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   1480篇
  免费   69篇
  国内免费   1篇
  1550篇
  2022年   10篇
  2021年   22篇
  2020年   9篇
  2019年   6篇
  2018年   22篇
  2017年   23篇
  2016年   32篇
  2015年   50篇
  2014年   61篇
  2013年   71篇
  2012年   101篇
  2011年   107篇
  2010年   64篇
  2009年   73篇
  2008年   89篇
  2007年   117篇
  2006年   108篇
  2005年   97篇
  2004年   86篇
  2003年   99篇
  2002年   85篇
  2001年   12篇
  2000年   13篇
  1999年   13篇
  1998年   26篇
  1997年   22篇
  1996年   11篇
  1995年   18篇
  1994年   12篇
  1993年   7篇
  1992年   8篇
  1991年   12篇
  1990年   5篇
  1989年   4篇
  1988年   4篇
  1987年   4篇
  1986年   2篇
  1985年   9篇
  1984年   2篇
  1983年   5篇
  1982年   6篇
  1981年   4篇
  1980年   4篇
  1979年   1篇
  1978年   3篇
  1977年   3篇
  1974年   3篇
  1971年   1篇
  1967年   1篇
  1957年   1篇
排序方式: 共有1550条查询结果,搜索用时 0 毫秒
101.
102.
Nicotianamine synthase (NAS) is an enzyme that is critical for the biosynthesis of the mugineic acid family of phytosiderophores in graminaceous plants, and for the homeostasis of metal ions in nongraminaceous plants. We isolated one genomic NAS clone, ZmNAS3, and two cDNA NAS clones, ZmNAS1 and ZmNAS2, from maize (Zea mays cv Alice). In agreement with the increased secretion of phytosiderophores with Fe deficiency, ZmNAS1 and ZmNAS2 were positively expressed only in Fe-deficient roots. In contrast, ZmNAS3 was expressed under Fe-sufficient conditions, and was negatively regulated by Fe deficiency. This is the first report describing down-regulation of NAS gene expression in response to Fe deficiency in plants, shedding light on the role of nicotianamine in graminaceous plants, other than as a precursor in phytosiderophore production. ZmNAS1-green fluorescent protein (sGFP) and ZmNAS2-sGFP were localized at spots in the cytoplasm of onion (Allium cepa) epidermal cells, whereas ZmNAS3-sGFP was distributed throughout the cytoplasm of these cells. ZmNAS1 and ZmNAS3 showed NAS activity in vitro, whereas ZmNAS2 showed none. Due to its duplicated structure, ZmNAS2 was much larger (65.8 kD) than ZmNAS1, ZmNAS3, and previously characterized NAS proteins (30-38 kD) from other plant species. We reveal that maize has two types of NAS proteins based on their expression pattern and subcellular localization.  相似文献   
103.
Two novel tachykinins (OctTK-I: Lys-Pro-Pro-Ser-Ser-Ser-Glu-Phe-Ile-Gly-Leu-Met-NH(2) and OctTK-II: Lys-Pro-Pro-Ser-Ser-Ser-Glu-Phe-Val-Gly-Leu-Met-NH(2)) were isolated from the posterior salivary gland of the octopus (Octopus vulgaris) using a contraction assay of the carp rectum. These peptides had in common the pentapeptide sequence -Phe-X-Gly-Leu-Met-NH(2) at the C-terminal and induced immediate contractions on the carp rectum and the guinea-pig ileum. cDNAs encoding their precursor proteins were cloned. The OctTK gene was expressed in the posterior salivary gland and the expression was localized in mucus-secreting cells of the gland. The results suggested that OctTKs might be secreted as a venomous substance acting on vertebrates such as fishes, which are the prey or natural enemies of the octopus.  相似文献   
104.
We examined the effects of (-)-Hydroxycitrate (HCA) ingestion on fat oxidation during moderate intensity exercise in untrained men. Six subjects ingested 500 mg of HCA or a placebo for 5 days and did endurance exercise. Blood FFA concentrations were significantly increased and respiratory exchange ratio (RER) decreased by HCA ingestion. These results suggested short-term HCA ingestion increases fat oxidation in untrained men.  相似文献   
105.
Lactococcus lactis 61-14 isolated from river water produced a bacteriocin active against a wide range of Gram-positive bacteria. N-terminal amino acid sequencing, mass spectral analysis of the purified bacteriocin, and genetic analysis using nisin-specific primers showed that the bacteriocin was a new natural nisin variant, termed nisin Q. Nisin Q and nisin A differ in four amino acids in the mature peptide and two in the leader sequence.  相似文献   
106.
For the purpose of phylogeographic study of lucidophyllous (evergreen broad-leaved) forests in Japan, we surveyed intraspecific chloroplast DNA (cpDNA) variation in 41 component species of such forests. Intraspecific cpDNA variations were detected in 14 species. In 15 species and one species group, 16 non-coding cpDNA regions were examined to find intraspecific sequence variation. The extent of variation in these regions was compared. The largest amount of intraspecific variation was detected in the rps16 region. A relatively large amount of intraspecific variation was detected in the petD-rpoA, rpl16, and trnL-F regions. It is suggested that these regions of cpDNA would be useful for detecting intraspecific variation in plant species, and could provide valuable information for various research purposes.  相似文献   
107.
108.
Asai, T., Howe, D. K., Nakajima, K., Nozaki, T., Takeuchi, T., and Sibley, L. D.Neospora caninum: Tachyzoites Express Type-I Nucleoside Triphosphate Hydrolase1. But Lack Nucleoside Diphosphate Hydrolase Activity.Experimental Parasitology90,277–285. We have identified type I nucleoside triphosphate hydrolase (NTPase; EC 3.6.1.3) activity, previously thought to be restricted to the virulent strains ofToxoplasma gondii, in the cell extracts ofNeospora caninumtachyzoites. Sequence analysis of a complete cDNA from Nc-1 strain indicated thatN. caninumNTPases shared approximately 69% identity to the NTPases ofT. gondiiand are most similar to the NTPase-I isozyme. Southern blot analysis of genomic DNA and sequence analysis of two independentNTPclones from the Nc-1 strain revealed the presence of multiple genes, at least two of which are transcribed. Substrate specificity andKmvalues for MgATP2−and MgADPhydrolysis for recombinant or partially purified native NcNTPase were the same as those for the type I isozyme (NTPase-I). Significantly, no type II enzyme (NTPase-II) activity for NDP hydrolysis was detected in cell extracts ofN. caninum, although it is universally present in allT. gondiistrains that have been tested. This intriguing difference between these two closely related apicomplexan parasites may provide insight into the function of the NTPases during intracellular parasitism.  相似文献   
109.
Glycopeptides containing the N-linked oligosaccharide from human serum IgA1 were analyzed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS). Two glycopeptides, GP1 and GP2, prepared from the endoproteinase Asp-N digest of the IgA1 heavy chain, were derived from the CH2 domain (N-glycan site at Asn263) and the tailpiece portion (N-glycan site at Asn459), respectively. The structure of the attached sugar chain was deduced from the mass number of the glycopeptide and confirmed by a two-dimensional mapping technique for a pyridylaminated oligosaccharide. GP1 was composed of two major components having a fully galactosylated bianntena sugar chain with or without a bisecting N-acetylglucosamine (GlcNAc) residue. On the other hand, the GP2 fraction corresponded to the glycopeptides having a fully galactosylated and fucosylated bianntena sugar chain partly bearing a bisecting GlcNAc residue. Thus, the site-specific fucosylation of the N-linked oligosaccharide on the tailpiece of the 1 chain became evident for normal human serum IgA1.  相似文献   
110.
We cloned the aspT gene encoding the L-aspartate:L-alanine antiporter AspTCt in Comamonas testosteroni genomic DNA. Analysis of the nucleotide sequence revealed that C. testosteroni has an asp operon containing aspT upstream of the l-aspartate 4-decarboxylase gene, and that the gene order of the asp operon of C. testosteroni is the inverse of that of Tetragenococcus halophilus. We used proteoliposomes to confirm the transport processes of AspTCt. To elucidate the two-dimensional structure of AspTCt, we analysed its membrane topology by means of alkaline phosphatase (PhoA) and beta-lactamase (BlaM) fusion methods. The fusion analyses revealed that AspTCt has seven transmembrane segments (TMs), a large cytoplasmic loop containing approximately 200 amino acid residues between TM4 and TM5, a cytoplasmic N-terminus, and a periplasmic C-terminus. These results suggest that the orientation of the N-terminus of AspTCt differs from that of tetragenococcal AspT, even though these two AspT orthologues catalyse the same transport reactions.  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