THE CONTROL OF AZORHIZOBIUM-CAULINODANS NIFA EXPRESSION BY OXYGEN, AMMONIA AND BY THE HF-I-LIKE PROTEIN, NRFA

The control of Azorhizobium caulinodans nifA expression in response to oxygen and ammonia involves FixLJ, FixK, NtrBC, NtrXY and the HF-I-li ke protein NrfA. The regulation is thus complex and possibly involves post-transcriptional regulation by NrfA. The coding region of nifA was determined using a translational lacZ fusion and by site-directed mut agenesis to identify which of four in frame AUG codons was used. The m ajor NifA protein is translated from the second AUG codon and is predi cted to consist of 613 amino acids. Primer extension analysis showed a major transcript starting 34 bp downstream from the anaerobox in wild -type, nifA, rpoN, ntrC and nrfA strains, but not in a fixK mutant, Fi xK- and oxygen-dependent transcription of nifA was confirmed by the an alysis of four transcriptional nifA-lacZ fusions with fusion junctions at positions +1, +47, +110 and +181 with respect to the start site. R egulation by ammonia was independent of FixK and RpoN, NtrC being only partially required, Thus, there may be another type of nitrogen contr ol that does not involve NtrC in A, caulinodans, NrfA is not required for the initiation of nifA transcription but, most probably, has an ef fect on nifA mRNA stability and/or translation. NrfA also restores the defect in rpoS translation to an Escherichia coli hfq mutant, indicat ing that HF-I and NrfA have similar activities in both A. caulinodans and E. coli.