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151.
Seasonal fluctuations of zooplankton biomass (dry weight) were determined during a year in two localities of Lake Xolotlán (Managua). Biomass estimations of the most common species of rotifers, cladocerans and copepods were made. The maximal zooplankton biomass was observed in February–April (dry season) in coincidence with the period of highest phytoplankton abundance. Copepods contributed with 78% and 84% to the mean zooplankton biomass at points 1 and 7, respectively. Cladocera biomass was lowest during most of the year, and it was probably controlled by fish predation. Development of rotifer biomass was more intense during the rainy season, when detritus particles were more abundant. Daily fluctuations of zooplankton biomass were not pronounced. 相似文献
152.
Clostridium sticklandii glycine reductase selenoprotein A gene: cloning, sequencing, and expression in Escherichia coli.
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Gene grdA, which encodes selenoprotein A of the glycine reductase complex from Clostridium sticklandii, was identified and characterized. This gene encodes a protein of 158 amino acids with a calculated M(r) of 17,142. The known sequence of 15 amino acids around the selenocysteine residue and the known carboxy terminus of the protein are correctly predicted by the nucleotide sequence. An opal termination codon (TGA) corresponding to the location of the single selenocysteine residue in the polypeptide was found in frame at position 130. The C. sticklandii grdA gene was inserted behind the tac promotor of an Escherichia coli expression vector. An E. coli strain transformed with this vector produced an 18-kDa polypeptide that was not detected in extracts of nontransformed cells. Affinity-purified anti-C. sticklandii selenoprotein A immunoglobulin G reacted specifically with this polypeptide, which was indistinguishable from authentic C. sticklandii selenoprotein A by immunological analysis. Addition of the purified expressed protein to glycine reductase protein components B and C reconstituted the active glycine reductase complex. Although synthesis of enzymically active protein A depended on the presence of selenium in the growth medium, formation of immunologically reactive protein did not. Moreover, synthesis of enzymically active protein in a transformed E. coli selD mutant strain indicated that there is a nonspecific mechanism of selenocysteine incorporation. These findings imply that mRNA secondary structures of C. sticklandii grdA are not functional for UGA-directed selenocysteine insertion in the E. coli expression system. 相似文献
153.
Hydrolysis of butteroil by immobilized lipase using a hollow-fiber reactor: part I. lipase adsorption studies 总被引:2,自引:0,他引:2
Adsorption of proteins from a crude preparation containing a lipase from Aspergillus niger on microporous polypropylene hollow fibers was studied at six different temperatures. Langmuir isotherms accurately describe the overall adsorption equilibria. Lipase is selectively adsorbed relative to the other proteins in the crude preparation. Hence, immobilization also provides further purification of the lipase. The predictions of the Langmuir model for the change in the specific activity of lipase upon adsorption are consistent with experimental results. The loading capacity of the hollow fibers decreases and the adsorption constant increases as temperature is increased. This effect is more significant in the case of lipolytic activity than it is for the total amount of adsorbed protein. Small, positive enthalpy changes are associated with the adsorption of lipase on these hydrophobic membranes. 相似文献
154.
J C Garcia M Strube K Leingang K Keller M M Mueckler 《The Journal of biological chemistry》1992,267(11):7770-7776
All 6 tryptophan residues in the human HepG2-type glucose transporter (Glut1) were individually altered by site-directed mutagenesis to investigate the role of these residues in transport function. Tryptophan residues in positions 48, 65, 186, 363, 388, and 412 of Glut1 were changed to either a glycine or leucine residue. Mutant mRNAs were synthesized and injected into Xenopus laevis oocytes. Transporter function as assessed by uptake of 2-deoxy-D-[3H]glucose or transport of 3-O-[3H]methylglucose was decreased in the 388 and 412 mutants but was unaltered in all other mutants. The amount of the mutant transporters expressed in total membrane and plasma membrane fractions was measured using Glut1-specific antibodies. Calculation of the intrinsic transport activity of each of the mutants using these data demonstrated that the reduced transport activity of the 412 mutants was caused entirely by a dramatic decrease in the intrinsic activity of the mutant proteins whereas the reduced activity of the 388 mutants was a result of a decreased level of the protein in oocytes, decreased targeting to the plasma membrane, and a modest decrease in the intrinsic activity. Protease/glycosidase mapping of in vitro translation products indicated that the effects of the 388 and 412 point mutations could not be attributed to a disruption in the ability of the mutant proteins to insert properly into the membrane. The ID50 for cytochalasin B inhibition of 2-deoxyglucose uptake was increased from 5 x 10(-7) M for the wild-type Glut1 to 4 x 10(-6) M in the 388 mutants but was unaltered in the 412 mutants. These observations suggest that 1) Trp-412 may comprise part of a hexose binding site or is involved in maintaining a local tertiary structure critical for transport function; 2) Trp-388 is involved in stabilizing the equilibrium binding of cytochalasin B to the transporter. Trp-388 may therefore lie near a substrate binding site and also appears to participate in stabilization of local tertiary structure important for full catalytic activity and efficient targeting to the Xenopus plasma membrane. 相似文献
155.
The influence of irradiance on growth, photosynthesis and respiration of Gyrodinium cf. aureolum 总被引:1,自引:0,他引:1
The bloom-forming marine dinoflagellate Gyrodinium cf. aureolumwas grown in batch cultures over a range of irradiances (35380µmolm2 s1 and growth, photosynthesis and respirationrates determined. Saturation of growth occurred at irradiancesof 100µmol m2 s1 Below this light level,decreases in growth rates and cell size, and a relative increasein carbon specific respiration rates, were observed. On theother hand, photosynthesis-irradiance relationships determinedfrom dissolved oxygen incubations showed that on a cellularand carbon basis, cultures grown at low irradiances had higherrates of light-limited and light-saturated photosynthesis, mainlyas a result of large increases in cell chlorophyll content.This adaptation strategy enables low-light-grown organisms toexploit available high irradiance through a relatively highphotosynthetic capacity. In cells grown at higher light levels(>100µmol m2 s1), excess photosynthatemay be diverted to storage rather than used for growth. 相似文献
156.
R C Leif I D Rosello D M Simler G P Garcia S B Leif 《Analytical and quantitative cytology and histology / the International Academy of Cytology [and] American Society of Cytology》1991,13(6):440-450
Ada is a new general-purpose language that embodies the concepts of software engineering. Although it was initially developed for military purposes, it is suitable for developing software for cytometry and other health-related applications. A pilot study has demonstrated the feasibility of employing Ada for cytometry applications. Three packages were created. The first subtracts a control three-dimensional population from multiple individual experimental populations and presents the results in spread sheet form. A second package has the capability of finding aggregates of cells. The results of this package are visualized employing a commercially available program for three-dimensional presentation of the data that permits rotation in real time. A third package consists primarily of interface drivers for two commercially available personal computer boards, an ADC and a stepper motor controller. The major problems with the coding were due to incomplete implementation of the language. This pilot study, together with others, indicates that it would be both cost effective and beneficial to implement cytometry and other medical devices in Ada. 相似文献
157.
H M Frankel E Garcia F Malik J K Weiss H R Weiss 《Journal of applied physiology》1992,73(5):1756-1761
This study investigated the effects 2 h after administration of acetazolamide on cerebral blood flow and the pattern of cerebral capillary perfusion. Arterial blood pressure, heart rate, arterial blood gases, and pH were recorded in two groups of rats along with either regional cerebral blood flow or the percentage of capillary volume per cubic millimeter and number per square millimeter perfused as determined in cortical, thalamic, pontine, and medullary regions of the brain. Blood pressure, heart rate, and arterial PCO2 were not significantly different between the rats receiving acetazolamide (100 mg/kg) and the controls. Arterial blood pH was significantly lower in the acetazolamide rats. Blood flow increased significantly in the cortical (+ 102%), thalamic (+ 89%), and pontine (+ 88%) regions receiving acetazolamide. In control rats, approximately 60% of the capillaries were perfused in all of the examined regions. The percentage of capillaries per square millimeter perfused was significantly greater in the cortical (+ 52%), thalamic (+ 49%), and pontine (+ 47%) regions of acetazolamide rats compared with controls. In the medulla the increases in blood flow and percentage of capillaries perfused were not significant. Thus in the regions that acetazolamide increased cerebral blood flow, it also increased the percentage of capillaries perfused. 相似文献
158.
Structural studies of the lysozyme coded by the pneumococcal phage Cp-1. Conformational changes induced by choline 总被引:2,自引:0,他引:2
The CPL-1 lysozyme coded by the pneumococcal phage Cp-1 has been overproduced in Escherichia coli under the control of a modified lipoprotein lactose promoter. This result has provided the conditions to analyse the CPL-1 secondary structure by circular dichroism (CD). The CD spectra recorded in the far-ultraviolet region showed, at neutral pH, two minima at 210 nm and 230 nm and a shoulder at 217 nm, whereas two bands at 260 nm and 295 nm were observed in the near-ultraviolet region. It has been estimated, by using the CDPROT program, that the protein is composed of 19% alpha-helix, 32% beta-sheet, 28% beta-turn and 21% random coil. Minor changes in the CD spectra were detected either when the pH was varied over 6-10 or when the ionic strength was increased to 1 M NaCl. Choline, a well known modulator of the enzyme activity that is present in the pneumococcal cell wall, induced remarkable changes in the intensities of the bands at 210, 230 and 295 nm, with the appearance of an unusual positive band at 225 nm. The conformational change was reversible and correlated with the competitive inhibitory effect of choline on the lysozyme activity, supporting, by a new and direct experimental approach, the basic role of choline in the recognition of the cell wall substrate. The analyses of the secondary structure prediction and the CD data reported here are compatible with the two-domain structure of CPL-1 reinforce our hypothesis that the C-terminal region is directly involved in the binding of the enzyme to the pneumococcal teichoic and lipoteichoic acids. 相似文献
159.
Sequence of the Streptococcus pneumoniae bacteriophage HB-3 amidase reveals high homology with the major host autolysin. 总被引:10,自引:5,他引:5
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We have sequenced a DNA fragment containing the pneumococcal bacteriophage HB-3 hbl gene, which codes for the phage lytic amidase. A remarkable nucleotide similarity (87.1%) between the lytA gene, coding for the pneumococcal amidase, the major autolysin of Streptococcus pneumoniae, and the hbl gene was found. This similarity completely disappeared outside the open reading frames coding for both amidases. The hbl gene transformed amidase-deficient strains of S. pneumoniae to the wild-type phenotype, and Southern blotting experiments provided evidence for recombination between donor and recipient genes. A comprehensive evaluation of these and previous results on the peptidoglycan hydrolases of S. pneumoniae and its bacteriophages suggested that recombination mechanisms participate in the evolution of the genes coding for these enzymes. 相似文献
160.