Tj. Goss et al., Roles of glutamate synthase, gltBD, and gltF in nitrogen metabolism of Escherichia coli and Klebsiella aerogenes, J BACT, 183(22), 2001, pp. 6607-6619
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.