RESUMO
Ammonium ion repressed nitrogenase in cells fixing N2 gas. Immunological tests and electrophoresis in various gels show that components I (Fe-Mo-S protein) was completely repressed by ammonium, whereas component II (Fe-S protein) apoprotein was not markedly affected. Component II from ammonium-grown cells, however, was inactive since it did not cross react with component I to reduce C2H2 to C2H4. The inactive component II apoprotein is immunologically identical to its active counterpart from cells fixing N2. Identical protein patterns were also observed in various gel-electrophoresis systems. Oxygen-inactivated component II may be reactivated with FeSo4. This salt is preferable to ferrous ammonium sulphate which inactivated component I. Immunodiffusion under aerobic conditions shows that purified component I is composed of aggregated and non-aggregated forms which are antigenically distinct. The aggregate was dissociated by treatment with sodium dodecyl sulphate (SDS) into a single antigenic species which was further resolved into two subunits on SDS disc polyacrylamide gel electrophoresis.