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941.
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946.
Jennifer Willms S. B. Rood Walter Willms Melvin Tyree 《Trees - Structure and Function》1998,12(4):215-223
The conservation of riparian (river valley flood plain) forests relies on the provision of instream flows that are sufficient
to sustain tree growth. In the present study, annual branch growth increments were investigated as an indicator of environmental
favorability for riparian cottonwoods. Trees of three species, Populus angustifolia, P. balsamifera, and P. deltoides, and their natural interspecific hybrids, were studied at five sites along the Oldman and South Saskatchewan rivers in Alberta,
Canada. Annual branch growth increments for the interval from 1983 to 1992 were positively correlated with stream flows (r
2 = 0.79 at Lethbridge) and slightly negatively correlated with weather variables that contribute to water demand: evaporation,
temperature, wind, and/or sunshine. The combination of January to May stream flow (water supply) and June evaporation (water
demand) almost entirely accounted for the branch growth variation across years (r
2 = 0.91 at Lethbridge). Tree ring increments were also investigated but were less closely correlated than branch increments
across trees or with stream flow. Branch growth increments thus provide an accurate but short duration (1 or 2 decades) record
of environmental favorability for growth. The close correlation between branch growth and stream flow indicates that water
is the principal limitation to growth of these riparian cottonwoods and that these trees obtained their water from a source
linked to the stream, the riparian water table. Analyses of branch increments should provide a management tool for (i) determining
instream flow needs for riparian cottonwoods and (ii) analyzing impacts of stream flow alterations due to river damming or
water diversion.
Received: 8 May 1997 / Accepted: 23 September 1997 相似文献
947.
Alexander F. Arendsen Jonathan Hadden Graeme Card Alan S. McAlpine Susan Bailey Vjacheslav Zaitsev Elizabeth H. M. Duke P. F. Lindley Monika Kröckel Alfred X. Trautwein Martinus C. Feiters John M. Charnock C. David Garner Sophie J. Marritt Andrew J. Thomson Ingeborg M. Kooter Michael K. Johnson Willy A. M. van den Berg Walter M. A. M. van Dongen W. R. Hagen 《Journal of biological inorganic chemistry》1998,3(1):81-95
The three-dimensional structure of the native "putative prismane" protein from Desulfovibrio vulgaris (Hildenborough) has been solved by X-ray crystallography to a resolution of 1.72?Å. The molecule does not contain a [6Fe-6S] prismane cluster, but rather two 4Fe clusters some 12?Å apart and situated close to the interfaces formed by the three domains of the protein. Cluster 1 is a conventional [4Fe-4S] cubane bound, however, near the N-terminus by an unusual, sequential arrangement of four cysteine residues (Cys 3, 6, 15, 21). Cluster 2 is a novel 4Fe structure with two μ2-sulfido bridges, two μ2-oxo bridges, and a partially occupied, unidentified μ2 bridge X. The protein ligands of cluster 2 are widely scattered through the second half of the sequence and include three cysteine residues (Cys 312, 434, 459), one persulfido-cysteine (Cys 406), two glutamates (Glu 268, 494), and one histidine (His 244). With this unusual mixture of bridging and external type of ligands, cluster 2 is named the "hybrid" cluster, and its asymmetric, open structure suggests that it could be the site of a catalytic activity. X-ray absorption spectroscopy at the Fe K-edge is readily interpretable in terms of the crystallographic model when allowance is made for volume contraction at 10?K; no Fe··Fe distances beyond 3.1?Å could be identified. EPR, Mössbauer and MCD spectroscopy have been used to define the oxidation states and the magnetism of the clusters in relation to the crystallographic structure. Reduced cluster 1 is a [4Fe-4S]1+ cubane with S?=?3/2; it is the first biological example of a "spin-admixed" iron-sulfur cluster. The hybrid cluster 2 has four oxidation states from (formally) all FeIII to three FeII plus one FeIII. The four iron ions are exchange coupled resulting in the system spins S?=?0, 9/2, 0 (and 4), 1/2, respectively, for the four redox states. Resonance Raman spectroscopy suggests that the bridging ligand X which could not be identified unambiguously in the crystal structure is a solvent-exchangeable oxygen. 相似文献
948.
949.
The Saccharomyces cerevisiae Prenylcysteine Carboxyl Methyltransferase Ste14p Is in the Endoplasmic Reticulum Membrane 总被引:1,自引:0,他引:1
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Julia D. Romano Walter K. Schmidt Susan Michaelis 《Molecular biology of the cell》1998,9(8):2231-2247
Eukaryotic proteins containing a C-terminal CAAX motif undergo a series of posttranslational CAAX-processing events that include isoprenylation, C-terminal proteolytic cleavage, and carboxyl methylation. We demonstrated previously that the STE14 gene product of Saccharomyces cerevisiae mediates the carboxyl methylation step of CAAX processing in yeast. In this study, we have investigated the subcellular localization of Ste14p, a predicted membrane-spanning protein, using a polyclonal antibody generated against the C terminus of Ste14p and an in vitro methyltransferase assay. We demonstrate by immunofluorescence and subcellular fractionation that Ste14p and its associated activity are localized to the endoplasmic reticulum (ER) membrane of yeast. In addition, other studies from our laboratory have shown that the CAAX proteases are also ER membrane proteins. Together these results indicate that the intracellular site of CAAX protein processing is the ER membrane, presumably on its cytosolic face. Interestingly, the insertion of a hemagglutinin epitope tag at the N terminus, at the C terminus, or at an internal site disrupts the ER localization of Ste14p and results in its mislocalization, apparently to the Golgi. We have also expressed the Ste14p homologue from Schizosaccharomyces pombe, mam4p, in S. cerevisiae and have shown that mam4p complements a Δste14 mutant. This finding, plus additional recent examples of cross-species complementation, indicates that the CAAX methyltransferase family consists of functional homologues. 相似文献
950.
Walter E. Horton Jr. Imelda Udo Patricia Precht Richard Blakir Karen Hasty 《In vitro cellular & developmental biology. Animal》1998,34(5):378-384
Summary The objective of this study was to determine if an immortalized mammalian chondrocyte cell line had a profile of matrix metalloproteinase
(MMP) expression that was consistent with what has been reported for primary chondrocytes in vitro and in vivo. A combination of zymography, Western, and Northern analysis was used to examine the expression of MMPs that are relevant
to cartilage degradation. Both interleukin-1β and tumor necrosis factor α induced a 4- to 9-fold increase in the level of
MMP-9 expression in conditioned media, and a 17- to 24-fold increase in MMP-3 mRNA. Other compounds such as basic fibroblast
growth factor and staurosporine each increased MMP-9 expression individually and potentiated the effects of the two cytokines.
Transforming growth factor β had no positive or inhibitory effects. N-methyl arginine blocked the increase in nitric oxide observed following treatment with the cytokines but did not prevent
the increased expression of MMPs. The pattern of metalloproteinase expression observed in IRC cells and the response to cytokines
is very similar to what has been reported during the pathogenesis of osteoarthritis. The IRC cells should be useful as a model
system to study basic mechanisms controlling chondrocyte MMP expression and to identify pharmacological modulators of this
process. 相似文献