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排序方式: 共有481条查询结果,搜索用时 31 毫秒
81.
The enzymatic conversion of 3'-phosphate terminated RNA chains to 2',3'-cyclic phosphate derivatives 总被引:1,自引:0,他引:1
The enzyme, RNA cyclase, has been purified from cell-free extracts of HeLa cells approximately 6000-fold. The enzyme catalyzes the conversion of 3'-phosphate ends of RNA chains to the 2',3'-cyclic phosphate derivative in the presence of ATP or adenosine 5'-(gamma-thio)triphosphate (ATP gamma S) and Mg2+. The formation of 1 mol of 2',3'-cyclic phosphate ends is associated with the disappearance of 1 mol of 3'-phosphate termini and the hydrolysis of 1 mol of ATP gamma S to AMP and thiopyrophosphate. No other nucleotides could substitute for ATP or ATP gamma S in the reaction. The reaction catalyzed by RNA cyclase was not reversible and exchange reactions between [32P]pyrophosphate and ATP were not detected. However, an enzyme-AMP intermediate could be identified that was hydrolyzed by the addition of inorganic pyrophosphate or 3'-phosphate terminated RNA chains but not by 3'-OH terminated chains or inorganic phosphate. 3'-[32P](Up)10Gp* could be converted to a form that yielded, (Formula: see text) after degradation with nuclease P1, by the addition of wheat germ RNA ligase, 5'-hydroxylpolynucleotide kinase, RNA cyclase, and ATP. This indicates that the RNA cyclase had catalyzed the formation of the 2',3'-cyclic phosphate derivative, the kinase had phosphorylated the 5'-hydroxyl end of the RNA, and the wheat germ RNA ligase had catalyzed the formation of a 3',5'-phosphodiester linkage concomitant with the conversion of the 2',3'-cyclic end to a 2'-phosphate terminated residue. 相似文献
82.
Assemblage of the prespliceosome complex with separated fractions isolated from HeLa cells 总被引:5,自引:0,他引:5
R Pruzan H Furneaux P Lassota G Y Hong J Hurwitz 《The Journal of biological chemistry》1990,265(5):2804-2813
The first ATP-dependent complex formed in pre-mRNA splicing is the prespliceosome, a 30 S complex. This reaction was investigated using partially purified fractions isolated from nuclear extracts of HeLa cells. Previous studies (Furneaux, H. M., Perkins, K. K., Freyer, G. A., Arenas, J., and Hurwitz, J. (1985) Proc. Natl. Acad. Sci. U. S. A. 82, 4351-4355) have shown that DEAE-cellulose chromatography of nuclear extracts yielded two fractions (fractions I and II, eluted at 0.2 and 1 M NaCl, respectively) which carried out pre-mRNA splicing only when combined. Fraction II, alone and in the presence of ATP, supported the formation of the 30 S complex. In this report, we have separated fraction II into ribonucleoprotein and protein-rich fractions by isopycnic banding in CsCl. The combination of these two fractions completely replaced fraction II in prespliceosome formation; when supplemented with fraction Ib (1 M NaCl Biorex fraction derived from fraction I), the preparations supported spliceosome formation; when supplemented with fraction I, they yielded spliced products. The CsCl fractions, like fraction II, efficiently converted pre-mRNA to the 30 S complex with high yields (30-70%). The 30 S complex was shown to contain pre-mRNA complexed to U2 small ribonucleoproteins and small amounts of U1 small ribonucleoproteins. The 30 S complex protected a 50-nucleotide region at the 3'-end of the intron from T1 RNase attack. This region included sequences spanning the branch site, the polypyrimidine stretch and the AG dinucleotide of the 3'-splice site. When the 30 S complex was first generated with partially purified fractions, followed by the addition of a large amount of poly(U) or unlabeled pre-mRNA, the 30 S complex could be chased into a 55 S spliceosome complex by the addition of fraction Ib. These results support the conclusion, initially derived from kinetic data, that the 30 S complex is a precursor of the 55 S complex. 相似文献
83.
Immortalization of rat embryo fibroblasts by an adenovirus 2 mutant expressing a single functional E1a protein. 总被引:7,自引:3,他引:4
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Expression of the adenovirus E1a and E1b genes is required for transformation of nonpermissive rodent cells. Differential splicing of the E1a precursor RNA molecules results in the production of two early mRNAs, 13S and 12S, which encode a 289-amino-acid-residue (289R) and 243R protein, respectively. Previously we constructed a mutant virus, dl231, which can only produce normal 289R protein from the E1a gene. In this report we demonstrate that dl231 induced focal transformation of primary rat embryo fibroblasts at 20% of the level of wild-type virus. dl231 transformants were immortalized and produced normal levels of E1a 13S and E1b mRNAs but only minute levels of defective E1a 12S mRNA. These transformants only minimally expressed the transformation phenotype and were similar to untransformed cells. Unlike wild-type transformants, they had a more fibroblastic morphology, were contact inhibited, grew to only low saturation density, and were limited in their ability to grow in an anchorage-independent manner in soft agar. We conclude that the 289R E1a protein can mediate immortalization of primary cells and that the 243R E1a protein is required to elicit the full transformation phenotype. 相似文献
84.
85.
F. Suzette Smith Samantha D. Rencher Helen E. Heslop Julia L. Hurwitz 《Cancer immunology, immunotherapy : CII》1995,41(2):104-110
Allogeneic bone marrow transplantation (BMT) has become a therapy of choice for the treatment of certain malignancies and hematopoietic disorders. However, immunodeficiencies following BMT continue to cause significant morbidity and mortality. We have compared the T cell receptor (TCR) repertoire of BMT patients and healthy control individuals by staining peripheral blood mononuclear cells with fluorochrome-labeled TCR-specific antibodies. Several patients exhibited a biased pattern of TCR expression atypical of the healthy controls, yet no particular TCR bias characterized all patients. For example, we found that 2%–8% of T cell from healthy individuals expressed the V19 TCR. One BMT patient exhibited V19 expression on more than 60% of peripheral T cells, while additional patients expressed V19 on less than 1% of T cells. The patients with the most extreme skewing of TCR types suffered from graft-versus-host disease. The causes of skewed TCR V expression patterns in BMT patients are not fully understood, yet some researchers have suggested that an oligoclonal expansion of CD8+ T cell populations may be largely responsible. To test this hypothesis, we examined the TCR V repertoire of CD4+ and CD8+ T cell populations. We found that biased V expression characterized both CD4+ and CD8+ T cell populations, sometimes within a single individual. Thus, therapies directed toward CD8+ T cells alone may not fully correct repertoire abnormalities following BMT. 相似文献
86.
The 18S ribosomal RNAs of 21 tetrapods were sequenced and aligned with five
published tetrapod sequences. When the coelacanth was used as an outgroup,
Lissamphibia (living amphibians) and Amniota (amniotes) were found to be
statistically significant monophyletic groups. Although little resolution
was obtained among the lissamphibian taxa, the amniote sequences support a
sister-group relationship between birds and mammals. Portions of the 28S
ribosomal RNA (rRNA) molecule in 11 tetrapods also were sequenced, although
the phylogenetic results were inconclusive. In contrast to previous
studies, deletion or down- weighting of base-paired sites were found to
have little effect on phylogenetic relationships. Molecular evidence for
amniote relationships is reviewed, showing that three genes
(beta-hemoglobin, myoglobin, and 18S rRNA) unambiguously support a
bird-mammal relationship, compared with one gene (histone H2B) that favors
a bird- crocodilian clade. Separate analyses of four other genes (alpha-
crystallin A, alpha-hemoglobin, insulin, and 28S rRNA) and a combined
analysis of all sequence data are inconclusive, in that different groups
are defined in different analyses and none are strongly supported. It is
suggested that until sequences become available from a broader array of
taxa, the molecular evidence is best evaluated at the level of individual
genes, with emphasis placed on those studies with the greatest number of
taxa and sites. When this is done, a bird-mammal relationship is most
strongly supported. When regarded in combination with the morphological
evidence for this association, it must be considered at least as plausible
as a bird-crocodilian relationship.
相似文献
87.
Samuel Edelstein Arie Harell Arie Bar Shmuel Hurwitz 《Biochimica et Biophysica Acta (BBA)/General Subjects》1975,385(2):438-442
Radioactively labelled cholecalciferol was administered continuously to chicks that were fed normal, low-calcium and low-phosphorus diets. It has been possible to show that under such steady state conditions with regard to cholecalciferol, and mineral restriction, the animal reacts by increased accumulation of 1,25-dihydroxycholecalciferol in the intestinal and the kidney cell, which was associated in the intestine with an increased calcium-binding activity. A similar accumulation of 1,25-dihydroxycholecalciferol in bone was not noticed.It is proposed that the intestine and the kidney, but not bone, are the main target organs for cholecalciferol in the maintenance of calcium homeostasis, and that both calcium and phosphorus play a role in the regulation of the formation and subsequent function of 1,25-dihydroxycholecalciferol. 相似文献
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