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1.
The inner-dune zone between the villages of Bakkum and Egmond in the North-Holland Dune Reserve is one of the few remaining examples of the semi-natural boundary between the dunes and the polders. For several centuries this area has been used for various purposes, in particular for grazing. Large-scale excavating, especially after about 1900, has encroached upon the dunes. Vegetation and land-use history were studied and a provisional scheme on the vegetation development is presented. Management policies discussed are non-intervention, mowing in order to start natural processes such as rabbit grazing, and the continuation of traditional livestock grazing. Proposals for external management policies were made in order to maintain or enhance the original transition zone, based on comprehensive physical-geographical studies of the whole inner-dune zone. 相似文献
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A number of acetoxycarboxylic acid esters were hydrolysed enantioselectively by Saccharomyces cerevisiae Hansen leading to chiral hydroxycarboxylic acid esters of high optical purity. The scope and limitations of this method with respect to the substitutional pattern of substrates were investigated. Subcellular localization of the hydrolytic activity on the plasma membrane led to the assumption that unspecific carboxyl esterases are responsible for hydrolysis of this type of substrate. Comparative experiments using viable cells and lyophilized cells as source of enzyme revealed the latter to be superior with respect to enantioselection and ease of handling. 相似文献
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This article is concerned with the problem of the relation between the genetic information contained in the DNA and the emergence of visible structure in multicellular animals. The answer is sought in a reappraisal of the data of experimental embryology, considering molecular, cellular and organismal aspects. The presence of specific molecules only confers a tissue identity on the cells when their concentration exceeds the 'threshold of differentiation'. When this condition is not fulfilled the activity of the genes that code for the specific molecules in question only confers on them a histogenetic potency, i.e. the capacity to form the corresponding tissue in further development (or to trans-differentiate to that tissue). The progressive restriction of histogenetic potencies during development reflects the irreversible repression of more and more genes. The establishment of a given tissue identity under the influence of an inducing tissue (or a morphogenetic hormone) is only possible when the cells have acquired the competence to respond. Tissue differentiation proceeds progressively during development thanks to the cytoplasmic 'memory' that cells retain collectively (or sometimes individually) of the items of information successively registered by their ancestors cells. The increasing complexity of visible structure emerging during development results only from the progression of tissue differentiation. This involves continual exchange of information among the cells and leads to (1) cell displacements and rearrangements, particularly during organogenesis and (2) extreme diversification of cell individualities within tissues, particularly during postembryonic growth. A mutation (just as a teratogenic factor) evokes an anomaly that is localized in both space and time because it alters a certain aspect of cell behaviour (particularly cell surface adhesiveness or mitotic activity) at the time when this is involved in the establishment of a particular structural trait. Neither the organization of the adult nor the modalities of development are encoded in the DNA. The automatic concatenation of cell interactions in the embryo and the structural amplification it entails is conditioned by the specific biochemical composition of the cytoplasm of the egg and by the heterogeneous distribution of its inclusions. 相似文献
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Cloning of the human α2-macroglobulin gene and detection of mutations in two functional domains: the bait region and the thiolester site 总被引:3,自引:0,他引:3
Summary Overlapping genomic clones of the human 2-macroglobulin (2M) gene were isolated from a cosmid library and were used to map 80 kb of the chromosomal region of this gene. Fragments carrying the two exons encoding the bait region and the exon encoding the thiolester site were partially sequenced and PCR primers were designed for the amplification of both functional domains. By direct genomic sequencing of these domains in 30 healthy individuals and in 30 patients with chronic lung disease three mutations were detected. The first was a sequence polymorphism occurring near the thiolester site of the gene, changing Val1000(GTC) to Ile1000(ATC), with allele frequencies of 0.30 (GTC) and 0.70 (ATC), respectively. No difference of 2M serum levels was observed for these two alleles. The second mutation occured within the thiolester site of one patient, changing Cys972(TGT) to Tyr972(TAT). Since activation of the internal thiolester formed between Cys972 and Gln975 in each of the subunits of the tetrameric 2M is involved in the covalent cross-linking of the activating proteinase, this mutation is predicted to interfere with 2M function. The 2M serum level was within the normal range in this patient. In one healthy individual we detected an alteration of the bait region sequence, which is usually encoded by two different exons separated by an intron of size 1.6kb. In this individual, PCR amplification of genomic DNA using the bait region primers produced the common fragment of size 1.8 kb and an additional variant fragment of size 0.23kb. This finding, and the genomic sequencing data of this individual, indicate that he carries two different alleles of the 2M gene: one with the regular structure (bait exon I-intron-bait exon II), the other with the two bait exons fused into one. Direct genomic sequencing of the two 2M functional domains is a useful tool for the detection of the genetic, and possibly the functional, heterogeneity of 2M. This, in turn, may provide some insight into the hitherto unknown physiological role(s) of 2M, by studying in vivo effects of naturally ocurring mutations of the gene. 相似文献
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A. Shirai F. M. Bozeman J. W. Humphries B. L. Elisberg J. E. Faber 《Journal of bacteriology》1967,94(5):1334-1339
Studies of experimental infection of the cotton rat, Sigmodon hispidus, with the virulent Sheila Smith (R type) and the avirulent Si 7 (U type) strains of Rickettsia rickettsii were undertaken to evaluate the role of this native wild mammal in the ecology of Rocky Mountain spotted fever. The Sheila Smith strain, which was highly lethal for guinea pigs, was nonpathogenic for cotton rats. Serial passage of the R-type strain in the cotton rat did not alter the virulence of the agent for cotton rats or guinea pigs. The U-type strain, which was originally recovered from a wild cotton rat, could not be maintained beyond the first passage in this animal host. Rickettsemia in the cotton rat occurred over a 24-hr period after inoculation of the virulent strain but was detected only 1 hr after inoculation of the avirulent strain. The short period of rickettsemia suggests that the cotton rat probably is not an important reservoir of R. rickettsii. Specific complement-fixing antibodies developed rapidly after infection with either strain, but the antibodies evoked by the R strain attained higher titers and persisted longer. Cotton rats previously infected with the Sheila Smith strain developed rickettsemia after reinfection with the same strain, even though relatively high levels of antibody were still present. 相似文献
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Escherichia coli glycerol kinase, a major regulatory enzyme which catalyzes the reversible MgATP-dependent phosphorylation of glycerol has been crystallized by the hanging drop vapor diffusion method at room temperature. Three different crystal forms have been obtained in the presence of glycerol and appear to be suitable for X-ray crystallographic studies. Vapor diffusion against 55% ammonium sulfate and 1% beta-octyl glucoside (pH 7.0) yields rhombohedral crystals with space group R32, a = b = 277.1 A, c = 78.7 A (hexagonal indexing) containing a dimer of Mr 112,000 in the asymmetric unit (Vm = 2.64 A3/dalton). Vapor diffusion against sodium chloride in the presence of 10% (w/v) polyethylene glycol (pH 6.5 to 7.0) yields two different crystal forms, both with space group P2(1). The first form has a = 88.1 A, b = 99.3 A, c = 114.6 A, beta = 119 degrees, the second form has a = 92.5 A, b = 117.6 A, c = 108.3 A, beta = 93.64 degrees. Addition of ADP enhances growth of the monoclinic forms. These forms appear to contain an entire tetramer of Mr 224,000 in the asymmetric unit and have Vm values of 2.28 and 2.65 A3/dalton, respectively. All forms diffract to better than 3.0 A resolution while the second monoclinic form diffracts to approximately 1.8 A. 相似文献
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D D Belsham E Rosenmann F A Pereira S G Williams M K Turney W J Kovacs L E Faber K Wrogemann 《Journal of steroid biochemistry》1989,33(3):389-394
Analysis of soluble proteins from human genital skin fibroblasts by two-dimensional polyacrylamide gel electrophoresis reveals an abundant protein doublet of mol. wt 56,000 with isoelectric points (pI) of 6.7 and 6.5. This protein is absent in non-genital skin fibroblasts as well as in genital skin fibroblasts of most patients with complete forms of androgen insensitivity. The protein specifically binds androgen. A protein of similar estimated molecular weight (58,000) from human genital skin fibroblasts has recently been found to be covalently radiolabelled by the affinity ligand dihydrotestosterone 17 beta-bromoacetate (DHT-BA). In the present study these proteins have been found to be indistinguishable on one- and two-dimensional gel electrophoresis. Antibodies raised against the 56 kDa pI 6.7/6.5 protein also recognized the protein covalently radiolabelled by DHT-BA. A third protein of estimated mol. wt 59,000 has been found to be associated with several steroid hormone receptor complexes but has no known ligand binding activity. This protein was found to be clearly separable from the 56/58 kDa protein on two-dimensional gel electrophoresis as it has a more acidic pI of approximately 5.4. Furthermore, antibodies against the 59 kDa protein do not recognize the 56 kDa species, and vice versa. 相似文献