The expression of nifA in Azorhizobium caulinodans requires a gene product homologous to Escherichia coli HF-I, an RNA-binding protein involved in the replication of phage Q beta RNA.

We report the characterization of a mutant of Azorhizobium caulinodans, isolated after ethyl methanesulfonate mutagenesis. This Nod+ Nif- Fix- mutant is unable to synthesize 10 of 15 polypeptides normally induced under conditions of nitrogen fixation. By using lacZ fusions it was shown that nifA and nifA-regulated genes were not expressed in this strain. The mutation was complemented by a constitutively expressed nifA gene or by a 1.1-kb DNA fragment from the wild-type strain, whose nucleotide sequence revealed a single open reading frame of 255 bp coding for an 85-amino acid polypeptide. The deduced amino acid sequence is similar to that of HF-I, an RNA-binding protein of Escherichia coli, which is required for replication of bacteriophage Q beta RNA. The similarity can be extended to the function since hfq, the structural gene for HF-I, complemented the A. caulinodans mutant. The corresponding gene in A. caulinodans was termed nrfA (for nif regulatory factor). Inactivation of nrfA in the mutant was due to a missense mutation resulting in the replacement of a cysteine residue by arginine. A null mutant, constructed by disruption of nrfA, exhibited the same phenotype as the missense mutant. Thus, an additional factor can be added to the already complex system of nifA regulation in A. caulinodans.