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961.
We studied the winter resource selection of muskoxen Ovibos moschatus in the High Arctic using a nested hierarchy of spatial scales 1) population range, 2) travel routes, 3) feeding sites (l e clusters of feeding craters), 4) feeding craters, and 5) diet (I e plant species) We found that, generally, patterns of selection remained consistent across all levels At successively smaller scales, muskoxen selected for higher graminoid abundance and particularly for thinner, softer snow cover, although we did not reject the hypothesis of random travel route selection Muskoxen uncovered forages from beneath the snow cover, by cratering, near the flonstic and nival extremes of availability Selection was consistently biased toward use of water sedge, Carex aquatilis As scale changed, however, muskoxen showed reversals of preference for some other forage species Diet was dominated by C aquatilis and cotton sedge, Eriophorum angustifolium , species characteristic of lowland meadows During spring melt, muskoxen moved to snow-free uplands to feed Dietary quality, as revealed by fecal nitrogen, increased at this time The consistency of the results across scales implied that these local levels of habitat selection occurred within one scaling domain 相似文献
962.
Mauseth James D. Uozumi Yoriko Plemons Brandon J. Landrum James V. 《Journal of plant research》1995,108(4):517-526
Wide-band tracheids are a specialized tracheid type in which an annular or helical secondary wall projects deeply into the
cell lumen. They are short, wide and spindle-shaped, and their bandlike secondary walls cover little of the primary wall,
leaving most of it available for water diffusion. Wide-band tracheids appear to store and conduct water while preventing the
spread of embolisms. They may be the most abundant tracheary element in the xylem, but they are always accompanied by at least
a few vessels. Typically, fibers are absent wherever wide-band tracheids are present. Wide-band tracheids occur in the primary
and secondary xylem of succulent stems, leaves and roots in genera of all three subfamilies of Cactaceae but were not found
in the relictual genusPereskia, which lacks succulent tissues. In the large subfamily Cactoideae, wide-band tracheids occur only in derived members, and
wide-band tracheids of North American Cactoideae are narrower and are aligned in a more orderly radial pattern than those
of South American Cactoideae. Wide-band tracheids probably arose at least three times in Cactaceae. 相似文献
963.
He Liu H. Peter Spielmann Nikolai B. Ulyanov David E. Wemmer Thomas L. James 《Journal of biomolecular NMR》1995,6(4):390-402
Summary The effect of experimental and integration errors on the calculations in interproton distances from NOE intensities is examined. It is shown that NOE intensity errors can have a large impact on the distances determined. When multiple spin (spin diffusion) effects are significant, the calculated distances are often underestimated, even when using a complete relaxation matrix analysis. In this case, the bias of distances to smaller values is due to the random errors in the NOE intensities. We show here that accurate upper and lower bounds of the distances can be obtained if the intensity errors are properly accounted for in the complete relaxation matrix calculations, specifically the MARDIGRAS algorithm. The basic MARDIGRAS algorithm has been previously described [Borgias, B.A. and James, T.L. (1990) J. Magn. Reson., 87, 475–487]. It has been shown to provide reasonably good interproton distance bounds, but experimental errors can compromise the quality of the resulting restraints, especially for weak cross peaks. In a new approach introduced here, termed RANDMARDI (random error MARDIGRAS), errors due to random noise and integration errors are mimicked by the addition of random numbers from within a specified range to each input intensity. Interproton distances are then calculated for the modified intensity set using MARDIGRAS. The distribution of distances that define the upper and lower distance bounds is obtained by using N randomly modified intensity sets. RANDMARDI has been used in the solution structure determination of the interstrand cross-link (XL) formed between 4-hydroxymethyl-4,5,8-trimethylpsoralen (HMT) and the DNA oligomer d(5-GCGTACGC-3)2 [Spielmann, H.P. et al. (1995) Biochemistry, 34, 12937–12953]. RANDMARDI generates accurate distance bounds from the experimental NOESY cross-peak intensities for the fixed (known) interproton distances in XL. This provides an independent internal check for the ability of RANDMARDI to accurately fit the experimental data. The XL structure determined using RANDMARDI-generated restrains is in good agreement with other biophysical data that indicate that there is no bend introduced into the DNA by the cross-link. In contrast, isolated spin-pair approximation calculations give distance restraints that, when applied in a restrained molecular dynamics protocol, produce a bent structure.Abbreviations NOE
nuclear Overhauser effect
- SD
standard deviation
- HMT
4-hydroxymethyl-4,5,8-trimethylpsoralen
- XL
psoralen-DNA interstrand cross-link 相似文献
964.
965.
966.
967.
Genetic regulation of gibberellin deactivation in Pisum 总被引:2,自引:0,他引:2
John J. Ross James B. Reid Stephen M. Swain Omar Hasan rew T. Poole Peter Hedden Christine L. Willis 《The Plant journal : for cell and molecular biology》1995,7(3):513-523
The regulation of gibberellin (GA) deactivation was examined using the sin (slender) mutation in the garden pea (Pisum sativum L.). This mutation blocks the deactivation of GA20, the precursor of the bioactive GA1. Firstly, crosses were made to combine sin with the GA biosynthesis mutations na, lhi and le-3. The combination sin na produced a novel phenotype, with long (‘slender’) basal internodes and extremely short (‘nana’) upper internodes. In contrast, the double mutant sin lhi was phenotypically dwarf. The mutation sin causes an accumulation of GA20 in maturing seeds, and this was unaffected by na, since the na mutation is not expressed in seeds. In contrast, lhi seeds did not accumulate GA20, since lhi imposes an early block on GA biosynthesis. Secondly, the effects of sin on several steps in GA deactivation were investigated. In maturing seeds, the mutation sin blocks two steps in GA20 metabolism, namely, GA20 to GA29, and GA29 to GA29-catabolite. In the vegetative plant, on the other hand, sin blocked the step GA20 to GA29, but not GA29 to GA29-catabolite; the steps GA20 to GA81 and GA20 to GA1 were also not impaired in this mutant. It is clear that the effects of sin, like those of na, are strongly organ-specific. The presence of separate enzymes for the steps GA20 to GA29 and GA29 to GA29-catabolite was suggested by the observation that GA8 inhibited the latter step, but not the former, and by the inability of GA20 and GA29 to inhibit each other's metabolism. It is suggested that the Sin gene may be a regulatory gene controlling the expression of two structural genes involved in GA deactivation. 相似文献
968.
Water molecules participate in proteinase-inhibitor interactions: crystal structures of Leu18, Ala18, and Gly18 variants of turkey ovomucoid inhibitor third domain complexed with Streptomyces griseus proteinase B. 总被引:3,自引:3,他引:0 下载免费PDF全文
K. Huang W. Lu S. Anderson M. Laskowski Jr M. N. James 《Protein science : a publication of the Protein Society》1995,4(10):1985-1997
Crystal structures of the complexes of Streptomyces griseus proteinase B (SGPB) with three P1 variants of turkey ovomucoid inhibitor third domain (OMTKY3), Leu18, Ala18, and Gly18, have been determined and refined to high resolution. Comparisons among these structures and of each with native, uncomplexed SGPB reveal that each complex features a unique solvent structure in the S1 binding pocket. The number and relative positions of water molecules bound in the S1 binding pocket vary according to the size of the side chain of the P1 residue. Water molecules in the S1 binding pocket of SGPB are redistributed in response to the complex formation, probably to optimize hydrogen bonds between the enzyme and the inhibitor. There are extensive water-mediated hydrogen bonds in the interfaces of the complexes. In all complexes, Asn 36 of OMTKY3 participates in forming hydrogen bonds, via water molecules, with residues lining the S1 binding pocket of SGPB. For a homologous series of aliphatic straight side chains, Gly18, Ala18, Abu18, Ape18, and Ahp18 variants, the binding free energy is a linear function of the hydrophobic surface area buried in the interface of the corresponding complexes. The resulting constant of proportionality is 34.1 cal mol-1 A-2. These structures confirm that the binding of OMTKY3 to the preformed S1 pocket in SGPB involves no substantial structural disturbances that commonly occur in the site-directed mutagenesis studies of interior residues in other proteins, thus providing one of the most reliable assessments of the contribution of the hydrophobic effect to protein-complex stability. 相似文献
969.
M. Fujinaga M. M. Chernaia N. I. Tarasova S. C. Mosimann M. N. James 《Protein science : a publication of the Protein Society》1995,4(5):960-972
The three-dimensional crystal structure of human pepsin and that of its complex with pepstatin have been solved by X-ray crystallographic methods. The native pepsin structure has been refined with data collected to 2.2 A resolution to an R-factor of 19.7%. The pepsin:pepstatin structure has been refined with data to 2.0 A resolution to an R-factor of 18.5%. The hydrogen bonding interactions and the conformation adopted by pepstatin are very similar to those found in complexes of pepstatin with other aspartic proteinases. The enzyme undergoes a conformational change upon inhibitor binding to enclose the inhibitor more tightly. The analysis of the binding sites indicates that they form an extended tube without distinct binding pockets. By comparing the residues on the binding surface with those of the other human aspartic proteinases, it has been possible to rationalize some of the experimental data concerning the different specificities. At the S1 site, valine at position 120 in renin instead of isoleucine, as in the other enzymes, allows for binding of larger hydrophobic residues. The possibility of multiple conformations for the P2 residue makes the analysis of the S2 site difficult. However, it is possible to see that the specific interactions that renin makes with histidine at P2 would not be possible in the case of the other enzymes. At the S3 site, the smaller volume that is accessible in pepsin compared to the other enzymes is consistent with its preference for smaller residues at the P3 position. 相似文献
970.
John J. Malinowski Bruce L. Grasberger Gary Trakshel Edward E. Huston Tracey M. Banks Patricia G. Brake Richard B. Ciccarelli Barry N. Jones James A. Koehn Diane Kratz Nicole Lundberg Panayiotis E. Stevis Carla T. Helaszek Mark A. Ator Angela M. Small Wood Travis Stams Byron Rubin Richard S. Alexander 《Protein science : a publication of the Protein Society》1995,4(10):2149-2155