Dual control of the nodA operon of Azorhizobium caulinodans ORS571 by a nod box and a NifA-σ54-type promoter

Earlier studies have shown that the Azorhizobium caulinodans nodA promoter is controlled by a host plant-derived flavonoid signal via the transcription activator NodD. Here, we report that the transcription of the nodA operon is also under the control of NifA-RpoN. A NifA-σ54-type promoter, P2nodA, 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 bacteria under nitrogen fixation conditions and in bacteroids. Mutation of P2nodA reduced, under certain conditions, the efficiency of nodulation and accelerated nodule senescence, suggesting that the dual control may help to optimize nodule initiation and function in the natural context of the symbiosis.