The isolation and characterization of a Rhizobium etli glutamate auxotroph,
TAD12, harbouring a single Tn5 insertion, is reported. This mutant produce
d no detectable glutamate synthase (GOGAT) activity. The cloning and physic
al characterization of a 7.2 kb fragment of R. etli DNA harbouring the stru
ctural genes gltB and gltD encoding the two GOGAT subunits GltB and GltD is
also reported. In comparison with the wild-type strain (CFN42), the GOGAT
mutant strain utilized less succinate and glutamate and grew less with this
and other amino acids as nitrogen source. R, etli assimilates ammonium by
the glutamine synthetase (GS)-GOGAT pathway and a GOGAT mutant prevents the
cycling of glutamine by this pathway, something that impairs nitrogen and
carbon metabolism and explains the decrease in the amino-nitrogen during ex
ponential growth, with glutamate as nitrogen source. GOGAT activity also ha
s a role in ammonium turnover and in the synthesis of amino acids and prote
ins, processes that are necessary to sustain cell viability in non-growing
conditions. The assimilation of ammonium is important during symbiosis and
glutamate constitutes 20-40% of the total amino-nitrogen. In symbiosis, the
blockage of ammonium assimilation by a GOGAT mutation significantly decrea
ses the amino-nitrogen pool of the bacteroids and may explain why more N-2
is fixed in ammonium, excreted to the plant cell, transported to the leaves
and stored in the seeds.