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111.
A tetrahedral intermediate is the prominent feature of the generally accepted mechanism for aspartate transcarbamoylase. We have synthesized N-pyrophosphoryl-L-aspartate as a charged analogue of the postulated intermediate. Surprisingly, its affinity for the enzyme from Escherichia coli was substantially lower than that of the previously known inhibitor phosphonoacetyl-L-aspartate which contained a trigonal carbonyl group. Similar results were obtained with the corresponding mercaptosuccinate derivatives. We also tested a number of new pyrophosphate analogues as inhibitors. Our results cast doubt on some aspects of the current model for the mechanism of this enzyme. 相似文献
112.
Addition of manganese, at levels of 50 ppm, to a liquid growth medium simulating adverse silage conditions had no effect on the growth or on the fermentation pattern of Enterobacter cloacae and Proteus vulgaris. Yet, the manganese strongly enhanced the growth of Lactobacillus plantarum. Co-cultures of L. plantarum and E. cloacae or P. vulgaris were, by addition of manganese ions, significantly altered in the favour of the former. This finding can be of use in mixed cultures where Enterobacteriaceae act as spoiler microorganisms. 相似文献
113.
Richard H. Elderkin 《Journal of mathematical biology》1982,13(3):283-303
A model of seed population dynamics proposed by S. A. Levin, A. Hastings, and D. Cohen is presented and analyzed. With the environment considered as a mosaic of patches, patch age is used along with time as an independent variable. Local dynamics depend not only on the local state, but also on the global environment via dispersal modelled by an integral over all patch ages. Basic technical properties of the time varying solutions are examined; necessary and sufficient conditions for nontrivial steady states are given; and general sufficient conditions for global asymptotic stability of these steady states are established. Primary tools of analysis include a hybrid Picard iteration, fixed point methods, monotonicity of solution structure, and upper and lower solutions for differential equations.This work was supported in part by National Science Foundation Grants MCS-7903497 and MCS-790349701 相似文献
114.
115.
Cloning of the ColE3-CA38 colicin and immunity genes and identification of a plasmid region which enhances colicin production 总被引:8,自引:0,他引:8
The colicin and immunity genes of plasmid ColE3-CA38 have been localized by characterization of bacteria carrying its cloned restriction fragments. They are within a 3.14-kb EcoRI segment, such that the immunity gene contains the KpnI site, and the colicin gene is adjacent to it within a 2.1-kb KpnI-HincII segment. The immunity gene and one end of the colicin gene are in the region of ColE3-CA38 which is not homologous to the closely related plasmid ColE2-P9. A 0.64-kb PvuI-EcoRI segment of the plasmid adjacent to that containing the colicin and immunity genes was found to augment colicin production on solid media, and also affected the morphology of clearing zones produced by the cells when used as indicators in overlays of stabs of colicin E2 or E7 producers. The 0.64-kb segment was required in its native orientation relative to the 3.14-kb EcoRI segment to cause its effects. 相似文献
116.
The origin of replication of the IncW plasmid pSa has been cloned and the function of this origin in Escherichia coli examined. A 1.9-kb region of DNA is required for efficient autonomous replication, and a 0.47-kb fragment within this region can initiate replication only in the presence of an autonomously replicating derivative of pSa. An Mr 35,000 protein (repA) is encoded adjacent to the origin and is required for efficient initiation of replication. The derivatives examined provide information suggesting a direct role of partition factors in plasmid replication and incompatibility. 相似文献
117.
The Lactobacillus casei gene for dihydrofolate reductase has been cloned in Escherichia coli using the multicopy vector pBR322. A restriction map of the cloned DNA has been prepared. The cloned DNA directs the synthesis of L. casei dihydrofolate reductase in E. coli and confers trimethoprim and methotrexate resistance. 相似文献
118.
(1) Current models for the mechanism of cyclic electron transport in Rhodopseudomonas sphaeroides and Rhodopseudomonas capsulata have been investigated by observing the kinetics of electron transport in the presence of inhibitors, or in photosynthetically incompetent mutant strains. (2) In addition to its well-characterized effect on the Rieske-type iron sulfur center, 5-(n-undecyl)-6-hydroxy-4,7-dioxobenzothiazole (UHDBT) inhibits both cytochrome b50 and cytochrome b?90 reduction induced by flash excitation in Rps. sphaeroides and Rps. capsulata. The concentration dependency of the inhibition in the presence of antimycin (approx. 2.7 mol UHDBT/mol reaction center for 50% inhibition of extent) is very similar to that of its inhibition of the antimycin-insensitive phase of ferricytochrome c re-reduction. UHDBT did not inhibit electron transfer between the reduced primary acceptor ubiquinone (Q?I) and the secondary acceptor ubiquinone (QII) of the reaction center acceptor complex. A mutant of Rps. capsulata, strain R126, lacked both the UHDBT and antimycin-sensitive phases of cytochrome c re-reduction, and ferricytochrome b50 reduction on flash excitation. (3) In the presence of antimycin, the initial rate of cytochrome b50 reduction increased about 10-fold as the Eh(7.0) was lowered below 180 mV. A plot of the rate at the fastest point in each trace against redox potential resembles the Nernst plot for a two-electron carrier with Em(7.0) ≈ 125 ± 15 mV. Following flash excitation there was a lag of 100–500 μs before cytochrome b50 reduction began. However, there was a considerably longer lag before significant reduction of cytochrome c by the antimycin-sensitive pathway occurred. (4) The herbicide ametryne inhibited electron transfer between Q?I and QII. It was an effective inhibitor of cytochrome b50 photoreduction at Eh(7.0) 390 mV, but not at Eh(7.0) 100 mV. At the latter Eh, low concentrations of ametryne inhibited turnover after one flash in only half of the photochemical reaction centers. By analogy with the response to o-phenanthroline, it is suggested that ametryne is ineffective at inhibiting electron transfer from Q?I to the secondary acceptor ubiquinone when the latter is reduced to the semiquinone form before excitation. (5) At Eh(7.0) > 200 mV, antimycin had a marked effect on the cytochrome b50 reduction-oxidation kinetics but not on the cytochrome c and reaction center changes or the slow phase III of the electrochromic carotenoid change on a 10-ms time scale. This observation appears to rule out a mechanism in which cytochrome b50 oxidation is obligatorily and kinetically linked to the antimycin-sensitive phase of cytochrome c reduction in a reaction involving transmembrane charge transfer at high Eh values. However, at lower redox potentials, cytochrome b50 oxidation is more rapid, and may be linked to the antimycin-sensitive reduction of cytochrome c. (6) It is concluded that neither a simple linear scheme nor a simple Q-cycle model can account adequately for all the observations. Future models will have to take account of a possible heterogeneity of redox chains resulting from the two-electron gate at the level of the secondary quinone, and of the involvement of cytochrome b?90 in the rapid reactions of the cyclic electron transfer chain 相似文献
119.
Paul Jensén 《Physiologia plantarum》1981,52(4):437-441
Uptake of Rb+ from a complete nutrient solution with 2.0 mM Rb+ was studied in roots of spring wheat seedlings ( Triticum aestivum L. cv. Svenno) with different K+ levels. The relationship between Rb+ uptake and concentration of K+ in the roots indicated a negative feedback mechanism operating through allosteric control. The Rb+ uptake process in root cells was divided into two steps: (1) binding of the ion in the free space, and (ii) transmembrane transport into the cytoplasm. Metabolic and non-metabolic components of uptake were separated by addition of the metabolic inhibitor 2,4-dinitrophenol (DNP) to the nutrient solution. It is suggested that metabolic Rb+ uptake requires energy in two uptake steps (for binding to the carrier entity in the free space and for transmembrane transport) or in one step only (for transmembrane transport), dependent on the K+ status of the roots. The change from metabolic to non-metabolic binding in the free space is accomplished by changing the conformational state of the carrier (slow/fast transitions). There may be a hysteretic effect on metabolic Rb+ uptake through a slow transition between carrier states. This is superimposed on the negative cooperativity, strengthening further cooperativity at intermediate K+ levels in the roots. Non-metabolic Rb+ uptake probably consists of two components, a carrier-mediated (facilitated diffusion) and a parallel diffusive component. 相似文献
120.
The intensity of light scattered at 90° to the incident beam and the effective hydrodynamic radii of mitochondria incubated under a variety of conditions have been measured. Addition of high concentrations of uncouplers to respiring mitochondria resulted in a decrease in scatter which was not due to swelling. Addition of valinomycin to mitochondria depleted of substrate in K+-free medium produced an increase in scatter that was not due to shrinking. It is concluded that changes in the intensity of scattered light are not reliable indices of changes of volume of mitochondria, and that changes in conformation with changes in metabolic state dominate changes in light scatter. A molecular mechanism for the effect of metabolic state upon the scattered intensity is suggested. 相似文献