Roles of glutamate synthase, gltBD, and gltF in nitrogen metabolism of Escherichia coli and Klebsiella aerogenes

Mutants of Escherichia coli and Klebsiella aerogenes that are deficient in glutamate synthase (glutamate-oxoglutarate amidotransferase [GOGAT]) activi ty have difficulty growing with nitrogen sources other than ammonia. Two mo dels have been proposed to account for this inability to grow. One model po stulated an imbalance between glutamine synthesis and glutamine degradation that led to a repression of the Ntr system and the subsequent failure to a ctivate transcription of genes required for the use of alternative nitrogen sources. The other model postulated that mutations in gltB or gltD (which encode the subunits of GOGAT) were polar on a downstream gene, gltF, which is necessary for proper activation of gene expression by the Ntr system. Th e data reported here show that the gltF model is incorrect for three reason s: first, a nonpolar gltB and a polar gltD mutation of K. aerogenes both sh ow the same phenotype; second, K. aerogenes and several other enteric bacte ria lack a gene homologous to gltF; and third, mutants of E. coli whose glt F gene has been deleted show no defect in nitrogen metabolism. The argument that accumulated glutamine represses the Ntr system in gltB or gltD mutant s is also incorrect, because these mutants can derepress the Ntr system nor mally so long as sufficient glutamate is supplied. Thus, we conclude that g ltB or gltD mutants grow slowly on many poor nitrogen sources because they are starved for glutamate. Much of the glutamate formed by catabolism of al ternative nitrogen sources is converted to glutamine, which cannot be effic iently converted to glutamate in the absence of GOGAT activity. Finally, GO GAT-deficient E. coli cells growing with glutamine as the sole nitrogen sou rce increase their synthesis of the other glutamate-forming enzyme, glutama te dehydrogenase, severalfold, but this is still insufficient to allow rapi d growth under these conditions.