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991.
A nucleosidase activity has been isolated from Lactobacillus acidophilus which rapidly hydrolyses N-6 (delta-2-isopentenyl) adenosine to its corresponding base, N-6(delta-2-isopentenyl) adenine. The activity can be distinguished from the spleen exzyme (EC. 2.4.2.1), a purine nucleoside transferase, on the basis of its substrate specificity, electrophoretic behavior, and nondependence on phosphate. The bacterial enzyme hydrolyzes both inosine and isopentenyl adenosine, giving Km values of 63.3muM and 177 muM respectively. The presence of this enzyme in bacteria counts for the rapid conversion of the parent nucleoside to isopentenyl adenine, which has been observed in these cells. The enzyme thus assumes importance as one of the catabolic activities available to the cell for metabolizing the cytokinin, N-6-(delta-2-isopentenyl) adenosine.  相似文献   
992.
A thermal perturbation curve (TPC) is defined to be the derivative of the fractional degree of saturation, f, with respect to temperature, considered as a function of the natural logarithm of free ligand concentration, y. The theoretical framework for the use of such curves in the thermodynamic analysis of ligand binding to macromolecules is presented. The thermal perturbation curve either provides or complements the information obtained from the derivative binding isotherm ?f/?y. For a single set of identical and independent sites the TPC is identical to the derivative binding isotherm. Analysis of such a curve directly yield ΔH0 and ΔG0 for the binding reaction. In actual experimental work, however, the TPC can only be approximated because of “self-buffering” effects relations between the parameter of the approximate curve and the thermodynamic quantities have been developed. This technique is applied to the proflavin-α-chymotryspin system to demonstrate its usefulness. The general features of thermal perturbation curves for cases of multiple sets of independent sites and cooperatively interacting sites have also been developed. The analysis of thermal perturbation curves in combination with other methods should provide a more powerful approach to the characterization of ligand-macromolecule interactions.  相似文献   
993.
A sulphotransferase preparation from hen's uterus catalysed the transfer of sulphate from adenosine 3′-phosphate 5′-sulphatophosphate to N-desulphated heparan sulphate, heparan sulphate, N-desulphated heparin and dermatan sulphate. Heparin, chondroitin sulphate and hyaluronic acid were inactive as substrates for the enzyme. N-desulphated heparin was a much poorer substrate for the enzyme than N-desulphated heparan sulphate suggesting that properties of the substrate other than available glucosaminyl residues influenced enzyme activity. N-acetylation of N-desulphated heparin and N-desulphated heparan sulphate reduced their sulphate acceptor properties so it was unlikely that the N-acetyl groups of heparan sulphate facilitated its sulphatiion. Direct evidence for the transfer of [35S]sulphate to amino groups of N-desulphated haparan sulphate was obtained by subsequent isolation of glucosamine N-[35S]sulphate from heparan [35S]sulphate product. This was made possible through the use of a flavobacterial enzyme preparation which contained “heparitinase” activity but had been essentially freed of sulphatases. Attempts to transfer [35S]sulphate to glucosamine or N-acetylglucosamine were unsuccessfull.  相似文献   
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To detect the genomic constitutions and investigate the evolutionary relationships between Campeiostachys Drobov and Elymus L. species, we have cloned and analyzed 271 5S nuclear ribosomal DNA sequences from 27 accessions of these species, mostly of Chinese origin. We identified Long H1, Short S1, and Long Y1 unit classes in nine Campeiostachys or Elymus species. The identification of the three orthologous unit classes was confirmed by the neighbor‐joining tree of each unit class from PAUP and the phylogeny tree of three unit classes from MrBayes. The results suggested that these Elymus species comprise StYH haplomes and should be included in Campeiostachys. The phylogeny tree showed a clear separation between the S1 unit class and Y1 unit class. However, Y1 unit class sequences formed a sister clade to the S1 unit class, implying that although the St and Y haplomes might have some affinity, they are distinct from one another. The phylogeny tree also indicated that the five species in sect. Turczaninovia (C. dahurica var. cylindrica, C. dahurica var. dahurica, C. dahurica var. tangutorum, E. purpuraristatus, and E. dahuricus Turcz. ex Griseb. var. violeus C. P. Wang & H. L. Yang) might share a more recent common ancestor, whereas the four species in sect. Elymus (C. nutans, E. breviaristatus (Keng) Keng ex Keng f., E. sinosubmuticus (Keng) Keng f., and E. atratus (Nevski) Hand.‐Mazz.) share a close relationship. By identifying only one type of unit class for each haplome, we propose that the 5S nuclear ribosomal DNA sequences of species within Campeiostachys might have undergone haplome‐specific concerted evolution.  相似文献   
999.
Alginate is an important polysaccharide that is commonly used as a gelling agent in foods, cosmetics and healthcare products. Currently, all alginate used commercially is extracted from brown seaweed. However, with environmental changes such as increasing ocean temperature and the increasing number of biotechnological uses of alginates with specific properties, there is an emerging need for more reliable and customizable sources of alginate. An alternative to seaweed for alginate production is Pseudomonas aeruginosa, a common Gram-negative bacterium that can form alginate-containing biofilms. However, P. aeruginosa is an opportunistic pathogen that can cause life-threatening infections in immunocompromised patients. Therefore, we sought to engineer a non-pathogenic P. aeruginosa strain that is safe for commercial production of alginate. Using a homologous recombination strategy, we sequentially deleted five key pathogenicity genes from the P. aeruginosa chromosome, resulting in the marker-free strain PGN5. Intraperitoneal injection of mice with PGN5 resulted in 0% mortality, while injection with wild-type P. aeruginosa resulted in 95% mortality, providing evidence that the systemic virulence of PGN5 is highly attenuated. Importantly, PGN5 produces large amounts of alginate in response to overexpression of MucE, an activator of alginate biosynthesis. The alginate produced by PGN5 is structurally identical to alginate produced by wild-type P. aeruginosa, indicating that the alginate biosynthetic pathway remains functional in this modified strain. The genetic versatility of P. aeruginosa will allow us to further engineer PGN5 to produce alginates with specific chemical compositions and physical properties to meet different industrial and biomedical needs.  相似文献   
1000.

Strain MG, isolated from an acidic pond sediment on the island of Milos (Greece), is proposed as a novel species of ferrous iron- and sulfur-oxidizing Acidithiobacillus. Currently, four of the eight validated species of this genus oxidize ferrous iron, and strain MG shares many key characteristics with these four, including the capacities for catalyzing the oxidative dissolution of pyrite and for anaerobic growth via ferric iron respiration. Strain MG also grows aerobically on hydrogen and anaerobically on hydrogen coupled to ferric iron reduction. While the 16S rRNA genes of the iron-oxidizing Acidi-thiobacillus species (and strain MG) are located in a distinct phylogenetic clade and are closely related (98–99% 16S rRNA gene identity), genomic relatedness indexes (ANI/dDDH) revealed strong genomic divergence between strain MG and all sequenced type strains of the taxon, and placed MG as the first cultured representative of an ancestral phylotype of iron oxidizing acidithiobacilli. Strain MG is proposed as a novel species, Acidithiobacillus ferrianus sp. nov. The type strain is MGT (= DSM 107098T = JCM 33084T). Similar strains have been found as isolates or indicated by cloned 16S rRNA genes from several mineral sulfide mine sites.

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