Transformation of Azotobacter vinelandii strains unable to fix nitrogen with Rhizobium spp. DNA.

The phenotypes of Azotobacter vinelandii ATCC 12837 strains defective in nitrogen fixation (Nif-) were characterized by intrageneric transformation with known Nif- strains of A. vinelandii OP. These former mutant strains were used as recipients for intergeneric transformation by deoxyribonucleic acid (DNA) prepared from Rhizobium spp. to determine if the rhizobia would transform the Azotobacter Nif- phenotypes to Nif+. The frequency of Nif+ transformants using Rhizobium DNA was always less than the frequency using Azotobacter wild-type DNA but was greater than the spontaneous reversion frequency. The Azotobacter Nif+ recombinants also were stable. DNA from all of the Rhizobium spp. transformed to Nif+ Azotobacter mutants defective in the nitrogenase component I (molybdoferredoxin); however, some recombinants had a lower nitrogenase activity and a delayed nitrogenase depression time. Mutants defective in the pleiotrophic transcriptional control of both nitrogenase components were transformed to Nif+ by the asymbiotic nitrogen fixing Rhizobium sp. 32H1 and 41A1, but not the symbiotic nitrogen-fixing species. The significance of these results and the possible future applications of this system are discussed.