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Ammonium uptake in Rhodopseudomonas capsulata
Authors:Barbara R. Sharak Genthner  Judy D. Wall
Affiliation:(1) Department of Biochemistry, University of Missouri, 65211 Columbia, MO, USA;(2) Present address: Batelle, Columbus Laboratories, 505 King Avenue, 43201 Columbus, OH, USA
Abstract:Investigations of the uptake of ammonium (NH4+) by Rhodopseudomonas capsulata B100 supported the presence of an NH4+transport system. Experimentally NH4+was determined by electrode or indophenol assay and saturation kinetics were observed with two apparent Km's of 1.7 mgrM and 11.1 mgrM (pH 6.8, 30°) and a Vmax at saturation of 50–60 nmol/min·mg protein. The optimum pH and temperature were 7.0 and 33° C, respectively. The Q10 quotient was calculated to be 1.9 at 100 mgrM NH4+, indicating enzymatic involvement. In contrast to the wild type, B100, excretion of NH4+, not uptake, was observed in a glutamine auxotroph, R. capsulata G29, which is derepressed for nitrogenase and lacks glutamine synthetase activity. G29R1, a revertant of G29, also took up NH4+at the same rate as wild type and had fully restored glutamine synthetase activity. Partially restored derivatives, G29R5 and G29R6, grew more slowly than wild type on NH4+as the nitrogen source, remained derepressed for nitrogenase in the presence of NH4+, and displayed rates of NH4+uptake in proportion to their glutamine synthetase activity. Ammonium uptake and glutamine synthetase activity were also restored in R. capsulata G29 exconjugants which had received the plasmid pPS25, containing the R. capsulata glutamine synthetase structural gene. These data suggest that NH4+transport is tightly coupled to assimilation.Abbreviations used CHES cyclohexylaminoethanesulfonic acid - GS glutamine synthetase - SDS sodium dodecylsulfate
Keywords:Ammonium uptake  Rhodopseudomonas capsulata  Phototroph  Glutamine synthetase  glnA  Ammonium export
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