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61.
Fabrizio Arigoni P. Alexandre Kaminski Hauke Hennecke Claudine Elmerich 《Molecular & general genetics : MGG》1991,225(3):514-520
Summary The nucleotide sequence of a 4.1 kb DNA fragment containing the fixABC region of Azorhizobium caulinodans was established. The three gene products were very similar to the corresponding polypeptides of Rhizobium meliloti. The C-terminal domains of both fixB products displayed a high degree of similarity with the -subunits of rat and human electron transfer flavoproteins, suggesting a role for the FixB protein in a redox reaction. Two open reading frames (ORF) were found downstream of fixC. The first ORF was identified as fixX on the basis of sequence homology with fixX from several Rhizobium and Bradyrhizobium strains. The second ORF potentially encoded a 69 amino acid product and was found to be homologous to a DNA region in the Rhodobacter capsulatus nif cluster I. Insertion mutagenesis of the A. caulinodans fixX gene conferred a Nif– phenotype to bacteria grown in the free-living state and a Fix– phenotype in symbiotic association with the host plant Sesbania rostrata. A crude extract from the fixX mutant had no nitrogenase activity. Furthermore, data presented in this paper also indicate that the previously identified nifO gene located upstream of fixA was probably a homologue of the nifW gene of Klebsiella pneumoniae and Azotobacter vinelandii. 相似文献
62.
Determination of the chromosomal site for the human radiosensitive ataxia telangiectasia gene by chromosome transfer 总被引:3,自引:0,他引:3
The chromosomal localization of the gene which complements radiation hypersensitivity of AT cells was studied by microcell-mediated chromosome transfer. A 6-thioguanine-resistant derivative of an immortalized AT cell line, AT2KYSVTG, was used as a recipient for microcell-mediated chromosome transfer from 4 strains of mouse A9 cells, 3 of which carried a human X/11 recombinant chromosome containing various regions of chromosome 11, while the other carried an intact X chromosome. HAT-resistant microcell hybrids were isolated and examined for their radiosensitivity and chromosome constitution. The microcell hybrid clones obtained from the transfer of an intact X chromosome or an X/11 chromosome bearing the pter → q13 region of chromosome 11 did not show a difference in radiosensitivity from parental AT cells, while those obtained from the transfer of X/11 chromosomes bearing either the p11 → qter or the pter → q23 region of chromosome 11 exhibited a marked radioresistance which was comparable to normal human fibroblasts. A HAT-resistant but radiosensitive variant was further obtained from the microcell fusion with an A9 cell strain carrying an X/11 chromosome bearing the 11p11 → qter region, in which a deletion at the 11q23 region was found. The results indicate that the gene which complements a radiosensitive phenotype of AT is located at the q23 region of chromosome 11. 相似文献
63.
The effects of the local anesthetic dibucaine on coupling between electron transport and ATP synthesis-hydrolysis by the coupling-factor complex (CF0CF1 ATPase) were investigated in thylakoid membranes from Spinacia oleracea L. cv. Monatol. Evidence is presented that inhibition of ATP synthesis was produced by a specific uncoupling mechanism which was based on dibucaine-membrane surface interactions rather than on the interaction of dibucaine with the ATPase complex. Dibucaine reduced the osmotic space of thylakoid vesicles. At low pH of the medium it stimulated ATP hydrolysis beyond the rates obtained with optimum concentrations of ‘classical’ uncouplers. After addition of dibucaine, there was displacement of membrane-bound Mg2+ and strong thylakoid stacking in the presence of only low Mg2+ concentrations. Inhibition of ATP synthesis and transmembrane pH gradient increased with medium pH. Hydrolysis of ATP by isolated CF1 and the CF0CF1 complex was only slightly affected by dibucaine. The data are discussed assuming the involvement of localized proton channels on the membrane surface in protonic coupling of electron transport and ATP synthesis. A hypothesis for the mechanisms of action of local anesthetics at the thylakoid membrane is presented. 相似文献
64.
D. T. WILLIAMSON 《African Journal of Ecology》1991,29(2):105-117
Information on condition, growth and reproduction was collected from a sample of 155 female lechwe in the Linyanti Swamp, northern Botswana. Condition varied significantly with age, reproductive state and season and indications were found of relationships between body mass and the attainment of puberty in young females and body mass and the fertility of adult females. There were also indications that the population was under nutritional stress because of high water levels and that this had caused reductions in the growth rate and fertility of young females. It is suggested that the sensitivity of young females to adverse environmental conditions could be exploited to optimize population monitoring for conservation purposes by using them as an indicator class, rather than monitoring the entire population. Because of relationships between condition, growth and reproduction, it is also suggested that long-term monitoring of condition should form part of any effort to understand population dynamics. 相似文献
65.
The general structure of cytochrome oxidase is reviewed and evidence that the enzyme acts as a redox-linked proton pump outlined. The overall H+/e
– stoichiometry of the pump is discussed and results [Wikström (1989),Nature
338, 293] which suggest that only the final two electrons which reduce the peroxide adduct to water are coupled to protein translocated are considered in terms of the restrictions they place on pump mechanisms. Direct and indirect mechanisms for proton translocation are discussed in the context of evidence for redox-linked conformational changes in the enzyme, the role of subunit III, and the nature of the CuA site. 相似文献
66.
Summary
Tylosema esculentum, a perennial geophyte bearing yellow distylous flowers in racemes, maintains a high degree of outbreeding through reciprocal herkogamy. In addition, a viscous liquid, the anther-mucilage, is produced by the anther connective tissue and released concurrently with the pollen. The polysaccharide- and lipid-rich mucilage, which is functional in the shedding and transfer of pollen, is available for more than 1 day due to the gradual solidification of the mucilage. The assimilation of the pollen with the liquid substance significantly affects the pollination biology of T. esculentum. This is the first report on the unique phenomenon of wet pollen in the Caesalpiniaceae. 相似文献
67.
X-ray crystal structures of the oxidized and reduced forms of the rubredoxin from the marine hyperthermophilic archaebacterium Pyrococcus furiosus. 总被引:10,自引:8,他引:2
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M. W. Day B. T. Hsu L. Joshua-Tor J. B. Park Z. H. Zhou M. W. Adams D. C. Rees 《Protein science : a publication of the Protein Society》1992,1(11):1494-1507
The structures of the oxidized and reduced forms of the rubredoxin from the archaebacterium, Pyrococcus furiosus, an organism that grows optimally at 100 degrees C, have been determined by X-ray crystallography to a resolution of 1.8 A. Crystals of this rubredoxin grow in space group P2(1)2(1)2(1) with room temperature cell dimensions a = 34.6 A, b = 35.5 A, and c = 44.4 A. Initial phases were determined by the method of molecular replacement using the oxidized form of the rubredoxin from the mesophilic eubacterium, Clostridium pasteurianum, as a starting model. The oxidized and reduced models of P. furiosus rubredoxin each contain 414 nonhydrogen protein atoms comprising 53 residues. The model of the oxidized form contains 61 solvent H2O oxygen atoms and has been refined with X-PLOR and TNT to a final R = 0.178 with root mean square (rms) deviations from ideality in bond distances and bond angles of 0.014 A and 2.06 degrees, respectively. The model of the reduced form contains 37 solvent H2O oxygen atoms and has been refined to R = 0.193 with rms deviations from ideality in bond lengths of 0.012 A and in bond angles of 1.95 degrees. The overall structure of P. furiosus rubredoxin is similar to the structures of mesophilic rubredoxins, with the exception of a more extensive hydrogen-bonding network in the beta-sheet region and multiple electrostatic interactions (salt bridge, hydrogen bonds) of the Glu 14 side chain with groups on three other residues (the amino-terminal nitrogen of Ala 1; the indole nitrogen of Trp 3; and the amide nitrogen group of Phe 29). The influence of these and other features upon the thermostability of the P. furiosus protein is discussed. 相似文献
68.
René Roy François D. Tropper Anna Romanowska Marie Letellier Luc Cousineau Serge J. Meunier Janusz Boratyński 《Glycoconjugate journal》1991,8(2):75-81
Starting from peracetylated chloro- or bromo-glycosyl donors ofN-acetylneurmainic acid,N-acetylglucosamine, glucose and lactose, the correspondingp-formylphenyl glycosides were synthesized stereospecifically under phase transfer catalysed conditions at room temperature in yields of 38–67%. After Zemplén de-O-acetylation, the formyl groups were directly and chemoselectively coupled to the lysine residues of bovine serum albumin by reductive amination using sodium cyanoborohydride. The conjugation reactions were followed as a function of time and under a series of different molar ratios of the reactants to provide glycoconjugates of varying degree of antigenicities. Thus, carbohydrate protein conjugates were made readily available using essentially two key reactions.Presented in part at the 15th International Carbohydrate Symposium, Yokohama, Japan, August 12–17, 1990. 相似文献
69.
Curtis J. Condon Su-Hua Chang Albert S. Feng 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1991,168(6):709-725
Summary Response characteristics of 130 single neurons in the superior olivary nucleus of the northern leopard frog (Rana pipiens pipiens) were examined to determine their selectivity to various behaviorally relevant temporal parameters [rise-fall time, duration, and amplitude modulation (AM) rate of acoustic signals. Response functions were constructed with respect to each of these variables. Neurons with different temporal firing patterns such as tonic, phasic or phasic-burst firing patterns, participated in time domain analysis in specific manners. Phasic neurons manifested preferences for signals with short rise-fall times, thus possessing low-pass response functions with respect to this stimulus parameter; conversely, tonic and phasic-burst units were non-selective and possessed all-pass response functions. A distinction between temporal firing patterns was also observed for duration coding. Whereas phasic units showed no change in the mean spike count with a change in stimulus duration (i.e., all-pass duration response functions), tonic and phasic-burst units gave higher mean spike counts with an increase in stimulus duration (i.e., primary-like high-pass response functions). Phasic units manifested greater response selectivity for AM rate than did tonic or phasic-burst units, and many phasic units were tuned to a narrow range of modulation rates (i.e., band-pass). The results suggest that SON neurons play an important role in the processing of complex acoustic patterns; they perform extensive computations on AM rate as well as other temporal parameters of complex sounds. Moreover, the response selectivities for rise-fall time, duration, and AM rate could often be shown to contribute to the differential responses to complex synthetic and natural sounds.Abbreviations
SON
superior olivary nucleus
-
DMN
dorsal medullary nucleus
-
TS
torus semicircularis
-
FTC
frequency threshold curve
-
BF
best excitatory frequency
-
PAM
pulsatile amplitude modulation
-
SAM
sinusoidal amplitude modulation
-
SQAM
square-wave amplitude modulation
-
MTF
modulation transfer function
-
PSTH
peri-stimulus time histogram 相似文献
70.
Absorption changes at 325 nm (delta A325) induced by 15 ps laser flashes (lambda = 650 nm) in PS II membrane fragments were measured with picosecond time-resolution. In samples with the reaction centers (RCs) kept in the open state (P I QA) the signals are characterized by a very fast rise (not resolvable by our equipment) followed by only small changes within our time window of 1.6 ns. In the closed state (PI QA-) of the reaction center the signal decays with an average half-life time of about 250 ps. It is shown that under our excitation conditions (E = 2 x 10(14) photons/cm2 per pulse) subtraction of the absorption changes in closed RCs (delta A closed 325) from those in open RCs (delta A open 325) leads to a difference signal which is dominated by the reduction kinetics of QA. From the rise kinetics of this signal and by comparison with data in the literature it is inferred that QA becomes reduced by direct electron transfer from Pheo- with a time constant of about 350 +/- 100 ps. 相似文献