Rhizobium sp. strain ORS571 ammonium assimilation and nitrogen fixation

Abstract

Among rhizobia studied, only Rhizobium sp. strain ORS571 grew unambiguously on N2 as sole N source. In ORS571, only the glutamine synthetase (GS)-glutamate synthase (GOGAT) pathway assimilated NH4+. ORS571 exhibited 2 unique physiological aspects of this pathway: ORS571 had only GS I, whereas all other Rhizobiaceae studied had GS I and GS II, and NADPH- and NADH-dependent GOGAT activities were present. ORS571 GS-affected and NADPH-GOGAT-affected mutant strains were defective in NH4+ assimilation (Asm-) and N2 fixation (Nif-) in culture and in plants [Sesbania rostrata]; NADH-GOGAT mutants were Asm- but Nif+. Bacteroid GS activity was essentially nil, suggesting symbiotic NH4+ export. Physiological studies on effects of glutamine, NH4+, methionine sulfoximine and diazo-oxo-norleucine on nitrogenase induction in culture implied a regulatory role for the intracellular glutamine pool.