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61.
The sibling species of the Echinogammarus berilloni-group are endemic for the Iberian Peninsula and southern France. These species show wide morphological variability with some overlap in their dianostic characters making their distinction difficult. Reroductive isolation and enzmatic jivergence amon allopatric and sympatric populations of four species sharing the same chromosome number has been studied. The results show a clear genetic differentiation of E. longiserosus and E. calvus versus the other two species. However, E. margalefi and E. echinosetosus show no clear genetic differentiation between them, confirming their crose relationship. All four species often coexist in the same drainage system. Isozme analysis was employed to check the hypothesis Of Margalef that sympathy would occur age, long-term phenomena of speciation inside of a given basin with subsequent contact and overlap between the differentiated forms. Electrophoretic data were also used to determine whether one flow among gammarids populations exists. A model proosed by other authors according to which the heterozyosity decreases towards the headwaters foes not fit to the data we have obtained from E. calvus. Thus, populations of this species from sources and springs of the Duero basin show the hiFhest values of mean heterozygosity. The differentiation in this basin can be explained by drift. Migration between populations of different rivers is prevented by natural barriers. The lowest river stretches are without amhipods interrupting the gene flow amon populations. A correction between genetic and geographic fistances among subbasins and basins was found applying a double logarithmic model. A model of migration of E. calvus in the Duero basin is proposed on the basis of allelic frequencies and on the distribution of mean hetero-zygosities.  相似文献   
62.
In mast cells, basophils, and the RBL-2H3 tumor mast cell model, crosslinking cell surface IgE-receptor complexes by multivalent ligands activates a signal transduction pathway that leads to the secretion of histamine, serotonin, and other inflammatory mediators. Receptor crosslinking in RBL-2H3 cells also changes cell surface morphology and increases F-actin assembly. Previously, Robertson et al. demonstrated that crosslinked IgE-receptor complexes become associated with the Triton X-100-insoluble fraction (the "cytoskeleton") of RBL-2H3 cells and raised the possibility that receptor-cytoskeletal association may be a required step in the stimulation of secretion. The studies reported here confirm by flow cytometry that crosslinking cell surface IgE by antigen induces the association of the crosslinked complexes with the detergent-insoluble fraction. Dose-response studies, also reported here, indicate that the detergent insolubility of the complexes does not correlate with secretion. Thus, secretion increases with antigen concentration to a maximum beyond which more antigen causes less, not more, secretion. There is little residual detergent-insoluble IgE at the concentrations of antigen that promote optimal secretion, whereas the association of IgE with the detergent-insoluble fraction is maximal at the high antigen concentrations that result in reduced secretion. The addition of monovalent hapten to reduce the amount of crosslinking caused by high concentrations of antigen increases secretion and simultaneously reduces the association of IgE with the detergent-insoluble fraction. Dihydrocytochalasin B, an inhibitor of antigen-stimulated actin polymerization, also increases the rate and extent of secretion and simultaneously delays the association of crosslinked IgE-receptor complexes with the detergent-insoluble fraction. From these data, we propose that the association of crosslinked IgE receptors with the detergent-insoluble fraction of RBL-2H3 cells increases with increased receptor crosslinking, is enhanced by antigen-induced actin polymerization, and is more likely related to the termination than the stimulation of secretion. The ligand-induced conversion of receptors to a detergent-insoluble form is not restricted to mast cells but occurs in a variety of cell types. Its general function may be to limit the generation or transmission of transmembrane signals.  相似文献   
63.
RBL-2H3 cells have been widely used to study histamine release in vitro. It was previously shown that these cells undergo striking morphological changes after IgE-mediated secretion. The present study was undertaken to examine if the morphological changes were dependent on activation of the Fc epsilon receptor. Therefore, the cells were stimulated to release histamine by two different mechanisms: activation of the Fc epsilon receptor by antigen and treatment with the calcium ionophore A23187. Cell surface and cytoskeletal changes were examined by fluorescence microscopy and scanning electron microscopy after either IgE- or ionophore-mediated histamine release. After exposure of the cells to either secretagogue, the cells spread over the surface of the culture dish and underwent rearrangement of the cytoskeleton. In addition, scanning electron microscopy revealed that deep ruffles developed on the surface of the cells undergoing IgE-mediated release. The surface changes were not as pronounced with the ionophore. The distribution of the cytoskeletal elements was examined by immunofluorescence using FITC-phalloidin and antibodies against vimentin and tubulin. In unstimulated cells actin was localized at the cell periphery, just under the plasma membrane. In the stimulated cells it was associated with the cell periphery and concentrated in the surface ruffles. As the stimulated cells spread, intermediate filaments and microtubules became distributed throughout the cell body, but there was no obvious association with the membrane ruffles. These morphological changes were dependent on the presence of extracellular calcium and on the concentration of ionophore or antigen, and were also correlated with the amount of histamine released. Additionally, IgE-mediated stimulation led to increased uptake of the soluble-phase tracer Lucifer yellow, whereas stimulation with the ionophore A23187 showed no increase in Lucifer yellow internalization. Ionophore A23187 produced changes similar but not identical to those seen in the RBL-2H3 cells after IgE-mediated histamine release. The differences may be owing to the involvement of the Fc epsilon receptor in IgE-mediated secretion.  相似文献   
64.
65.
Elemental composition and feeding rate of hydromedusae Phialidium sp. on copepods were studied in the laboratory. Regression equations for both mature and immature medusae allowed the estimation of their dry weight (DW), total C and N content as a function of their diameter. The mean C content as percentage of the DW varied from 13.13% ( ) for the immature to 19.38% (5.68) for the mature individuals. The mean N content is 4.03% (2.49) of DW of immatures and 5.85% (2.70) of the matures. Ingestion rate of Phialidium sp. fed on copepods (200–500 μm) increased with prey density but reached a maximum at high prey concentrations. A maximum ingestion rate of 8.55 (1.6) copepods · medusa −1 · h−1 was reached for prey concentrations of > 140 copepods · 1 −1 for both immature and mature medusae. Maximum daily consumption of prey weight varied from 1.41 to 978% C body weight for mature medusae and from 2.90 to 975% for the immature individuals.  相似文献   
66.
Summary The sequence of the PcnB protein of Escherichia coli, a protein required for copy number maintenance of ColE1-related plasmids, was compared with the PIR sequence database. Strong local similarities to the sequence of the E. coli protein tRNA nucleotidyltransferase were found. Since a substrate of the latter protein, tRNA, structurally resembles the RNAs that control ColE1 copy number we believe that we may have identified a region in PcnB that interacts with these RNAs. Consistent with this idea is our observation that PcnB is required for the replication of R1, a plasmid whose replication is also regulated by a small RNA.  相似文献   
67.
68.
69.
Several classes ofsecA mutants have been isolated which reveal the essential role of this gene product forE. coli cell envelope protein secretion. SecA-dependent,in vitro protein translocation systems have been utilized to show that SecA is an essential, plasma membrane-associated, protein translocation factor, and that SecA's ATPase activity appears to play an essential but as yet undefined role in this process. Cell fractionation studies suggested that SecA protein is in a dynamic state within the cell, occurring in soluble, peripheral, and integral membraneous states. These data have been used to argue that SecA is likely to promote the initial insertion of secretory precursor proteins into the plasma membrane in a manner dependent on ATP hydrolysis. The protein secretion capability of the cell has been shown to translationally regulatesecA expression with SecA protein serving as an autogenous repressor, although the exact mechanism and purpose of this regulation need to be defined further.  相似文献   
70.
Barley seedlings (Hordeum vulgare L. Boone) were grown at 20°C with 16 h/8 h light/dark cycle of either high (H) intensity (500 mole m-2 s-1) or low (L) intensity (55 mole m-2 s-1) white light. Plants were transferred from high to low (H L) and low to high (L H) light intensity at various times from 4 to 8 d after leaf emergence from the soil. Primary leaves were harvested at the beginning of the photoperiod. Thylakoid membranes were isolated from 3 cm apical segments and assayed for photosynthetic electron transport, Photosystem II (PS II) atrazine-binding sites (QB), cytochrome f(Cytf) and the P-700 reaction center of Photosystem I (PS I). Whole chain, PS I and PS II electron transport activities were higher in H than in L controls. QB and Cytf were elevated in H plants compared with L plants. The acclimation of H L plants to low light occurred slowly over a period of 7 days and resulted in decreased whole chain and PS II electron transport with variable effects on PS I activity. The decrease in electron transport of H L plants was associated with a decrease in both QB and Cytf. In L H plants, acclimation to high light occurred slowly over a period of 7 days with increased whole chain, PS I and PS II activities. The increase in L H electron transport was associated with increased levels of QB and Cytf. In contrast to the light intensity effects on QB levels, the P-700 content was similar in both control and transferred plants. Therefore, PS II/PS I ratios were dependent on light environment.Abbreviations Asc ascorbate - BQ 2,5-dimethyl-p-benzoquinone - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCIP 2,6-dichlorophenolindophenol - H control plants grown under high light intensity - H L plants transferred from high to low light intensity - L low control plants grown under low light intensity - L H plants transferred from low to high light intensity - MV methyl viologen - P-700 photoreaction center of Photosystem I - QB atrazine binding site - TMPD N,N,N,N-tetramethyl-p-phenylenediamine Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 11990 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643, USA.  相似文献   
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