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J. Bacteriol., 08 1996, 4830-4838, Vol 178, No. 16
F Arsene, PA Kaminski and C Elmerich
Azospirillum brasilense NifA, which is synthesized under all physiological
conditions, exists in an active or inactive from depending on the
availability of ammonia. The activity also depends on the presence of PII,
as NifA is inactive in a glnB mutant. To investigate further the mechanism
that regulates NifA activity, several deletions of the nifA coding sequence
covering the amino-terminal domain of NifA were constructed. The ability of
these truncated NifA proteins to activate the nifH promoter in the absence
or presence of ammonia was assayed in A. brasilense wild-type and mutant
strains. Our results suggest that the N-terminal domain is not essential
for NifA activity. This domain plays an inhibitory role which prevents NifA
activity in the presence of ammonia. The truncated proteins were also able
to restore nif gene expression to a glnB mutant, suggesting that PII is
required to activate NifA by preventing the inhibitory effect of its
N-terminal domain under conditions of nitrogen fixation. Low levels of
nitrogenase activity in the presence of ammonia were also observed when the
truncated gene was introduced into a strain devoid of the ADP- ribosylation
control of nitrogenase. We propose a model for the regulation of NifA
activity in A. brasilense.
Copyright © 1996, American Society for Microbiology
Modulation of NifA activity by PII in Azospirillum brasilense: evidence for a regulatory role of the NifA N-terminal domain
Centre National de la Recherche Scientifique Unite Recherche Associee 1300. Departement des Biotechnologies, Institut Pasteur, Paris, France. arsene@pasteur.fr
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