TRANSCRIPTION OF THE AZOSPIRILLUM-BRASILENSE NIFH GENE IS POSITIVELY REGULATED BY NIFA AND NTRA AND IS NEGATIVELY CONTROLLED BY THE CELLULAR NITROGEN STATUS

The expression of a translational Azospirillum brasilense nifH-uidA fu sion was studied in A. brasilense and in Rhizobium meliloti strains wi th mutations in nifA, ntrA and ntrC. Induction of the fusion was obser ved in the R. meliloti wild-type and NtrC- strains on incubation under microaerobic conditions but not in the NifA- and NtrA- strains, showi ng the absolute requirement of both sigma-54 and NifA for activation o f the nifH promoter. Histochemical analysis of the root nodules elicit ed by R. meliloti wild-type showed expression of the fusion in the lat e symbiotic zone but not in the meristematic and the early symbiotic z ones. No induction of the nifH-uidA fusion was observed in the R. meli loti wild-type or NifA- strains incubated aerobically in nitrogen-free medium, indicating that, in contrast to R. meliloti nifH, A. brasilen se nifH cannot be activated directly by NtrC. Expression of the nifH g ene in A. brasilense only occurs under nitrogen-limiting, microaerobic conditions, suggesting the presence of a nitrogen-dependent control s ystem for nif gene expression.