全文获取类型
收费全文 | 4811篇 |
免费 | 358篇 |
国内免费 | 3篇 |
出版年
2022年 | 45篇 |
2021年 | 59篇 |
2020年 | 51篇 |
2019年 | 70篇 |
2018年 | 98篇 |
2017年 | 84篇 |
2016年 | 93篇 |
2015年 | 136篇 |
2014年 | 207篇 |
2013年 | 300篇 |
2012年 | 314篇 |
2011年 | 348篇 |
2010年 | 205篇 |
2009年 | 166篇 |
2008年 | 199篇 |
2007年 | 242篇 |
2006年 | 211篇 |
2005年 | 189篇 |
2004年 | 163篇 |
2003年 | 152篇 |
2002年 | 124篇 |
2001年 | 127篇 |
2000年 | 133篇 |
1999年 | 102篇 |
1998年 | 54篇 |
1997年 | 45篇 |
1996年 | 49篇 |
1995年 | 41篇 |
1994年 | 36篇 |
1993年 | 36篇 |
1992年 | 75篇 |
1991年 | 90篇 |
1990年 | 88篇 |
1989年 | 84篇 |
1988年 | 44篇 |
1987年 | 64篇 |
1986年 | 70篇 |
1985年 | 74篇 |
1984年 | 66篇 |
1983年 | 43篇 |
1982年 | 48篇 |
1981年 | 34篇 |
1980年 | 25篇 |
1979年 | 33篇 |
1978年 | 31篇 |
1977年 | 32篇 |
1976年 | 24篇 |
1975年 | 21篇 |
1974年 | 23篇 |
1972年 | 26篇 |
排序方式: 共有5172条查询结果,搜索用时 0 毫秒
141.
J H Freisheim L H Ericsson K G Bitar R B Dunlap A V Reddy 《Archives of biochemistry and biophysics》1977,180(2):310-317
Dihydrofolate reductase from amethopterin-resistant Lactobacillus casei contains three tryptophan residues and the amino acid sequence surrounding each tryptophan has been determined. Oxidation of one of these residues by N-bromosuccinimide at pH 6.5 can be correlated with the complete loss of enzymatic activity. Following denaturation in urea, the oxidized enzyme was alkylated with dimethyl(2-hydroxy-5-nitrobenzyl) sulfonium bromide. Based on amino acid analyses and absorbance measurements at 410 nm, 2.2 mol of hydroxynitrobenzyl groups was incorporated per mol of protein. Presumably, hydroxynitrobenzyl adducts are formed with the two nonessential tryptophans. From the amino acid compositions of the two major thermolytic peptides containing the hydroxynitrobenzyl label and the partial sequences of two cyanogen bromide peptides containing the tryptophans, it was deduced that tryptophan-5 and tryptophan-129 were modified and, therefore, by difference, tryptophan-21 is the functional residue which becomes oxidized. The amino acid sequence surrounding tryptophan-21 is -Leu--Trp-His-Leu-Pro-. In reductases from four other species, this region of the sequence is highly homologous; such a conservation in this vicinity of the primary structure may indicate a functional involvement. The proline residues at positions 20 and 24 may serve to position tryptophan-21 into the appropriate configuration for optimum substrate-binding interactions. 相似文献
142.
R. Casper Sylke Meyer D.-E. Lesemann D. V. D. Reddy Ramanan Rajeshwari S. M. Misari S. S. Subbarayudu 《Journal of Phytopathology》1983,108(1):12-17
In groundnut rosette diseased groundnut plants collected near Zaria, Nigeria, a luteovirus was detected by ELISA and ISEM. In ELISA only beet western yellows virus antiserum reacted, while in ISEM luteovirus particles were trapped by antisera beet western yellows virus, potato leafroll virus, pea leafroll virus and barley yellow dwarf virus. The data are in agreement with the interpretation that the assistor of groundnut rosette virus is possibly a member of the luteovirus group. 相似文献
143.
E N Benveniste M Vidovic R B Panek J G Norris A T Reddy D J Benos 《The Journal of biological chemistry》1991,266(27):18119-18126
Astrocytes can be induced by interferon-gamma (IFN-gamma) to express class II major histocompatibility complex (MHC) antigens. This study was undertaken to elucidate the intracellular signaling pathways involved in IFN-gamma induction of class II MHC. We examined the effects of Na+/H+ antiporter and protein kinase C (PKC) inhibitors on class II expression and Na+ influx in astrocytes. We found that amiloride and ethyl isopropylamiloride, inhibitors of Na+/H+ exchange, blocked IFN-gamma-induced class II gene expression. IFN-gamma stimulated Na+ influx, and this increased influx was inhibited by amiloride. Treatment of astrocytes with the PKC inhibitor H7 also blocked the increase in Na+ uptake induced by IFN-gamma, indicating that IFN-gamma-induced PKC activation is required for subsequent Na+ influx. IFN-gamma treatment produced an increase of total PKC activity, which was associated with a rapid translocation of PKC activity from cytosolic to particulate fraction. H7 and another PKC inhibitor, staurosporine, inhibited IFN-gamma-induced class II gene expression. However, 4 beta-phorbol 12 beta-myristate 13 alpha-acetate, a potent PKC activator, did not affect class II expression. Taken together, our data indicate that both IFN-gamma-induced PKC activation and Na+ influx are required for class II MHC expression in astrocytes but that activation of PKC alone is not sufficient for ultimate expression of this gene. 相似文献
144.
145.
Vitamin D receptor (VDR) is a nuclear protein which mediates the physiological actions of its hormone ligand, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). While it appears that the receptor-hormone complex regulates the expression of hormone-dependent genes involved in mineral homeostasis, its role in induction of differentiation of leukaemic cells is less clear. We have studied the expression of the VDR gene in several sublines of HL-60 leukaemic cells with varying responsiveness to 1,25(OH)2D3. Sublines which rapidly differentiated to monocytic forms were shown to contain elevated steady-state levels of VDR mRNA within 1 h of exposure to high concentration of 1,25(OH)2D3. This up-regulation of the expression of VDR was not apparent in sublines in which monocytic differentiation occurred after a delay of several days. Beginning at approximately 3 h after exposure to 1,25(OH)2D3 in most cases, there was a gradual decline in VDR mRNA levels. Measurement of steady-state levels of mRNA for c-myc and c-fos showed that in sublines of HL-60 cells which respond rapidly to 1,25(OH)2D3, elevation of VDR mRNA is evident prior to the changes in proto-oncogene expression. These data are consistent with the hypothesis that a change in VDR gene expression is one of the steps that promote monocytic differentiation. 相似文献
146.
147.
Effect of different carbon and nitrogen sources on the production of penitrem B was studied.d-Xylose induced maximum penitrem B production, while melibiose, glycerol, citric acid and succinic acid were poor substrates.
Potassium nitrate,l-asparagine, sodium nitrate, glycine,dl-aspartic acid andl-tryptophan supported good production of penitrem B. Conversely zirconyl nitrate, barium nitrate, aluminum nitrate, acetanilide,
4-aminobenzoic acid, 4-nitrobenzoic acid and 4-nitroaniline were toxic and did not even permit the growth of the fungus. 相似文献
148.
Transformation of chicken myelomonocytic cells by a retrovirus expressing the v-myb oncogene from the long terminal repeats of avian myeloblastosis virus but not Rous sarcoma virus. 下载免费PDF全文
To test the effect of long terminal repeat (LTR) regulatory sequences on the transforming capability of the v-myb oncogene from avian myeloblastosis virus (AMV), we have constructed replication-competent avian retroviral vectors with nearly identical structural genes that express v-myb from either AMV or Rous sarcoma virus (RSV) LTRs. After transfection into chicken embryo fibroblasts, virus-containing cell supernatants were used to infect chicken myelomonocytic target cells from preparations of 16-day-old embryonic spleen cells. Both wild-type AMV and the virus expressing v-myb from AMV LTRs (RCAMV-v-myb) were able to transform the splenocyte cultures into a population of immature myelomonocytic cells. The transformed cells expressed the p48v-Myb oncoprotein and formed compact foci when grown in soft agar. In contrast, the virus expressing v-myb from RSV LTRs (RCAS-v-myb) was repeatedly unable to transform the same splenocyte cells, despite being able to infect fibroblasts with high efficiency. This difference in the transforming activities of v-myb-expressing viruses with different LTRs most likely results from the presence of a factor (or factors) within the appropriate myelomonocytic target cell that promotes specific expression from the AMV but not from the RSV LTR. 相似文献
149.
S Shimba B Buckley R Reddy T Kiss W Filipowicz 《The Journal of biological chemistry》1992,267(19):13772-13777
U3 small nucleolar RNA (snoRNA) is an abundant small RNA involved in the processing of pre-ribosomal RNA of eukaryotic cells. U3 snoRNA has been previously characterized from several sources, including human, rat, mouse, frog, fruit fly, dinoflagellates, slime mold, and yeast; in all these organisms, U3 snoRNA contains trimethylguanosine cap structure. In all instances where investigated, the trimethylguanosine-capped snRNAs including U3 snoRNA, are synthesized by RNA polymerase II. However, in higher plants, the U3 snoRNA is synthesized by RNA polymerase III and contains a cap structure different from trimethylguanosine (Kiss, T., and Solymosy, F. (1990) Nucleic Acids Res. 18, 1941-1949; Marshallsay, C., Kiss, T., and Filipowicz, W. (1990) Nucleic Acids Res. 18, 3451-3458; Kiss, T., Marshallsay, C., and Filipowicz, W. (1991) Cell 65, 517-526). In this study, we present evidence that cowpea and, most likely, tomato plant U3 snoRNA contains a methyl-pppA cap structure. These data show that the same U3 snoRNA contains different cap structures in different species and suggest that the kind of cap structure that an uridylic acid-rich small nuclear RNA contains is dependent on the RNA polymerase responsible for its synthesis. In vitro synthesized plant U3 snoRNA, with pppA or pppG as its 5' end, was converted to methyl-pppA/G cap structure in vitro when incubated with extracts prepared from wheat germ or HeLa cells. These data show that the capping machinery is conserved in organisms as evolutionarily distant as plants and mammals. Nucleotides 1-45 of tomato U3 snoRNA, which are capable of forming a stem-loop structure, are sufficient to direct the methyl cap formation in vitro. 相似文献
150.
In vivo cytotoxic T lymphocyte induction with soluble proteins administered in liposomes. 总被引:5,自引:0,他引:5
R Reddy F Zhou S Nair L Huang B T Rouse 《Journal of immunology (Baltimore, Md. : 1950)》1992,148(5):1585-1589
The in vivo induction of a CTL response usually requires that Ag be endogenously synthesized so that appropriate processing can occur. In most of the few examples where successful CTL induction was reported with proteins and peptides, unacceptable adjuvants or means of Ag formulation were used. In the present report, liposomes were used to incorporate the soluble proteins OVA and beta-galactosidase. This simple and convenient to use approach, which requires minimal amounts of Ag, results in priming for a CD8+ CTL response and the establishment of immunologic memory. The liposome approach may not only prove a convenient means of inducing CTL responses in vivo but may also be useful to study the mechanisms of Ag processing. 相似文献