Patterns of [13N]ammonium uptake and assimilation by Frankia HFPArl3
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Nitrogen-starved cells of Frankia strain HFPArl3 incorporated [13N]-labeled ammonium into glutamine ≫ serine ≫ (glutamate, alanine, aspartate), after five-minute radioisotope exposures. High initial endogenous pools of glutamate were reduced, while total glutamine increased, during short term NH inf4 sup+ incubation. Preincubation of cells in methionine sulfoximine (MSX) resulted in [13N]glutamine reduced by more than 80%, while [13N]glutamate and [13N]alanine levels increased. The results suggest that glutamine synthetase is the primary enzyme of ammonium assimilation, and that glutamate dehydrogenase and alanine dehydrogenase may also function in ammonium assimilation at low levels. Efflux of [13N]serine and lesser amounts of [13N]glutamine was detected from the Frankia cells. The identity of both Ser and Gln in the extracellular compartment was confirmed with gas chromatography/mass spectrometry. Serine efflux may be of significance in nitrogen transfer in Frankia.
Key wordsFrankia Nitrogen fixation Glutamine synthetase Ammonium assimilation Serine
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