MODULATION OF NIFA ACTIVITY BY P-II IN AZOSPIRILLUM-BRASILENSE - EVIDENCE FOR A REGULATORY ROLE OF THE NIFA N-TERMINAL DOMAIN

Azospirillum brasilense NifA, which is synthesized under all physiolog ical conditions, exists in an active or inactive form depending on the availability of ammonia. The activity also depends on the presence of P-II, as NifA is inactive in a glnB mutant. To investigate further th e meechanism that regulates NifA activity, several deletions of the ni fA coding sequence covering the amino-terminal domain of NifA were con structed. 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 pla ys an inhibitory role which prevents NifA activity in the presence of ammonia. The truncated proteins were also aisle to restore nif gene ex pression to a glnB mutant, suggesting that P-II is required to activat e NifA by preventing the inhibitory effect of its N-terminal domain un der conditions of nitrogen fixation. Low levels of nitrogenase activit y in the presence of ammonia were also observed when the truncated gen e 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.