Purification and properties of the nitrogenase of Azospirillum amazonense

The nitrogenase of the free-living, microaerobic, N2-fixing bacterium Azospirillum amazonense (strain Y1) was purified by chromatography on DEAE-52 cellulose, by heat treatment, and by preparative polyacrylamide gel electrophoresis. The specific nitrogenase activities were 2,400 nmol of C2H4 formed per min per mg of protein for dinitrogenase (MoFe protein) and 1,800 nmol of C2H4 formed per min per mg of protein for dinitrogenase reductase (Fe protein). The MoFe protein was composed of a minimum of 1,852 amino acid residues, had an isoelectric point of 5.2, and contained 2 atoms of Mo, 24 atoms of Fe, and 28 atoms of acid-labile sulfide per molecule. The Fe protein had 624 amino acid residues and an isoelectric point of 4.6 and contained four atoms of Fe and six atoms of acid-labile sulfide per molecule. The purified MoFe protein showed two subunits with molecular weights of 55,000 and 50,000. The purified Fe protein revealed two polypeptides on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with apparent molecular weights of 35,000 and 31,000. The two Fe protein polypeptides were demonstrated with immunological techniques in the purified, highly active enzyme as well as in extracts. Also, Azotobacter vinelandii Fe protein showed two closely migrating polypeptides that migrated differently from the Fe protein polypeptides of Azospirillum brasilense or Rhodospirillum rubrum. The nitrogenase activity of Azospirillum amazonense Y1 was independent of Mn2+, and the addition of activating enzyme had no effect. No activating enzyme could be found in Azospirillum amazonense. Obviously, the nitrogenase system of Azospirillum amazonense Y1 is different from that of Azospirillum brasilense Sp7 and resembles the Azotobacter system.