Regulation of Rhizobium nitrogen fixation by the unadenylylated glutamine synthetase I system.

Rhizobium sp. 32H1 glutamine auxotrophs have a complex phenotype: a highly adenylylated glutamine synthetase [GS; L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] I and an undetectable GS II (GS II(-)). They are defective in the adenylylation cascade system for GS I. Prototrophic revertants are of two classes: those (3204 type) which retain the adenylylated GS I phenotype but become GS II(+), and those (3205 type) which acquire a constitutive unadenylylated GS I but remain GS II(-). Like the parent auxotroph, 3204 remains incapable of nitrogen fixation both in culture and in root nodules of Macroptileum atropurpureum. In contrast, 3205 is nitrogenase constitutive. This implies that GS I or associated adenylylation proteins are involved in the control of Rhizobium 32H1 nif gene expression and that GS II is not so involved. Normally, rhizobia fix atmospheric N(2) only during symbiosis and, in so doing, only transiently synthesize nitrogenase. Moreover, whereas wild-type strains export ammonium, constitutive strains can assimilate ammonium produced by nitrogenase. This phenotype allows direct selection of nitrogen fixation-defective mutants in Rhizobium.