Excretion of ammonium by Azospirillum brasilense mutants resistant to ethylenediamine

Abstract

Several spontaneous ethylenediamine-resistant mutants of Azospirillum brasilense strain FP2 (Sp7, NalR SmR) were isolated. Four mutants, HM053, HM14, HM26, and HM210, were found to fix nitrogen constitutively in the presence of high concentration of NH4+ and to excrete NH4+ derived from nitrogen fixation. They also showed lower rates of NH4+ uptake than the wild-type strain, FP2. All of the mutants were prototrophic for glutamine or glutamate. Their glutamate synthase and glutamate dehydrogenase activities were similar to those of the wild-type strain. However, they presented different patterns of glutamine synthetase activity. Mutant HM14 showed low levels of normally regulated glutamine synthetase activity, while the other mutants showed low levels (HM053) or wild-type levels (HM26 and HM210) of constitutively adenylylated glutamine synthetase activity. The mutants are probably defective in the adenylylation system. Key words: Azospirillum brasilense, ammonium excretion, ethylenediamine resistance, glutamine synthetase.

Mutagenesis and Functional Characterization of the glnB , glnA , and nifA Genes from the Photosynthetic Bacterium Rhodospirillum rubrum

“…Indeed, several mutants have been found in A. brasilense and R. sphaeroides that show high nitrogenase activity even in the presence of high concentration of NH 4 ϩ , indicating a perturbation of both regulatory systems. Some of these mutants in A. brasilense also showed other phenotypes such as low GS activity, a defect in histidine transport, or a low rate of NH 4 ϩ uptake (8,15,18,43). Unfortunately, genes linked to these phenotypes have not been identified.…”

Posttranslational regulation of nitrogenase activity in Azospirillum brasilense ntrBC mutants: ammonium and anaerobic switch-off occurs through independent signal transduction pathways

“…Several regulatory A. brasilense mutants, which showed high nitrogenase activity in the presence of NH4+ 'Iwere isolated. Some of these mutants lacked GS activity (11), and some had effects on histidine transport (9) or NH4' uptake and excretion (22). Unfortunately, neither the nature of these mutations nor their effects on the DRAT/DRAG system were determined.…”

History on the biological nitrogen fixation research in graminaceous plants: special emphasis on the Brazilian experience

“…This work led to an intensified effort at the molecular level in order to understand the structural organization and regulation of the nif genes of this endophytic diazotrophic bacterium. The work on Azospirillum brasilense was initially centered on the isolation and characterization of regulatory mutants able to constitutively fix nitrogen in presence of NH 4 + (Machado et al 1991). These mutants were also able to excrete ammonia, the product of the nitrogen fixation process (Machado et al 1991, Vitorino et al 2001.…”

“…The work on Azospirillum brasilense was initially centered on the isolation and characterization of regulatory mutants able to constitutively fix nitrogen in presence of NH 4 + (Machado et al 1991). These mutants were also able to excrete ammonia, the product of the nitrogen fixation process (Machado et al 1991, Vitorino et al 2001. The group has also dedicated considerable effort on studying the regulation of the nitrogen fixation process in A. brasilense.…”