Modulation of NifA activity by PII in Azospirillum brasilense: evidence for a regulatory role of the NifA N-terminal domain.
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.
Arsene
F
F
Centre National de la Recherche Scientifique Unité Recherche Associée 1300. Département des Biotechnologies, Institut Pasteur, Paris, France. arsene@pasteur.fr
Kaminski
P A
PA
Elmerich
C
C
D000595:Amino Acid Sequence; D000641:Ammonia / Q000494:pharmacology; D015806:Azospirillum brasilense / Q000235:genetics* / Q000378:metabolism*; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D003001:Cloning, Molecular; D017931:DNA Primers; D004926:Escherichia coli; D015964:Gene Expression Regulation, Bacterial; D005809:Genes, Regulator; D005838:Genotype; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009591:Nitrogenase / Q000235:genetics; D010088:Oxidoreductases; D051922:PII Nitrogen Regulatory Proteins; D010641:Phenotype; D010957:Plasmids; D016133:Polymerase Chain Reaction; D011401:Promoter Regions, Genetic; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000378:metabolism; D017384:Sequence Deletion; D014157:Transcription Factors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism*; D001616:beta-Galactosidase / Q000096:biosynthesis
D001426:Bacterial Proteins; D017931:DNA Primers; C061338:NifA protein, Bacteria; D051922:PII Nitrogen Regulatory Proteins; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; C404143:PIID regulatory protein, Bacteria; D000641:Ammonia; D010088:Oxidoreductases; D009591:Nitrogenase; C029130:nitrogenase reductase; D001616:beta-Galactosidase