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31.
Cells transformed by Polyoma virus (Py) can undergo a high rate of excision or amplification of integrated viral DNA sequences, and these phenomena require the presence of homology (i.e., repeats) within the viral insertion as well as a functional viral large T antigen (T-Ag). To determine whether the main role of large T-Ag in excision and amplification was replicative or recombination-promoting, we studied transformed rat cell lines containing tandem insertions of a ts-a Py molecule (encoding a thermolabile large T-Ag) with a deletion of the origin of viral DNA replication. Culturing of these cells at the temperature permissive for large T-Ag function did not result in any detectable excision or amplification of integrated Py sequences. We then introduced into origin-defective lines a recombinant plasmid containing the viral origin of replication and the gene coding for resistance to the antibiotic G418. All G418-resistant clones analyzed readily amplified the integrated plasmid molecules when grown under conditions permissive for large T-Ag function, showing that these cells produced viral large T-Ag capable of promoting amplification in trans of DNA sequences containing the Py origin. These observations strongly suggest that Polyoma large T antigen promotes excision or amplification of viral DNA by initiating replication at the integrated origin, providing a favorable substrate for subsequent recombination. 相似文献
32.
From ligulate flowers of Matricaria chamomilla was isolated a mixture of apigenin 7-O-β-glucoside diacetates, which was shown to be based on (2″, 3″)- and (3″, 4″)-diacetates. 相似文献
33.
Ettore Benedetti Benedetto Di Blasio Vincenzo Pavone Carlo Pedone Claudio Toniolo Gian Maria Bonora 《International journal of biological macromolecules》1980,2(4):217-224
The X-ray diffraction and i.r. absorption conformational analysis of has showed the absence of intramolecularly hydrogen-bonded peptide conformations in the solid state. The molecules are held together in rows of ‘cyclic dimer’ motifs through intermolecular NH … OC (acid) and OH … OC {urethane} hydrogen bonds, the secondary amide-like group of the urethane moiety being in the unusual cis conformation, whereas the carboxylic acid group in the common syn conformation. The two molecules in the unit cell present a centrosymmetric set of ?, ψ1, and ψ2 values. In polar solvents solvated species largely predominate. In saturated hydrocarbon solution non-associated and associated (mostly involving the carboxylic acid CO as the proton acceptor) species simultaneously occur. The extent of association decreases with dilution. The amount of intramolecularly hydrogen-bonded oxy-C7 and C5 forms if any, should be extremely small. The type of association at saturation seems to differ from that found in the crystalline compound obtained by precipitation with saturated aliphatic hydrocarbons (from a diethyl ether solution). 相似文献
34.
Elisabetta Colletti Laura Frontali Claudio Palleschi Micheline Wesolowski Hiroshi Fukuhara 《Molecular & general genetics : MGG》1979,175(1):1-4
Summary In S. cerevisiae four isoacceptor mitochondrial tRNAs for serine have been separated by reversed phase chromatography. At least two of these species are products of different genes. In this work the deletion mapping technique has been used to locate two genes for tRNAser. The gene for tRNAser previously localized in the oli I region of the mitochondrial genome has been found to code for tRNA
ser
2
, and another gene coding for tRNA
ser
1
has been detected in the region where most of other tRNA genes are found. Results of fine mapping experiments allowed to localize this gene in the proximity of the gene for tRNAarg. 相似文献
35.
A temperature-sensitive mutant of BHK, designated is BN-2, shows a rapid drop in 3H-thymidine incorporation along with accumulation of the cells in the G1 phase of the cycle when asynchronous cultures are shifted from 33.5°C to the nonpermissive temperature of 39.5°C. Synchronized cultures of ts BN-2 cells did not enter DNA synthesis when shifted up in G1. Shift-up of cultures at the beginning of the S phase resulted in an approximately normal rate of DNA synthesis for about 2 hr. The rate of DNA synthesis then quickly declined, and the cells became arrested in mid-S after completion of approximately 0.5 rounds of DNA replication. At the same time, the majority of the cells were observed to lose the nuclear membrane and displayed premature chromosome condensation. These events were followed by the appearance of cells containing several micronuclei and eventual cell disruption and death. The nonpermissive temperature appeared to have no effect on either the elongation of short fragments of DNA or the execution of mitosis after the completion of the S phase under permissive conditions. The ts defect in this mutant may directly limit the initiation of DNA synthesis or alter the regulation of chromatin condensation. 相似文献
36.
Giancarlo Lunazzi Claudio Tiribelli Bruno Gazzin Gianluigi Sottocasa 《生物化学与生物物理学报:生物膜》1982,685(2):117-122
Bilitranslocase, a plasma membrane protein involved in bilirubin and other organic anion uptake by the liver, exhibits a high molecular weight (170 000) when isolated in the presence of deoxycholate. This value is decreased to approx. 100 000 if deoxycholate is not included in the isolation medium. Both preparations can be resolved into two kinds of subunit, α and β, of 37 000 and 35 500, respectively, by reduction with 2-mercaptoethanol and addition of sodium dodecyl sulfate. Under these conditions the two subunits are still capable of high-affinity sulfobromophthalein binding and, despite the presence of the detergent, may be isolated by preparative polyacrylamide gel electrophoresis still associated with the dye. It may be suggested that the physiological subunit composition of bilitranslocase is α2-β. 相似文献
37.
Claudio D. Stern 《Experimental cell research》1982,140(2):468-471
The dorso-ventral polarity of cells within sheets of epiblast from early chick embryos can be experimentally reversed by applying low voltages across them in experimental chambers. The induced reversal is stable and affects the position of histochemically and ultrastructurally demonstrable markers. 相似文献
38.
Extracellular synaptic factors induce clustering of acetylcholine receptors stably expressed in fibroblasts 下载免费PDF全文
The clustering of nicotinic acetylcholine receptors (AChRs) is one of the first events observed during formation of the neuromuscular junction. To determine the mechanism involved in AChR clustering, we established a nonmuscle cell line (mouse fibroblast L cells) that stably expresses just one muscle-specific gene product, the AChR. We have shown that when Torpedo californica AChRs are expressed in fibroblasts, their immunological, biochemical, and electrophysiological properties all indicate that fully functional cell surface AChRs are produced. In the present study, the cell surface distribution and stability of Torpedo AChRs expressed in fibroblasts (AChR-fibroblasts) were analyzed and shown to be similar to nonclustered AChRs expressed in muscle cells. AChR-fibroblasts incubated with antibodies directed against the AChR induced the formation of small AChR microclusters (less than 0.5 micron 2) and caused an increase in the internalization rate and degradation of surface AChRs (antigenic modulation) in a manner similar to that observed in muscle cells. Two disparate sources of AChR clustering factors, extracellular matrix isolated from Torpedo electric organ and conditioned media from a rodent neuroblastoma-glioma hybrid cell line, each induced large (1-3 microns 2), stable AChR clusters with no change in the level of surface AChR expression. By exploiting the temperature-sensitive nature of Torpedo AChR assembly, we were able to demonstrate that factor-induced clusters were produced by mobilization of preexisting surface AChRs, not by directed insertion of newly synthesized AChRs. AChR clusters were never observed in the absence of extracellular synaptic factors. Our results suggest that these factors can interact directly with the AChR. 相似文献
39.
Mario Maggi Guido Fantoni Alessandro Peri Stefano Giannini Maria Luisa Brandi Claudio Orlando Mario Serio 《The Journal of steroid biochemistry and molecular biology》1991,40(4-6):481-491
Oxytocin (OT) and V1 vasopressin (VP) receptors are present simultaneously in several tissues, including the uterus. In myometrium these receptors mediate contractility, while in endometrium they mediate the release of other uterotonic substances as endothelin (ET). In rabbit myometrium, estrogens increase, while progesterone blunts neurohypophysial hormone receptors. However, the action of sex steroids on OT and V1 VP receptors differs in terms of the ED50 and maximal effect. Therefore, at parturition, only OT receptors show a dramatic rise, while V1 VP receptors do not change, suggesting a major role for OT in labor. ET is a potent stimulator of uterine activity acting through specific receptors present on myometrial cells. These receptors as well as the endometrial localization of ET are modulated by sex steroids, indicating that ET might represent a paracrine regulator of uterine activity. In humans, OT but not V1 VP receptors increase as pregnancy progresses, confirming the primary relevance of OT in timing delivery. 相似文献
40.
Pierangelo Luporini Cristina Miceli Claudio Ortenzi Adriana Vallesi 《Genesis (New York, N.Y. : 2000)》1992,13(1):9-15
Euplotes raikovi, like other ciliates, passes through a postconjugal immaturity, operatively identified by an apparent cell inability to form mating pairs under experimental conditions that are the same as those used for inducing mating at maturity. In cells homozygous for the gene mat-2, which controls the pheromone Er-2, Er-2 mRNA synthesis and mature Er-2 secretion were shown to start from the very beginning of the life cycle and continue throughout immaturity, although to extents estimated to be 5- to 10-fold lower than at maturity. In addition, experiments of 125 I-Er-2 binding and crosslinking provided evidence that autocrine pheromone-binding sites, showing values of the dissociation constant of the order of 10?9 M, are on the surface of immature cells. The number of these sites per cell was estimated to increase from less than 106 per cell of 5–7 fissions of age, to about 16 × 106 at maturity. These results were taken to suggest that a pheromone-receptor production is stimulated during immaturity by autocrine pheromone binding to cells and that this production might be essential for the development of a pheromone-receptor density high enough to transform the cell from “immature” to “adult,” that is competent to respond as well to pheromones of conspecific, genetically different cells. © 1992 Wiley-Liss, Inc. 相似文献