THE ENHANCEMENT OF AMMONIUM ASSIMILATION IN RHIZOBIUM-ETLI PREVENTS NODULATION OF PHASEOLUS-VULGARIS

The modification of the ammonium assimilation pathway of Rhizobium etl i (GS-GOGAT) by adding an additional ammonium assimilation enzyme, GDH , strongly affects its symbiotic interaction with beans, The plasmid p AM1a, based in the stable vector pTR101 (M. Weinstein, R. C. Roberts, and D. R. Helsinki, J. Bacteriol. 174,7486-7489, 1992), containing the Escherichia coli gdhA gene flanked by two transcription-translation t erminators was constructed, The expression of GDH in both, the wild ty pe (CFN42/pAM1a) and a ntrC(-) mutant (CFN2012/pAM1a) R. etli strains, gave a similar metabolic effect, i.e., high GDH and reduced GOGAT act ivities, and an increased synthesis and excretion of several amino aci ds, The total inhibition of bean nodulation was observed when the mini mum optimal inoculum of R. etli CFN42/pAM1a was used; however, an effe ctive symbiosis occurred with the CFN2012/pAM1a mutant strain, While a total inhibition of the induction of the nodA gene by bean root exuda te or by naringenin was observed in the CFN42/pAM1a strain, at 10 mM a mmonium, the CFN2012/pAM1a showed an optimal nodA gene induction, A co rrelation between nodA gene induction, Nod factor production, and nodu lation was observed, We conclude that in R. etli, there is a down-regu lation of nod gene expression and nodulation when a high internal nitr ogen content is built up by the presence of a functional GDH and that NtrC is involved in such regulation, An instability of the plasmid har boring the gdhA gene was observed during symbiosis, indicating a stron g selection against cells containing this plasmid.