Dual control of the nodA operon of Azorhizobium caulinodans ORS571 by a nod box and a NifA-sigma(54)-type promoter

Earlier studies have shown that the Azorhizobium caulinodans nodA promoter is controlled by a host plant-derived flavonoid signal via the transcriptio n activator NodD. Here, we report that the transcription of the nodA operon is also under the control of NifA-RpoN. A NifA-sigma (54)-type promoter, P 2nodA, is present upstream of the nod-box consensus motif of the nodA gene and directs expression of a nodA-uidA reporter gene both in free-living bac teria under nitrogen fixation conditions and in bacteroids. Mutation of P2n odA reduced, under certain conditions, the efficiency of nodulation and acc elerated nodule senescence, suggesting that the dual control may help to op timize nodule initiation and function in the natural context of the symbios is.