CONSERVED MOTIFS IN A DIVERGENT NOD BOX OF AZORHIZOBIUM-CAULINODANS ORS571 REVEAL A COMMON STRUCTURE IN PROMOTERS REGULATED BY LYSR-TYPE PROTEINS

Nodulation of leguminous plants by Rhizobium, Bradyrhizobium, and Azorhizobium spp. is dependent on the induction by the plant host of different bacterial nodulation (nod) loci. The transcription of these nod loci is activated in the presence of plant-produced flavonoids upon binding of the NodD protein-a LysR-type activator-to specific sequences present in the nod promoters. Originally, a 47-base-pair (bp) region called the nod box was shown to be the target sequence for binding of NodD. From the comparison of the nod box sequences of (brady)rhizobia with a more divergent nod box from Azorhizobium caulinodans, we now propose a modular build-up of the nod box with the sequence A-T-C-N9-G-A-T as the binding target of the NodD protein (the NodD box). More generally, we show that LysR-type-regulated promoters contain the characteristic sequence T-N11-A as the core of an inverted repeat and propose this to be the "LysR motif" involved in specific binding to LysR-type proteins. Data obtained upon site-specific mutagenesis of this motif in the NodD box sustains this proposal. Further, we provide strong arguments that the inducer flavonoid, involved in transcriptional activation of Azorhizobium nod genes, interacts directly with the NodD protein, thereby increasing its binding affinities for the NodD box.