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21.
An important component of computer programs for determining the solution conformation of proteins and other flexible molecules
from nuclear magnetic resonance data are the so-called “bound smoothing algorithms”, which compute lower and upper limits
on the values of all the interatomic distances from the relatively sparse set which can usually be measured experimentally.
To date, the only methods efficient enough for use in large problems take account of only the triangle inequality, but an
appreciable improvement in the precision of the limits is possible if the algebraic relations between the distances among
each quadruple of atoms are also considered. The goal of this paper is to use a recently improved algorithm for computing
these “tetrangle inequality limits” to determine just how much improvement really is possible, given the types of experimental
data that are usually available. 相似文献
22.
Timothy Troy 《Dialectical Anthropology》1990,15(1):74-81
Timothy Troy is a Librarian at the Center for Creative Photography, University of Arizona. 相似文献
23.
Mahan James R.; Sherman Timothy D.; Funkhouser Edward A. 《Plant & cell physiology》1988,29(4):735-737
The thermal dependence of two of the reactions catalyzed bythe nitrate reductase from Chlorella vulgaris was determined.The activation energies for NADH:nitrate oxidoreductase (EC1.6.6.1
[EC]
) and NADH:Cytochrome c oxidoreductase (EC 1.6.99.3
[EC]
)are 42.1 kJ?mol1 and 21.5 kJ?mol1, respectively.Since the thermal dependency of the two enzymes is different,ratios of the activities will vary with temperature. The importanceof both rigorous thermal control during nitrate reductase assaysas well as the need to specify the temperature at which theratio of activities for the enzyme are clearly established.
1Present Address: Cropping Systems Research Laboratory, USDA-ARS,Route 3, Box 215, Lubbock, TX 79401, U.S.A. (Received November 25, 1987; Accepted March 2, 1988) 相似文献
24.
25.
Timothy J. Fahey 《Mycorrhiza》1992,1(2):83-89
Summary Mycorrhizae play an important role in regulating patterns of energy and nutrient flux in terrestrial ecosystems. To conceptualize this role I develop the theory behind a simple index of the efficiency of soil resource acquisition by plant root systems (E). The morphological, physiological and demographic characteristics of mycorrhizae that define E appear to vary with environment and with plant community composition. This theory is elaborated with examples drawn from forest ecology literature. Some inconsistencies among observations of fine root dynamics are particularly revealing: (1) belowground carbon allocation vs soil fertility; (2) causes of root mortality; (3) root longevity vs decomposition rates. A comprehensive theory of mycorrhizal and ecosystem dynamics must await resolution of these inconsistencies and better quantitative information on mycorrhizal features affecting E. 相似文献
26.
A homologue of the Escherichia coli DsbA protein involved in disulphide bond formation is required for enterotoxin biogenesis in Vibrio cholerae 总被引:22,自引:0,他引:22
A strain of Vibrio cholerae, which had been engineered to express high levels of the non-toxic B subunit (EtxB) of Escherichia coli heat-labile enterotoxin, was subjected to transposon (TnphoA) mutagenesis. Two chromosomal TnphoA insertion mutations of the strain were isolated that showed a severe defect in the amount of EtxB produced. The loci disrupted by TnphoA in the two mutant derivatives were cloned and sequenced, and this revealed that the transposon had inserted at different sites in the same gene. The open reading frame of the gene predicts a 200-amino-acid exported protein, with a Cys-X-X-Cys motif characteristic of thioredoxin, protein disulphide isomerase, and DsbA (a periplasmic protein required for disulphide bond formation in E. coli). The V. cholerae protein exhibited 40% identity with the DsbA protein of E. coli, including 90% identity in the region of the active-site motif. Introduction of a plasmid encoding E. coli DsbA into the V. cholerae TnphoA derivatives was found to restore enterotoxin formation, whilst expression of Etx or EtxB in a dsbA mutant of E. coli confirmed that DsbA is required for enterotoxin formation in E. coli. These results suggest that, since each EtxB subunit contains a single intramolecular disulphide bond, a transient intermolecular interaction with DsbA occurs during toxin subunit folding which catalyses formation of the disulphide in vivo. 相似文献
27.
Estimators of the Mantel-Haenszel variance consistent in both sparse data and large-strata limiting models 总被引:5,自引:0,他引:5
This paper proposes a new estimator of the variance of the Mantel-Haenszel estimator of common odds ratio that is easily computed and consistent in both sparse data and large-strata limiting models. Monte Carlo experiments compare its performance to that of previously proposed variance estimators. 相似文献
28.
The influence of subterranean termites on the hydrological characteristics of a Chihuahuan desert ecosystem 总被引:4,自引:0,他引:4
Summary Rainfall simulation at an average intensity of 124 mm·h-1 was used to compare infiltration and run off on arid areas where subterranean termites had been eliminated four years prior to the initiation of the study (termite free) with adjacent areas populated by subterranean termites (termites present). Infiltration rates on termite free plots with less than 5% perennial plant cover were significantly lower 51.3±6.8 mm·h-1 than rates on comparable termites present plots 88.4±5.6 mm·h-1. On plots centered on Larrea tridentata shrubs, there were no differences in infiltration rates with or without termites. Plots with shrub cover had the highest infiltration rates 101±6 mm·h-1. Highest run-off volumes were recorded from termite free <5% grass cover plots and the lowest from plots with shrubs. There were no differences in suspended sediment concentrations from termites present and termite free plots. Average bed load concentration was more than three times greater from termite free, <5% cover plots than from termites present, <5% cover plots.The reduction in infiltration, high run-off volumes and high bedloads from termite free areas without shrub cover is related to increased soil bulk density resulting from the collapse of subterranean galleries of the termites that provide avenues of bulk flow into the soil. Subterranean termites affect the hydrology of Chihuahuan desert systems by enhancing water infiltration and retention of top soil. The presence of a shrub canopy and litter layer cancels any effect of subterranean termites on hydrological parameters. Since approximately 2/3 of the area is not under shrub canopies, subterranean termites are considered to be essential for the maintenance of the soil water characteristics that support the present vegetation. 相似文献
29.
30.
Local names of vascular plants, accompanied by botanical specimens, were collected in the John Crow Mountains, Portland Parish, Jamaica. Each of the names was volunteered by one local informant; many were independently corroborated by others. We recorded 91 names of forest trees and shrubs, 20 names of climbers, and 33 names of other plants. The uneven distribution of local names among different categories of plants is discussed. The names are mostly of English origin, with a scattering from Spanish, Amerindian and West African languages. Many names are not listed in the standard floras; 20 names are apparently hitherto unrecorded in print. The same name was often found to be applied to a different species from the usage given in the floras, showing the variation in vernacular names from one part of the island to another. 相似文献