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991.
Escherichia coli strain CAN is unable to support the growth of bacteriophage T4 strains requiring the suppressor function of T4 tRNASer. Biochemical analysis of the mutant strain revealed that it is deficient in a RNase which acts on the artificial tRNA precursor tRNA-C-U. 相似文献
992.
Kinetics of beta-glucuronidase induction by androgen. Genetic variation in the first order rate constant 总被引:3,自引:0,他引:3
Measurements of enzyme activity, rates of protein synthesis, and mRNA activity suggest that the induction of beta-glucuronidase in mouse kidney in response to androgen is regulated at a pretranslational level. Following an initial lag period, the rate and extent of induction follow the rules of simple turnover kinetics and can be described in terms of a zero order rate constant for acquisition of mRNA activity (ka) and a first order rate constant for loss of activity (kb). Genetic variation in kb, described here for the first time, alters the half-time and extent of induction. Variation in kb is independent of previously described variation in ka and, unlike changes in ka, is not associated with change in the lag time. The DNA sequences determining kb, like those determining ka, are genetically linked to the structural gene for beta-glucuronidase. Following the removal of androgen, beta-glucuronidase activity, rate of synthesis, and mRNA activity all decline rapidly with half-lives of 1-2 days. Even in the most rapidly inducing strains, this is significantly faster than the half-time for induction determined by kb. Furthermore, genetic variation in kb does not affect the rate of de-induction. These facts suggest that kb may not describe the turnover of beta-glucuronidase mRNA, but rather the turnover of another step in the induction process. 相似文献
993.
A simple enucleation technique that facilitates autoradiographic and electron microscopic examination of cytoplasms is described. Cells were grown on commercially available plastic Leighton coverslips and these were centrifuged in the presence of cytochalasin B. The centrifugation requires no special holders and only a high speed centrifuge. Enucleation frequencies of greater than 90% were obtained for Chinese hamster fibroblasts and mouse B-82 cells. 相似文献
994.
M. von Balthazar K. Raunsgaard Pedersen E. M. Friis 《Plant Systematics and Evolution》2005,255(1-2):55-75
A charcoalified fossil flower bud of a new genus and species (Teixeiria lusitanica) is described from the Early Cretaceous of Portugal. The flower is actinomorphic and unisexually male. At the base of the
bud there are several bracts of different sizes, which are followed by sepal-like and petal-like tepals. Bracts and perianth
organs seem to be arranged spirally and to exhibit transitions between different organ categories. The androecium has numerous
stamens in two sizes, but with unclear arrangement. Pollen is small and tricolpate with a perforate tectum and a densely columellate
infratectal layer. No carpels or remains of carpels could be observed on the floral axis. Teixeiria lusitanica shows most affinities to members of Ranunculales. There are also some similarities with Berberidopsis (Berberidopsidaceae, Berberidopsidales) and members of the Saxifragales (Hamamelidaceae and Daphniphyllaceae). 相似文献
995.
996.
Management of the Wadden Sea 总被引:1,自引:0,他引:1
The Wadden Sea situated along the North Sea coasts of Denmark, the Federal Republic of Germany and The Netherlands represents
one of the world's largest bar-built type of estuaries. The area is a typical sedimentation and mineralization basin, with
a large influx of organic matter from the adjoining North Sea, consequently a delicate oxygen balance and a rich benthic macrofauna,
poor in species, which serves as food for juveniles of some commercially important North Sea fishes and for large numbers
of migrating and wintering waders and waterfowl. Past and present activities of the human society in the area include fisheries
(mainly for shrimp and mussels, semi-culture), shipping, land reclamation, recreation, dredging for sand and shells, and waste
discharge from industries and human communities. Until the present these activities, although sometimes conflicting, did not
fundamentally affect the area and its biota (pollution excluded), but future claims, including the construction of large deep-sea
harbours, drilling for natural gas and oil, large-scale land reclamation and increased industrialization etc., might gradually
induce degradation. For instance, area reduction by continued land reclamation could lead to irreversible losses of specific
biotopes (e. g. salt-marshes, mud-flats), which could affect the size of bird and fish populations in a much wider region.
Increased pollution, which has already inflicted damage on bird and seal populations, could reduce the fauna and hence the
value of the area as a natural sanctuary. In the event of a proposal for a new human activity in the area, the present standing
practice in the countries concerned requires an evaluation of its safety and economic aspects and its environmental impact.
However, the various plans are considered separately and there is a general need for integrated management of the area. 相似文献
997.
O. S. Mashkina T. M. Tabatskaya A. I. Gorobets K. A. Shestibratov 《Applied Biochemistry and Microbiology》2010,46(8):769-775
Conditions of cultivation and micropropagation of selected biotypes of five willow species (Salyx dasyclados Wimm., S. caspica Pall., S. triandra L., S. purpurea L., and S. viminalis L.) and two hybrids (×S. acuminata S. and ×S. palustris Host.) were optimized. Data on in vitro propagation of S. caspica, S. triandra, S. purpurea together with hybrids S. acuminata and S. palustris were obtained for the first time. It has been demonstrated that the outcome of cultivation and propagation of willows strongly
depends on genotypic peculiarities of initial plants. The optimal terms of isolation and sterilization of single-node segments
for obtaining 50–75% of aseptic viable developing cultures were estimated. The nutritive media were selected providing induction
of stem development (to 67%), their rooting (to 91%), elongation (to 3–6 cm), and multiplication (propagation coefficient
of 4). The designed method (adopted to different genotypes) can be applied for obtaining aseptic in vitro cultures serving
as initial plant material for genetic transformation and mass propagation of plants with new agriculturally valuable characteristics
which are of interest for construction of bioenergetic plantations and for needs of the paper industry. 相似文献
998.
S L Rakestraw W E Ford R G Tompkins M A Rodgers W P Thorpe M L Yarmush 《Biotechnology progress》1992,8(1):30-39
A set of anti-melanoma immunoconjugates were prepared which contained chlorin e6: antibody molar ratios of 18.9:1, 11.2:1, 6.8:1, and 1.7:1. All immunoconjugates retained antigen binding activity regardless of the chromophore:antibody substitution ratio that was attained. In contrast, the ground-state absorption spectra of the immunoconjugates showed features which appeared to be dependent on the chromophore:antibody molar ratio. In addition, the quantum yield of singlet oxygen generated by the conjugated chromophores was observed to be significantly less than that observed with the unbound dye. Time-resolved absorbance spectroscopy of the chromophore excited triplet state indicated that the loss of singlet oxygen quantum yield resulted from diminished chromophore triplet yield. Analysis of data obtained from in vitro photolysis of target melanoma cells, in combination with that obtained from the immunochemical and photochemical studies, indicates that the observed immunoconjugate phototoxicity can be reasonably quantitatively represented by (1) the ability of the immunoconjugate to bind SK-MEL-2 cell surface antigen, (2) the amount of chromophore localized at the target cells by immunoconjugate binding, (3) the delivered dose of light at 634 nm, and (4) the singlet oxygen quantum yield of the antibody-bound photosensitizer. Though these data argue strongly for photolysis by the cumulative dosage of singlet oxygen at the cell membrane, nonetheless, the concurrent photoinduced release of other cytotoxic agents should not be ruled out. 相似文献
999.
K Cureton P Bishop P Hutchinson H Newland S Vickery L Zwiren 《European journal of applied physiology and occupational physiology》1986,54(6):656-660
Ten men and 11 women were studied to determine the effect of experimentally equating haemoglobin concentration ([Hb]) on the sex difference in maximal oxygen uptake (VO2max). VO2max was measured on a cycle ergometer using a continuous, load-incremented protocol. The men were studied under two conditions: 1) with normal [Hb] (153 g X L-1) and 2) two days following withdrawal of blood, which reduced their mean [Hb] to exactly equal the mean of the women (134 g X L-1). Prior to blood withdrawal, VO2max expressed in L X min-1 and relative to body weight and ride time on the cycle ergometer test were greater (p less than .01) in men by 1.11 L X min-1 (47%), 4.8 ml X kg-1 min-1 (11.5%) and 5.9 min (67%), respectively, whereas VO2max expressed relative to fat-free weight (FFW) was not significantly different. Equalizing [Hb] reduced (p less than .01) the mean VO2max of the men by 0.26 L X min-1 (7.5%), 3.2 ml X kg-1 min-1 (6.9%) or 4.1 ml X kg FFW-1 min-1 (7.7%), and ride time by 0.7 min (4.8%). Equalizing [Hb] reduced the sex difference for VO2max less than predicted from proportional changes in the oxygen content of the arterial blood and arteriovenous oxygen content difference during maximal exercise. It was concluded that the sex difference in [Hb] accounts for a significant, but relatively small portion of the sex difference in VO2max (L X min-1). Other factors such as the dimensions of the oxygen transport system and musculature are of greater importance. 相似文献
1000.
A simple modification of nuclear staining after acid hydrolysis has been made which provides easy identification of quail nuclear markings in a chick-quail chimera. This method also improves the histologic detail normally seen with hematoxylin and eosin when compared to the more commonly used Feulgen reaction. Embryonic tissues can be fixed in Zenker's or Helly's solution and the sections obtained are hydrolyzed in acid (3.5 N HCl at 37 C for 40-50 min). After acid hydrolysis the sections are stained with hematoxylin and eosin rather than Schiff reagent and fast green. The interphase nuclei of chick cells show homogeneous or mottled purplish blue staining, while quail nuclei contain a dark blue spot. This staining corresponds to the reddish purple staining of the quail's heterochromatin seen adjacent to the nucleolus in the standard Feulgen stain. This new technique facilitates identification of quail cell types in the chick host and provides superior histology of the chick tissues by demonstrating cytoplasmic detail. 相似文献