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Ammonium Assimilation (ammonium + assimilation)
Selected AbstractsMETABOLIC AND ECOLOGICAL CONSTRAINTS IMPOSED BY SIMILAR RATES OF AMMONIUM AND NITRATE UPTAKE PER UNIT SURFACE AREA AT LOW SUBSTRATE CONCENTRATIONS IN MARINE PHYTOPLANKTON AND MACROALGAE,JOURNAL OF PHYCOLOGY, Issue 2 2007T. Alwyn Marine phytoplankton and macroalgae acquire important resources, such as inorganic nitrogen, from the surrounding seawater by uptake across their entire surface area. Rates of ammonium and nitrate uptake per unit surface area were remarkably similar for both marine phytoplankton and macroalgae at low external concentrations. At an external concentration of 1 ,M, the mean rate of nitrogen uptake was 10±2 nmol·cm,2·h,1 (n=36). There was a strong negative relationship between log surface area:volume (SA:V) quotient and log nitrogen content per cm2 of surface (slope=,0.77), but a positive relationship between log SA:V and log maximum specific growth rate (,max; slope=0.46). There was a strong negative relationship between log SA:V and log measured rate of ammonium assimilation per cm2 of surface, but the slope (,0.49) was steeper than that required to sustain ,max (,0.31). Calculated rates of ammonium assimilation required to sustain growth rates measured in natural populations were similar for both marine phytoplankton and macroalgae with an overall mean of 6.2±1.4 nmol·cm,2·h,1 (n=15). These values were similar to maximum rates of ammonium assimilation in phytoplankton with high SA:V, but the values for algae with low SA:V were substantially less than the maximum rate of ammonium assimilation. This suggests that the growth rates of both marine phytoplankton and macroalgae in nature are often constrained by rates of uptake and assimilation of nutrients per cm2 surface area. [source] CONTRASTING EFFECTS OF METHIONINE SULFOXIMINE ON UPTAKE AND ASSIMILATION OF AMMONIUM IN ULVA INTESTINALIS (CHLOROPHYCEAE),JOURNAL OF PHYCOLOGY, Issue 4 2004Neill G. Barr Ammonium is assimilated in algae by the glutamine synthetase (GS),glutamine:2-oxoglutarate aminotransferase pathway. In addition to the assimilation of external ammonium taken up across the cell membrane, an alga may have to reassimilate ammonium derived from endogenous sources (i.e. nitrate reduction, photorespiration, and amino acid degradation). Methionine sulfoximine (MSX), an irreversible inhibitor of GS, completely inhibited GS activity in Ulva intestinalis L. after 12 h. However, assimilation of externally derived ammonium was completely inhibited after only 1,2 h in the presence of MSX and was followed by production of endogenous ammonium. However, endogenous ammonium production in U. intestinalis represented only a mean of 4% of total assimilation attributable to GS. The internally controlled rate of ammonium uptake (Vi) was almost completely inhibited in the presence of MSX, suggesting that Vi is a measure of the maximum rate of ammonium assimilation. After complete inhibition of ammonium assimilation in the presence of MSX, the initial or surge (Vs) rate of ammonium uptake in the presence of 400 ,M ammonium chloride decreased by only 17%. However, the amount that the rate of ammonium uptake decreased by was very similar to the uninhibited rate of ammonium assimilation. In addition, the decrease in the rate of ammonium uptake in darkness (in the absence of MSX) in the presence of 400 ,M ammonium chloride matched the decrease in the rate of ammonium assimilation. However, in the presence of 10 ,M ammonium chloride, MSX completely inhibited ammonium assimilation but had no effect on the rate of uptake. [source] Mechanism of insoluble phosphate solubilization by Pseudomonas fluorescens RAF15 isolated from ginseng rhizosphere and its plant growth-promoting activitiesLETTERS IN APPLIED MICROBIOLOGY, Issue 2 2009K.-H. Park Abstract Aims:, To investigate the mechanism of insoluble phosphate (P) solubilization and plant growth-promoting activity by Pseudomonas fluorescens RAF15. Methods and Results:, We investigated the ability of Ps. fluorescens RAF15 to solubilize insoluble P via two possible mechanisms: proton excretion by ammonium assimilation and organic acid production. There were no clear differences in pH and P solubilization between glucose-ammonium and glucose-nitrate media. P solubilization was significantly promoted with glucose compared to fructose. Regardless of nitrogen sources used, Ps. fluorescens RAF15 solubilized little insoluble P with fructose. High performance liquid chromatography analysis showed that Ps. fluorescens RAF15 produced mainly gluconic and tartaric acids with small amounts of 2-ketogluconic, formic and acetic acids. During the culture, the pH was reduced with increase in gluconic acid concentration and was inversely correlated with soluble P concentration. Ps. fluorescens RAF1 showed the properties related to plant growth promotion: pectinase, protease, lipase, siderophore, hydrogen cyanide, and indoleacetic acid. Conclusion:, This study indicated that the P solubility was directly correlated with the organic acids produced. Significance and Impact of the Study:,Pseudomonas fluorescens RAF15 possessed different traits related to plant growth promotion. Therefore, Ps. fluorescens RAF15 could be a potential candidate for the development of biofertilizer or biocontrol agent. [source] Regulation of AmtR-controlled gene expression in Corynebacterium glutamicum: mechanism and characterization of the AmtR regulonMOLECULAR MICROBIOLOGY, Issue 2 2005Gabriele Beckers Summary AmtR, the master regulator of nitrogen control in Corynebacterium glutamicum, represses transcription of a number of genes during nitrogen surplus. Repression is released by an interaction of AmtR with signal transduction protein GlnK. As shown by pull-down assays and gel retardation experiments, only adenylylated GlnK, which is present in the cells during nitrogen limitation, is able to bind to AmtR. The AmtR regulon was characterized in this study by a combination of bioinformatics, transcriptome and proteome analyses. At least 33 genes are directly controlled by the repressor protein including those encoding transporters and enzymes for ammonium assimilation (amtA, amtB, glnA, gltBD), urea and creatinine metabolism (urtABCDE, ureABCEFGD, crnT, codA), a number of biochemically uncharacterized enzymes and transport systems (NCgl1099, NCgl1100, NCgl 1915,1918) as well as signal transduction proteins (glnD, glnK). For the AmtR regulon, an AmtR box has been defined which comprises the sequence tttCTATN6AtAGat/aA. Furthermore, the transcriptional organization of AmtR-regulated genes and operons was characterized. [source] Molecular characterization, function and regulation of ammonium transporters (Amt) and ammonium-metabolizing enzymes (GS, NADP-GDH) in the ectomycorrhizal fungus Hebeloma cylindrosporumMOLECULAR MICROBIOLOGY, Issue 2 2003Arnaud Javelle Summary External hyphae, which play a key role in nitrogen nutrition of trees, are considered as the absorbing structures of the ectomycorrhizal symbiosis. Here, we have cloned and characterized Hebeloma cylindrosporum AMT1, GLNA and GDHA genes, which encode a third ammonium transporter, a glutamine synthetase and an NADP-dependent glutamate dehydrogenase respectively. Amt1 can fully restore the pseudohyphal growth defect of a Saccharomyces cerevisiae mep2 mutant, and this is the first evidence that a heterologous member of the Mep/Amt family complements this dimorphic change defect. Dixon plots of the inhibition of methylamine uptake by ammonium indicate that Amt1 has a much higher affinity than the two previously characterized members (Amt2 and Amt3) of the Amt/Mep family in H. cylindrosporum. We also identified the intracellular nitrogen pool(s) responsible for the modulation of expression of AMT1, AMT2, AMT3, GDHA and GLNA. In response to exogenously supplied ammonium or glutamine, AMT1, AMT2 and GDHA were downregulated and, therefore, these genes are subjected to nitrogen repression in H. cylindrosporum. Exogenously supplied nitrate failed to induce a downregulation of the five mRNAs after transfer of mycelia from a N-starved condition. Our results demonstrate that glutamine is the main effector for AMT1 and AMT2 repression, whereas GDHA repression is controlled by intracellular ammonium, independently of the intracellular glutamine or glutamate concentration. Ammonium transport activity may be controlled by intracellular NH4+. AMT3 and GLNA are highly expressed but not highly regulated. A model for ammonium assimilation in H. cylindrosporum is presented. [source] Suillus bovinus glutamine synthetase gene organization, transcription and enzyme activities in the Scots pine mycorrhizosphere developed on forest humusNEW PHYTOLOGIST, Issue 2 2004Jarmo T. Juuti Summary ,,Glutamine synthetase (GS) expression and activity is of central importance for cellular ammonium assimilation and recycling. Thus, a full characterization of this enzyme at the molecular level is of critical importance for a better understanding of nitrogen (N) assimilation in the mycorrhizal symbiosis. ,,Genomic and cDNA libraries of Suillus bovinus were constructed to isolate the GS gene, glnA, and corresponding cDNAs. The transcription initiation site was identified and transcription and enzyme activities were characterized in pure culture mycelium and mycorrhiza, and extramatrical mycelium samples harvested from Scots pine,Suillus bovinus microcosms grown on forest humus. ,,Pure culture mycelium, mycorrhiza and extramatrical mycelium all exhibited equivalent levels of GS transcription, translation and enzyme activities. However, levels of transcription and enzyme activity did not correlate as a large majority of detectable transcripts showed specific 5,-end truncation. ,,Our data suggest that GS is constitutively expressed and not directly affected by environmental conditions of the symbiotic N uptake. Any changes in the intracellular ammonium level are most likely handled by regulatory flexibility of GS at enzyme level. [source] | ||||||||||