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71.
72.
Summary An allele (oliC31) of the A. nidulans oliC gene has been cloned using homology with the equivalent gene from N. crassa. OliC31 codes for an oligomycin-resistant, triethyltin-hypersensitive form of subunit 9 of the mitochondrial ATP synthase complex. Direct selection for oligomycin-resistance was possible following transformation of A. nidulans with the oliC31 gene. The phenotypes of transformants cultured in the presence of oligomycin were indicative of the position of integration of the transforming plasmid within the genome. Subsequent recombination events involving the integrated oliC31 gene were also apparent from altered levels of resistance to oligomycin or triethyltin. This gene should prove useful as a marker for transformation of strains lacking auxotrophic lesions and in gene replacement or disruption experiments.  相似文献   
73.
Summary Rainfall simulation at an average intensity of 124 mm·h-1 was used to compare infiltration and run off on arid areas where subterranean termites had been eliminated four years prior to the initiation of the study (termite free) with adjacent areas populated by subterranean termites (termites present). Infiltration rates on termite free plots with less than 5% perennial plant cover were significantly lower 51.3±6.8 mm·h-1 than rates on comparable termites present plots 88.4±5.6 mm·h-1. On plots centered on Larrea tridentata shrubs, there were no differences in infiltration rates with or without termites. Plots with shrub cover had the highest infiltration rates 101±6 mm·h-1. Highest run-off volumes were recorded from termite free <5% grass cover plots and the lowest from plots with shrubs. There were no differences in suspended sediment concentrations from termites present and termite free plots. Average bed load concentration was more than three times greater from termite free, <5% cover plots than from termites present, <5% cover plots.The reduction in infiltration, high run-off volumes and high bedloads from termite free areas without shrub cover is related to increased soil bulk density resulting from the collapse of subterranean galleries of the termites that provide avenues of bulk flow into the soil. Subterranean termites affect the hydrology of Chihuahuan desert systems by enhancing water infiltration and retention of top soil. The presence of a shrub canopy and litter layer cancels any effect of subterranean termites on hydrological parameters. Since approximately 2/3 of the area is not under shrub canopies, subterranean termites are considered to be essential for the maintenance of the soil water characteristics that support the present vegetation.  相似文献   
74.
Four monoclonal antibodies that are directed against antigens present in sperm and absent from other worm tissues were characterized. Antibody TR20 is directed against the major sperm proteins, a family of small, abundant, cytoplasmic proteins that have been previously described (Klass, M. R., and D. Hirsh, 1981, Dev. Biol., 84:299-312; Burke, D. J., and S. Ward, 1983, J. Mol. Biol., 171:1-29). Three other antibodies, SP56, SP150, and TR11, are all directed against the same set of minor sperm polypeptides that range in size from 29 to 215 kD. More than eight different sperm polypeptides are antigenic by both immunotransfer and immunoprecipitation assays. The three antibodies are different immunoglobulin subclasses, yet they compete with each other for antigen binding so they are directed against the same antigenic determinant on the multiple sperm proteins. This antigenic determinant is sensitive to any of six different proteases, is insensitive to periodate oxidation or N-glycanase digestion, and is detectable on a polypeptide synthesized in vitro. Therefore, the antigenic determinant resides in the polypeptide chain. However, peptide fragments of the proteins are not antigenic, thus the determinant is likely to be dependent on polypeptide conformation. The antigenic determinant shared by these proteins could represent a common structural feature of importance to the localization or cellular specificity of these proteins.  相似文献   
75.
Heterodimers of tyrosyl-tRNA synthetase from Bacillus stearothermophilus have been produced by mutagenesis at the subunit interface. Oppositely charged groups have been engineered into the subunits so that they can form a complementary pair. Wild-type tyrosyl-tRNA synthetase is a symmetrical dimer in which the side chains of the 2 Phe-164 residues interact at the subunit interface. Phe-164 was mutated to Asp in tyrosyl-tRNA synthetase and to Lys in a truncated enzyme (des-(321-419)tyrosyl-tRNA synthetase) which lacks the two tRNA-binding sites, but which can catalyze pyrophosphate exchange. The size difference allows subunit association to be studied by gel filtration chromatography. These changes induce reversible dissociation from active dimers into inactive monomers at pH values which favor ionization at position 164. A mixture of the two mutants near neutral pH is apparently fully active in pyrophosphate exchange and consists of a heterodimer of [Asp164]tyrosyl-tRNA synthetase and [Lys164]des-(321-419)tyrosyl-tRNA synthetase. Despite having only one binding site for tRNA, heterodimer has full aminoacylation activity at high concentrations of tyrosine. We have therefore produced a family of dimers that differ in stability near neutral pH. This novel approach using protein engineering allows specific dimerization of subunits of the same size that have different defined mutations, each subunit being tagged by the charge. Such hybrid proteins can be used to study subunit interaction.  相似文献   
76.
When Arbacia punctulata spermatozoa are incubated in seawater containing ammonium hydroxide (pH 8.8), the sperm plasma membrane-bound guanylate cyclase is dephosphorylated, its electrophoretic mobility increases (from an apparent molecular mass of 160 to 150 kD), and its enzymatic activity decreases 3.5-fold. Transfer of these cells into ammonium-free seawater (pH 7.4) results in the rephosphorylation of the cyclase, its reconversion to 160 kD, and recovery of the enzymatic activity lost upon dephosphorylation. This is the first direct demonstration that the activity of membrane-bound guanylate cyclase can be regulated by phosphorylation. A plasma membrane preparation is described that specifically supports the in vitro phosphorylation of the guanylate cyclase. This preparation will be useful in more detailed studies on the relationship between phosphorylation state and enzymatic activity of membrane-bound guanylate cyclase.  相似文献   
77.
Summary Stem cells of the embryonal carcinoma cell line called H6 can be induced to differnetiate to endoderm-like cells by retinoic acid (3×10−6 M). We have detected a diffusible and stable factor which is secreted by H6 endoderm-like cells and stimulates the growth of H6 stem cells. The stimulation by the endoderm-like cells is considereably greater than that by mouse fibroblasts or H6 stem cells themselves. No reciprocal stimulation of endoderm-like cells by stem cells occurs. Part but not all of the stimulation might be due to extracellular matrix proteins or to insulin-like growth factor type 2, each of which also stimulates the growth of H6 stem cells. Insulin causes no such stimulation. This work was supported by research rant no. CA-16754 from the National Cancer Institute to J. W. L. E. L. G. was supported by an American Heart Association Medical Student Research Award. Editor's Statement This paper presents a good example of cooperativity between undifferentiated teratoma stem cells and differentiated parietal endoderm-derived countrparts in terms of growth support. It raises the interesting question of the relationship between factors produced by paprietal and visceral endoderm cells. Gordon H. Sato  相似文献   
78.
Immunofluorescence assays for marine ammonium- and nitrite-oxidizing bacteria were used to assess the diversity of nitrifying bacteria isolated from marine environments. The antisera show relatively broad specificity, in that each reacts with several strains of the same physiological type as the strain to which the antiserum was prepared. The antisera do not, however, react with any strains of differing physiological type. Seventy percent of the 30 unidentified ammonium-oxidizing isolates tested reacted with one or both of the antisera produced to marine ammonium-oxidizing strains, and 8 of the 9 unidentified nitrite-oxidizing strains tested reacted with 1 or more of the 3 nitrite oxidizer antisera used. Ammonium- and nitrite-oxidizing bacteria were enumerated in samples taken in a depth profile (to 750 m) in the Southern California Bight by immunofluorescence assays for two ammonium oxidizers and two nitrite oxidizers. Average abundances of the two types of nitrifiers were 3.5 × 105 and 2.8 × 105 cells liter−1, respectively. Nitrifiers constitute 0.1 to 0.8% of the total bacterial population in these samples.  相似文献   
79.
The primary antibody response of different mouse strains challenged with two antigenic forms of alpha (1-3) dextran, dextran B1355S and dextran-hemocyanin, was examined. Only BALB/c mice responded with both kappa and lambda antibodies. The kappa to lambda ratio was affected by factors such as the antigenic form of dextran, the time at which the serum was analyzed, and the priming regimen. Surprisingly, priming with hemocyanin in adjuvant increased the kappa portion of the response not only to dextran-hemocyanin but also to dextran B1355S. Other strains of mice responded with only lambda antibodies. These results extend our previous results on the analysis of dextran-specific B cell precursors.  相似文献   
80.
Immunological tolerance represents an individual's nonresponsiveness to self-antigens or foreign antigens. Inasmuch as the mechanism of self-nonself discrimination tolerance is unknown, it remains an area of active investigation because of its implied involvement in certain immune disorders. In this paper we discuss neonatal idiotype suppression and internal idiotope vaccination as models for B cell tolerance. The results demonstrate opposite aspects of idiotope regulation. Neonatal idiotype suppression represents an anti-idiotype-induced establishment of tolerance, whereas an internal antigen vaccine overcomes an established tolerance to self-antigens. New views on the functional importance of idiotype self-tolerance are discussed.  相似文献   
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