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排序方式: 共有671条查询结果,搜索用时 203 毫秒
1.
Evolution of the 28S ribosomal RNA gene in anurans: regions of variability and their phylogenetic implications 总被引:1,自引:0,他引:1
Fifteen restriction sites were mapped to the 28S ribosomal RNA gene of
individuals representing 54 species of frogs, two species of salamanders, a
caecilian, and a lungfish. Eight of these sites were present in all species
examined, and two were found in all but one species. Alignment of these
conserved restriction sites revealed, among anuran 28S rRNA genes, five
regions of major length variation that correspond to four of 12 previously
identified divergent domains of this gene. One of the divergent domains
(DD8) consists of two regions of length variation separated by a short
segment that is conserved at least throughout tetrapods. Most of the
insertions, deletions, and restriction-site variations identified in the
28S gene will require sequence-level analysis for a detailed reconstruction
of their history. However, an insertion in DD9 that is coextensive with
frogs in the suborder Neobatrachia, a BstEII site that is limited to
representatives of two leptodactylid subfamilies, and a deletion in DD10
that is found only in three ranoid genera are probably synapomorphies.
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2.
Characterization of cross-bridge elasticity and kinetics of cross-bridge cycling during force development in single smooth muscle cells 总被引:4,自引:2,他引:2
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Force development in smooth muscle, as in skeletal muscle, is believed to reflect recruitment of force-generating myosin cross-bridges. However, little is known about the events underlying cross-bridge recruitment as the muscle cell approaches peak isometric force and then enters a period of tension maintenance. In the present studies on single smooth muscle cells isolated from the toad (Bufo marinus) stomach muscularis, active muscle stiffness, calculated from the force response to small sinusoidal length changes (0.5% cell length, 250 Hz), was utilized to estimate the relative number of attached cross-bridges. By comparing stiffness during initial force development to stiffness during force redevelopment immediately after a quick release imposed at peak force, we propose that the instantaneous active stiffness of the cell reflects both a linearly elastic cross-bridge element having 1.5 times the compliance of the cross-bridge in frog skeletal muscle and a series elastic component having an exponential length-force relationship. At the onset of force development, the ratio of stiffness to force was 2.5 times greater than at peak isometric force. These data suggest that, upon activation, cross-bridges attach in at least two states (i.e., low-force-producing and high-force-producing) and redistribute to a steady state distribution at peak isometric force. The possibility that the cross-bridge cycling rate was modulated with time was also investigated by analyzing the time course of tension recovery to small, rapid step length changes (0.5% cell length in 2.5 ms) imposed during initial force development, at peak force, and after 15 s of tension maintenance. The rate of tension recovery slowed continuously throughout force development following activation and slowed further as force was maintained. Our results suggest that the kinetics of force production in smooth muscle may involve a redistribution of cross-bridge populations between two attached states and that the average cycling rate of these cross-bridges becomes slower with time during contraction. 相似文献
3.
A note on symbiosis of Legionella pneumophila and Tatlockia micdadei with human respiratory flora 总被引:1,自引:0,他引:1
Sixteen micro-organisms, representing clinically important respiratory microflora, were tested for their ability to stimulate the growth of Legionella pneumophila and Tatlockia micdadei in nutritionally-deficient agar media. Nutritional symbiosis, indicated by the appearance of satellite colonies of L. pneumophila or T. micdadei, was observed for H. influenzae and N. meningitidis. This interaction between normal flora and pathogens of the respiratory tract may have clinical relevance in the pathogenesis of Legionnaires' disease and Pittsburgh pneumonia. 相似文献
4.
Selective oxidative destruction of iron-sulfur clusters. Ferricyanide oxidation of Azotobacter vinelandii ferredoxin I 总被引:1,自引:0,他引:1
The destructive oxidation of aerobically isolated 7Fe Azotobacter vinelandii ferredoxin I [(7Fe)FdI] by Fe(CN)3-6 is examined using low-temperature magnetic circular dichroism (MCD) and EPR. The results demonstrate that oxidation of the [3Fe-3S] cluster occurs only after essentially complete destruction of the [4Fe-4S] cluster. It is therefore feasible by controlled Fe(CN)3-6 oxidation to obtain a partially metallated form of FdI, (3Fe)FdI, containing only a [3Fe-3S] cluster. The MCD and EPR data demonstrate that the [3Fe-3S] cluster in (3Fe)FdI is essentially identical in structure to that in the native protein. 相似文献
5.
Native x-ray diffraction data from single crystals of inactive aconitase from pig heart (Mr 80,000) have been collected on oscillation films to 2.7 A. Analysis shows that significant measurements of the anomalous scattering signal from the Fe-S cluster in the enzyme are available in the film data. The 5.0-A resolution anomalous difference Patterson function contains vectors for one Fe-S cluster (one aconitase molecule) per asymmetric unit in space group P2(1)2(1)2 with a = 173.6, b = 72.0, and c = 72.7 A. At 2.7-A resolution, the vector map is best interpreted by three Fe sites separated from each other by less than 3 A. The single-crystal diffraction data thus confirm the presence of a 3Fe center in the inactive form of aconitase. Furthermore, the data provide crystallographic evidence that 3Fe clusters exhibit structural heterogeneity. The Fe-Fe vectors cannot be interpreted in terms of 4-A distances as observed for the [3Fe-3S] cluster in Azotobacter ferrodoxin (Ghosh, D., O'Donnell, S., Furey, W., Robbins, A. H., and Stout, C. D. (1982) J. Mol. Biol. 158, 73-109). The results are therefore in agreement with a [3Fe-4S] cluster having 2.7-A Fe-Fe distances (Beinert, H., Emptage, M. H., Dreyer, J.-L., Scott, R. A., Hahn, J. E., Hodgson, K. O., and Thomson, A. J. (1983) Proc. Natl. Acad. Sci. U. S. A. 80, 393-396). However, the data do not unambiguously discriminate between this model and other 3Fe clusters having short Fe-Fe distances. 相似文献
6.
Disc Plate Method of Microbiological Antibiotic Assay: I. Factors Influencing Variability and Error 1
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Several factors are investigated that normally cause variation in zone diameters in conventional disc plate diffusion assay procedures. Of these factors the most serious is the unequal exposure of the individual plates at top or bottom of stacks to temperatures above and below room temperature. This unequal temperature exposure is avoided by novel handling and incubation procedures. A major variable, but one which can be controlled, is the varying time interval between pouring seeded agar and the time of applying the pads with antibiotic to the plates. This influence of time of setting and the effects of several other sequential operations are combined into a composite variable. This variable is then accounted for and normalized by interposing "external" reference plates set with a reference solution in the sequence of approximately 100 plates. No "internal" reference zones are employed. Such factors as volume of agar poured, wedge shape of agar in a dish, volumetric errors in dilutions, and timing considerations are studied and discussed. The results of this study form the basis for a test protocol which is presented in a following paper. 相似文献
7.
Effects of Dose on the Induction of Dominant-Lethal Mutations and Heritable Translocations with Ethyl Methanesulfonate in Male Mice 总被引:3,自引:2,他引:1
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W. M. Generoso W. L. Russell Sandra W. Huff Sandra K. Stout D. G. Gosslee 《Genetics》1974,77(4):741-752
Genetic damage by ethyl methanesulfonate (EMS) in male mice was measured at doses ranging from 50 to 300 mg/kg with dominant-lethal mutations and reciprocal translocations as endpoints. No appreciable increase in dominant-lethal mutations was detected following a dose of 100 mg/kg. Dominant lethals induced by EMS were convincingly detected only after a dose of 150 mg/kg, but in the translocation experiment an increase in the genetic effect was detectable at the 50 mg/kg dose. It is likely that dominant lethals had also been induced at the 50 and 100 mg/kg doses, but were not detected due to the relative insensitivity of the dominant..lethal procedure. Thus, for detection of low levels of EMS-induced chromosome breakage, translocations are a much more reliable endpoint than are dominant-lethal mutations. A procedure for large-scale screening of induced translocations is described.—The dominant-lethal dose-response curve, plotted on the basis of living embryos as a percentage of the control value, is clearly not linear as it is markedly concave downward. Similarly, the translocation dose-response curve showed a more rapid increase in the number of translocations with dose than would be expected on the basis of dose-square kinetics. It is clear for both of these endpoints that the effectiveness of EMS in inducing chromosome breakage is proportionately much lower at low doses. 相似文献
8.
C D Stout 《Journal of molecular biology》1989,205(3):545-555
The recently redetermined structure of the 7 Fe ferredoxin from Azotobacter vinelandii has been refined against a new 1.9 A data set. The crystallographic R-factor is 0.215 for all 9586 observed reflections 8.0 to 1.9 A. The model contains 106 amino acid residues, two Fe-S clusters and 21 water molecules. The root-mean-square deviations from ideality of bonds and angles are 0.014 A and 3.3 degrees, respectively. The refinement confirms the presence of two free cysteines: the thiol of C11 is in association with the side-chain of K100; the thiol of C24 is 3.35 A from inorganic sulfur of the [4 Fe-4 S] cluster. The refinement confirms a [3 Fe-4 S] model for the 3 Fe cluster. The two Fe-S clusters have similar bond distances and angles. The structure of the protein for residues 1 to 57 superposes within 0.85 A on residues 1 to 53 of the 8 Fe ferredoxin structure for main-chain N, CA and C atoms, if residues 9, 10, 29 and 30 of 7 Fe ferredoxin are omitted. These residues are part of two loops in contact with residues of the extended C-terminal chain of 7 Fe ferredoxin. 相似文献
9.
D L Stout 《Biochemical and biophysical research communications》1992,189(2):765-770
Porphyrin accumulation by proliferating cells, e.g., those associated with cancers or wounds, tends to correlate with increased transferrin receptor density. To determine whether transferrin might be implicated in porphyrin transport, fluorescence and absorption spectroscopy were used to study the interaction of porphyrins with transferrin. A single high-affinity binding site for heme and other porphyrins (Kd approximately 20-25 nM) was detected by fluorescence spectroscopy. Difference spectroscopy revealed three additional heme-binding sites. These sites were distinct from the iron-binding sites: 1) Apotransferrin and diferric transferrin bound porphyrins with equal affinity; 2) 59Fe was not displaced from transferrin by porphyrins. Murine erythroleukemia cells incubated with [59Fe]hemin-[125I]transferrin internalized both labels concomitantly. Accumulation of [59Fe]hemin could be blocked by a 100-fold excess of diferric transferrin but not by apotransferrin. These results indicate that cells can internalize exogenous heme, and possibly porphyrins, bound to transferrin via its receptor. 相似文献
10.
Michael J. Stout Kathi V. Workman Jeffrey S. Workman Sean S. Duffey 《Biochemical Systematics and Ecology》1996,24(7-8):611-625
Damage to foliage of the tomato, Lycopersicon esculentum, causes the induction of proteinase inhibitors and of the oxidative enzymes polyphenol oxidase, peroxidase, and lipoxygenase. The time courses of induction of these proteins by feeding of two caterpillar species (Manduca sexta and Helicoverpa zea) were studied in a series of experiments. In another series of experiments, the effects of plant age on the inducibility of these proteins were studied. In the time course experiments, induction of proteinase inhibitors and oxidative enzymes in the damaged leaflet was rapid, with higher protein activities evident in damaged leaflets within 12–24 h of damage, depending on the enzyme and the species of insect used to damage the plant. Systemic induction of proteinase inhibitors was also rapid, but systemic induction of polyphenol oxidase was delayed relative to systemic induction of proteinase inhibitors, possibly because high constitutive polyphenol oxidase activities obscured expression of systemic induction at earlier time points. Lipoxygenase and peroxidase were not induced systemically. Induction of all proteins persisted for at least 21 days. In the phenology experiments, inducibility of all proteins decreased in magnitude and was less consistent as plants aged. The results of these experiments exemplify the numerous constraints on induction in tomato plants. Knowledge of these physiological constraints is important to an understanding of the ecological role and causal basis of induced resistance. 相似文献