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P C Maloney 《BioEssays : news and reviews in molecular, cellular and developmental biology》1992,14(11):757-762
This article summarizes the study of anion exchange mechanisms in bacteria. Along with defining at least two different families of anion exchange, an examination of such carrier-mediated antiport reactions has led to techniques that considerably broaden the scope of biochemical methods for examining membrane proteins. Such advances have been exploited to show that anion exchange itself forms the mechanistic base of an entirely new kind of proton pump, one which may shed light on a variety of bacterial events, including methanogenesis. Perhaps most important, the study of exchange provided the final link in a chain of evidence pointing to a structural 'rhythm' that seems to characterize membrane carriers. These three issues--a biochemical tool, a new proton pump, and a common structural rhythm--are briefly examined in the context of their origins in the analysis of bacterial anion exchange. 相似文献
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Bacterial Community Structure in Relation to the Carbon Environments in Lettuce and Tomato Rhizospheres and in Bulk Soil 总被引:9,自引:0,他引:9
Abstract
Crop roots provide dynamic nutrient environments within agroecosystems that can influence the relative abundance and activity
of oligotrophic and copiotrophic microorganisms. Copiotrophic organisms grow in carbon (C)-rich environments and their distribution
implies that C abundance favors their survival. Survival of oligotrophic organisms is dependent on their ability to multiply
and maintain activity in habitats of low C flux. To determine if spatial variation in available C along the root coincides
with different physiological groups of bacteria, we isolated bacteria from the rhizosphere at different locations along the
tap root of lettuce and tomato plants grown under greenhouse and field conditions. In all five experiments, the overall numbers
of both oligotrophs and copiotrophs were high at the upper portions of the root and lower at tip locations and in the bulk
soil environment. Consistent patterns in the ratio of copiotrophic to oligotrophic (C:O) bacteria along the roots of lettuce
and tomato were obtained and clearly showed that the C:O ratio was different for these two crop species. With lettuce, C:O
ratios were high at the root tip (1.22 to 1.61) and upper mid-root locations (0.90 to 1.30), intermediate at the lower mid-root
locations (0.73 to 0.95), and low at the root base (0.56 to 0.76). With tomato, C:O ratios were low at root tip locations
(0.50 to 0.68) and high at mid and base locations along the root (1.20 to 1.28). These differences may reflect qualitative
and quantitative differences in root exudates between these crop species. In our experiments, nitrogen (N) concentrations
and lateral branch sites, providing C sources, were important factors influencing bacterial populations in the rhizosphere
of lettuce and tomato. Competitive interactions between microorganisms and physiological constraints with respect to substrate
affinity may be two important mechanisms influencing bacterial populations and structure of rhizosphere communities.
Received: 14 August 1996; Accepted: 10 December 1996 相似文献
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Maloney Judith A.; Tsygankova Oxana M.; Yang Lijun; Li Qiuyang; Szot Agnieszka; Baysal Kemal; Williamson John R. 《American journal of physiology. Cell physiology》1999,276(1):C221
In rat liver epithelial (WB) cells,Ca2+ pool depletion induced by twoindependent methods resulted in activation of extracellular signal-regulated protein kinase (ERK). In the first method,Ca2+ pool depletion bythapsigargin increased the activity of ERK, even when rise in cytosolicCa2+ was blocked with theCa2+ chelator BAPTA-AM. For thesecond method, addition of extracellular EGTA at a concentration shownto deplete intracellular Ca2+pools also increased ERK activity. In each instance, ERK activation, asmeasured by an immunocomplex kinase assay, was greatly reduced by thetyrosine kinase inhibitor genistein, suggesting thatCa2+ store depletion increased ERKactivity through a tyrosine kinase pathway. The intracellularCa2+-releasing agent thapsigarginincreased Fyn activity, which was unaffected by BAPTA-AM pretreatment,suggesting that Fyn activity was unaffected by increased cytosolic freeCa2+. Furthermore, depletion ofintracellular Ca2+ with EGTAcaused inactivation of protein phosphatase 2A and protein tyrosinephosphatases. ANG II-induced activations of Fyn, Raf-1, and ERK wereaugmented in cells pretreated with BAPTA-AM, but ANG II-inducedexpression of the dual-specificity phosphatase mitogen-activatedprotein kinase phosphatase-1 was blocked by BAPTA-AM pretreatment.Together these results indicate that ERK activity is regulated by thebalance of phosphorylation vs. dephosphorylation reactions in intactcells and that the amount of Ca2+stored in intracellular pools plays an important role in this regulation. 相似文献
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P Ringler M J Borgnia H Stahlberg P C Maloney P Agre A Engel 《Journal of molecular biology》1999,291(5):1181-1190
Molecular water channels (aquaporins) allow living cells to adapt to osmotic variations by rapid and specific diffusion of water molecules. Aquaporins are present in animals, plants, algae, fungi and bacteria. Here we present an electron microscopic analysis of the most ancient water channel described so far: the aquaporin Z (AqpZ) of Escherichia coli. A recombinant AqpZ with a poly(histidine) tag at the N terminus has been constructed, overexpressed and purified to homogeneity. Solubilized with octylglucoside, the purified AqpZ remains associated as a homotetramer, and assembles into highly ordered two-dimensional tetragonal crystals with unit cell dimensions a = b = 95 A, gamma = 90 degrees when reconstituted by dialysis in the presence of lipids. Three-dimensional reconstruction of negatively stained lattices revealed the p42(1)2 packing arrangement that is also observed with the human erythrocyte water channel (AQP1). The 8 A projection map of the AqpZ tetramer in frozen hydrated samples is similar to that of AQP1, consistent with the high sequence homology between these proteins. 相似文献