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71.
CO2 exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed
using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities
to savanna woodlands. A maximum CO2 flux of 0.207 mg m-2 s-1 was measured in the crusts of the Guanipa savannas, while in the other examined crusts (0.035–0.105 mg m-2 s-1) the flux was similar to values reported for terrestrial algae. The CO2 flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic
efficiencies of the crusts were compared. The activation energy calculated for the CO2 fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12
kcal mole-1), whereas they were evident in the other crust studied. An optimum CO2 incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the
basis of the results and characteristics of the habitats. 相似文献
72.
The primary structure of the HLA-A2 subtype A*0204 (isoelectric focusing variant A2.A) has been determined. cDNA encoding this subtype was amplified by the polymerase chain reaction. Four independent full-lenght cDNA clones encoding A*0204 were analyzed to obtain a consensus sequence for this subtype. A*0204 differs from A*0201 by a single nucleotide change of G to T through the coding regions, resulting in an Arg to Met change at position 97. This substitution accounts for the isoelectric focusing pattern of the subtype. The same change occurs in other HLA-A specificities in association with other changes in its vicinity. The absence of additional substitutions in A*0204 suggests that it could have arisen from A*0201 by point mutation, and that recurrent mutations may take place during HLA diversification. The spatial location of this change implies that A*0204 must be a functional variant. Comparison of its sequence with other HLA-A2 subtypes reveals that much of the HLA-A2 subtype polymorphism is generated by variations in four neighboring positions, including position 97, which are located in two adjacent -strands on the floor of the peptide binding site of the molecule.The nucleotide sequence data reported in this paper have been submitted to the EMBL nucleotide sequence database and have been assigned the accession number X57954.
Address correspondence and offprint requests to: J. A. López de Castro. 相似文献
73.
Ramon Latorre Juan Bacigalupo Ricardo Delgado Pedro Labarca 《Journal of bioenergetics and biomembranes》1991,23(4):577-597
Four different nucleotide-gated ion channels are discussed in terms of their biophysical properties and their importance in cell physiology. Channels activated directly by cGMP are present in vertebrate and invertebrate photoreceptors. In both cases cGMP increases the fraction of time the channel remains in the open state. At least three cGMP molecules are involved in channel opening in vertebrate photoreceptors and the concentration of the cyclic nucleotide to obtain the half maximal effect is about 15 µM. The light-dependent channel of both vertebrates and invertebrates is poorly cation selective. The vertebrate channel allows divalent cations to pass through 10–15-fold more easily than monovalent ions. In agreement with their preference for divalent cations, this channel is blocked byl-cis Dialtazem, a molecule that blocks certain types of calcium channels. In olfactory neurons a channel activated by both cAMP and cGMP is found and, as in the light-dependent channel, several molecules of the nucleotide are needed to open the channel with a half maximal effect obtained in the range of 1–40 µM. The channel is poorly cationic selective. A K+ channel directly and specifically activated by cAMP is found inDrosophila larval muscle. At least three cAMP molecules are involved in the opening reaction. Half-maximal effect is obtained at about 50 µM. This channel is blocked by micromolar amount of tetraethylammonium applied internally. Interestingly, this channel has a probability of opening 10–20-fold larger in the mutantdunce, a mutant that possesses abnormally elevated intracellular cAMP level, than in the wild type. 相似文献
74.
Cecilia Zazueta José A. Holguín Jorge Ramírez 《Journal of bioenergetics and biomembranes》1991,23(6):889-902
We describe a calcium transport that is sensitive to ruthenium red in liposomes reconstituted with mitochondrial extracts. This system is able to build an internally negative membrane potential, which allows the electrogenic influx of Ca2+ and Sr2+. Proteins with molecular weights higher than 35 kDa were incorporated to the vesicles, and enhanced the accumulation of the cation in an energy-dependent fashion. 相似文献
75.
Chloramphenicol acetyl transferase (CAT) gene was used as a reporter gene to assess the conditions for polyethylene glycol (PEG)-mediated transfection of kiwifruit protoplasts. The effect of plasmid concentration and the presence of carrier DNA were each assessed by analysing CAT activity in transfected protoplasts using thin-layer chromatography (TLC) autoradiographic detection of acetylated chloramphenicol. A gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) non-radioactive method was developed for monitoring CAT gene activity. This method provides a high speed of analysis (30 min) and precise means of detecting acetylated products at the nanomolar level, enabling quantification at very low transfection rates. Using this method we optimized plasmid and PEG concentration and also assessed the effect of heat shock on transfection. The best CAT activity was obtained using 30% polyethylene glycol 4000 and by submitting protoplasts to heat shock (45 °C, 5 min) prior to transfection. 相似文献
76.
Ignacio R. Rodriguez Pedro Gonzales J. Samuel Zigler Teresa Borrás 《生物化学与生物物理学报:疾病的分子基础》1992,1180(1):44-52
A congenital cataract present in guinea pigs provided a unique opportunity to study a hereditary lens diseases at the molecular level. ζ-crystallin, one of the most abundant guinea pig lens proteins, was found to be altered in the lens of cataractous animals. Several ζ-crystallin cDNA clones were isolated from a cataractous lens library and found to contain a 102-bp deletion towards the 3′ end of the coding region. The deletion does not interfere with the reading frame but results in a protein 34 amino acids shorter. Sequence analysis of a normal genomic ζ-crystallin clone revealed that the missing 102-bp fragment corresponds to an entire exon (exon 7). PCR analysis of the genomic DNA isolated from cataractous animals showed that exon 7, though missing from the mRNA, is intact in the cataractous genome. Further sequence analysis of the α-crystallin gene disclosed a dinucleotide delection of the universal AG at the acceptor splice-site of intron 6 of the mutant gene. The presence of this mutation results in the skipping of exon 7 during the mRNA processing which in turn results in the altered ζ-crystallin protein. This if the first time a genomic mutation in an enzyme/crytallin gene has been directly linked to a congenital cataract. 相似文献
77.
Interaction of two variable proteins (PilE and PilC) required for pilus-mediated adherence of Neisseria gonorrhoeae to human epithelial cells 总被引:16,自引:0,他引:16
Thomas Rudel Jos P. M. van Putten Carol P. Gibbs † Rainer Haas Thomas F. Meyer 《Molecular microbiology》1992,6(22):3439-3450
Pili confer the initial ability of Neisseria gonorrhoeae to bind to epithelial cells. Pilin (PilE), the major pilus subunit, and a minor protein termed PilC, reportedly essential for pilus biogenesis, undergo intra-strain phase and structural variation. We demonstrate here that at least two different adherence properties are associated with the gonococcal pili: one is specific for erythrocytes, which is virtually unaffected by PilE variation, and another is specific for epithelial cells, and is modulated in response to the variation of PilE. Based on this finding, mutants of a recA- strain were selected that had lost the ability to bind to human cornea epithelial cells (A-) but retained the ability to form pili (P+) and to agglutinate human erythrocytes (H+). The adherence-negative mutants failed to produce detectable levels of PilC1 or PilC2 proteins, representing piIC phase variants generated in the absence of RecA. The A- pilC phase variants were indistinguishable from their A+ parents and spontaneous A+ revertants with regard to the amount of PilE produced and its electrophoretic mobility, the degrees of piliation and haemagglutination, and the pilE nucleotide sequence. These data demonstrate a central role for PilC in pilus-mediated adherence of N. gonorrhoeae to human epithelial cells and further indicate that neither PilC1 nor PilC2 is obligatory for the assembly of gonococcal pili. 相似文献
78.
Arturo Manjón José L. Iborra Concepción Martínez-Madrid 《Applied microbiology and biotechnology》1991,35(2):176-179
Summary
Rhodococcus fascians cells were immobilized by entrapment in -carrageenan. The ability of the system to continuously degrade limonin was tested against pH. A burst of activity was observed when changing from pH 4.5 to 5.0, and a small increase could be seen above the latter value. Such behaviour was not only a response of the metabolic activity of the cells to changes in the medium pH, but to selectivity towards the chemical form of the limonin substrate, which also depends on pH. Additionally, the immobilized cells showed increased resistance against pH changes, since the system recovered almost full activity when the pH was restored to 7.0 after being operated for long periods at pH 4.0. The decrease in limonin-degrading capability of the immobilized cells at low pH values could be overcome by choosing an appropriate dilution rate.Offprint requests to: J. L. Iborra 相似文献
79.
A lacZ-pbpB gene fusion coding for an inducible hybrid protein that recognizes localized sites in the inner membrane of Escherichia coli. 总被引:4,自引:4,他引:0
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An in-phase gene fusion consisting of the 5'-terminal 1,314 base pairs (bp) of the structural gene for beta-galactosidase (lacZ) and the 3'-terminal 1,644 bp of the structural gene coding for penicillin-binding protein 3 (pbpB) of Escherichia coli was constructed and cloned in the plasmid pDIAM64. The product of the fusion gene was a remarkably stable protein with an apparent molecular weight of 110,000 (p110) that retained the ability to covalently interact with beta-lactam antibiotics. The fusion protein was found associated with the membrane at low levels of induction, but it accumulated in the cytoplasm of cells induced for a long time as inclusion bodies of high density. Inclusion bodies were localized at defined positions corresponding to septal sites in all of the pDIAM64-containing strains tested except PAT84 and GD113 (which carry the ftsZ84 mutant allele). These findings indicate a possible role of the FtsZ protein in the integration of Pbp3 into the membrane and in septum localization during the cell division cycle. 相似文献
80.
The ability of stationary-phase cells of Escherichia coli W7 to incorporate radioactive precursors into macromolecular murein has been studied. During the initial 6 h of the stationary phase, resting cells incorporated meso-[3H]diaminopimelic acid at a rate corresponding to the insertion of 1.3 X 10(4) disaccharide units min-1 cell-1. Afterwards, the rate of incorporation dropped drastically (90%) to a low but still detectable level. Incorporation during stationary phase did not result in an increased amount of total murein in the culture, suggesting that it was related to a turnover process. Analysis of the effects of a number of beta-lactam antibiotics indicated that incorporation of murein precursors in stationary-phase cells was mediated by penicillin-binding proteins, suggesting that the activity of penicillin-binding protein 2 was particularly relevant to this process. 相似文献