SYMBIOTIC NITROGEN-FIXATION BY A NIFA DELETION MUTANT OF RHIZOBIUM-MELILOTI - THE ROLE OF AN UNUSUAL NTRC ALLELE

In the N2-fixing alfalfa symbiont Rhizobium meliloti, the three sigma5 4 (NTRA)-dependent positively acting regulatory proteins NIFA, NTRC, a nd DCTD are required for activation of promoters involved in N2 fixati on (pnifHDKE and pfixABCX), nitrogen assimilation (pglnII), and C4-dic arboxylate transport (pdctA), respectively. Here, we describe an allel e of ntrC which results in the constitutive activation of the above NT RC-, NIFA-, and DCTD-regulated promoters. The expression and activatio n of wild-type NTRC occur in response to nitrogen availability, wherea s in cells carrying the ntrC283 allele, the NTRC283 protein appears co nstitutively active and is constitutively expressed. The ntrC283 allel e was shown to carry a single mutation resulting in the replacement of an Asp by a Tyr residue in the helix-turn-helix motif of ntrC283. Int roduction of the ntrC283 allele into a nifA deletion mutant restores t he N2-fixation ability to 70 to 80% of the wild-type level. Thus, the nifA gene is dispensable for symbiotic N2 fixation.