AMMONIUM REPRESSION OF THE NITRITE-NITRATE (NASAB) ASSIMILATORY OPERON OF AZOTOBACTER-VINELANDII IS ENHANCED IN MUTANTS EXPRESSING THE NIFOGENE AT HIGH-LEVELS

A number of Tn5 mutants were isolated which were unable to fix nitroge n and showed enhanced ammonium repression of the nitrate/nitrite assim ilation genes. They also had reduced nitrate reductase activity under fully inducing conditions. Insertions were localized within the nifB g ene, and inability to fix nitrogen was shown to be due to disruption o f the nifB gene. However, enhanced ammonium repression proved to be th e result of constitutive expression of the downstream nifO gene from a n 'out' promoter present in Tn5. Our results suggest that molydenum me tabolism might function as a regulatory factor that acts through the n itrate reductase.