associated with conjugative and mobilizable plasmids. 相似文献
6.
Julie A. Eclov Qingwen Qian Rebecca Redetzke Quanhai Chen Steven C. Wu Chastity L. Healy Steven B. Ortmeier Erin Harmon Gregory C. Shearer Timothy D. O’Connell 《Journal of lipid research》2015,56(12):2297-2308
Heart failure with preserved ejection fraction (HFpEF) is half of all HF, but standard HF therapies are ineffective. Diastolic dysfunction, often secondary to interstitial fibrosis, is common in HFpEF. Previously, we found that supra-physiologic levels of ω3-PUFAs produced by 12 weeks of ω3-dietary supplementation prevented fibrosis and contractile dysfunction following pressure overload [transverse aortic constriction (TAC)], a model that resembles aspects of remodeling in HFpEF. This raised several questions regarding ω3-concentration-dependent cardioprotection, the specific role of EPA and DHA, and the relationship between prevention of fibrosis and contractile dysfunction. To achieve more clinically relevant ω3-levels and test individual ω3-PUFAs, we shortened the ω3-diet regimen and used EPA- and DHA-specific diets to examine remodeling following TAC. The shorter diet regimen produced ω3-PUFA levels closer to Western clinics. Further, EPA, but not DHA, prevented fibrosis following TAC. However, neither ω3-PUFA prevented contractile dysfunction, perhaps due to reduced uptake of ω3-PUFA. Interestingly, EPA did not accumulate in cardiac fibroblasts. However, FFA receptor 4, a G protein-coupled receptor for ω3-PUFAs, was sufficient and required to block transforming growth factor β1-fibrotic signaling in cultured cardiac fibroblasts, suggesting a novel mechanism for EPA. In summary, EPA-mediated prevention of fibrosis could represent a novel therapy for HFpEF. 相似文献
7.
8.
T V Hancock S C Adolph T T Gleeson 《Comparative biochemistry and physiology. Part A, Molecular & integrative physiology》2001,130(1):67-79
The majority of elevated O(2) consumption associated with short and vigorous activity occurs during recovery, thus an assessment of associated metabolic costs should also examine the excess post-exercise oxygen consumption (EPOC). This study examined O(2) uptake during exercise, EPOC and distance traveled during 5-, 15-, 60- and 300-s sprints at maximal treadmill intensity in Dipsosaurus (N=10; 74.3+/-2.1 g). EPOC (0.08, 0.14, 0.23 and 0.18 ml O(2) g(-1), respectively) was large (80-99% of total elevated O(2) consumption) and increased significantly between 5 and 60 s. The cost of activity (C(act); ml O(2) g(-1) x km(-1)), intended to reflect the total net costs associated with the activity, was calculated as the total elevated O(2) consumption per unit distance traveled. C(act) decreased with activity duration due to proportionally larger increases in distance traveled relative to EPOC volume, and is predicted by the equation C(act)=14.7 x activity duration (s)(-0.24). The inclusion of EPOC costs provides an ecologically relevant estimate of the total metabolic cost of locomotor activity. C(act) exceeds standard transport costs at all durations examined due to the addition of obligate recovery costs. The differences are large enough to impact energy budget analyses for ectotherms. 相似文献
9.
The major outer membrane porin (PorB) of Neisseria gonorrhoeae is an essential protein that mediates ion exchange between the organism and its environment and also plays multiple roles in human host pathogenesis. To facilitate structure-function studies of porin''s multiple roles, we performed saturating mutagenesis at the porB locus and used deep sequencing to identify essential versus mutable residues. Random mutations in porB were generated in a plasmid vector, and mutant gene pools were transformed into N. gonorrhoeae to select for alleles that maintained bacterial viability. Deep sequencing of the input plasmid pools and the output N. gonorrhoeae genomic DNA pools identified mutations present in each, and the mutations in both pools were compared to determine which changes could be tolerated by the organism. We examined the mutability of 328 amino acids in the mature PorB protein and found that 308 of them were likely to be mutable and that 20 amino acids were likely to be nonmutable. A subset of these predictions was validated experimentally. This approach to identifying essential amino acids in a protein of interest introduces an additional application for next-generation sequencing technology and provides a template for future studies of both porin and other essential bacterial genes. 相似文献
10.
Xiaoling Tong Steven Hrycaj Ondrej Podlaha Aleksandar Popadic Antónia Monteiro 《Developmental biology》2014
In insects, forewings and hindwings usually have different shapes, sizes, and color patterns. A variety of RNAi experiments across insect species have shown that the hox gene Ultrabithorax (Ubx) is necessary to promote hindwing identity. However, it remains unclear whether Ubx is sufficient to confer hindwing fate to forewings across insects. Here, we address this question by over-expressing Ubx in the butterfly Bicyclus anynana using a heat-shock promoter. Ubx whole-body over-expression during embryonic and larvae development led to body plan changes in larvae but to mere quantitative changes to adult morphology, respectively. Embryonic heat-shocks led to fused segments, loss of thoracic and abdominal limbs, and transformation of head limbs to larger appendages. Larval heat-shocks led to reduced eyespot size in the expected homeotic direction, but neither additional eyespots nor wing shape changes were observed in forewings as expected of a homeotic transformation. Interestingly, Ubx was found to be expressed in a novel, non-characteristic domain – in the hindwing eyespot centers. Furthermore, ectopic expression of Ubx on the pupal wing activated the eyespot-associated genes spalt and Distal-less, known to be directly repressed by Ubx in the fly?s haltere and leg primordia, respectively, and led to the differentiation of black wing scales. These results suggest that Ubx has been co-opted into a novel eyespot gene regulatory network, and that it is capable of activating black pigmentation in butterflies. 相似文献