THE RHIZOBIUM SP. BR816 NODD(3) GENE IS REGULATED BY A TRANSCRIPTIONAL REGULATOR OF THE ARAC XYLS FAMILY/

Rhizobium sp. BR816 contains four nodD alleles of which nodD(3) is the most important transcriptional regulator for nodulation of Phaseolus vulgaris. Upstream of nodD3 an open reading frame, orf816, was identif ied. The deduced ORF816 protein shows homology with transcriptional re gulators of the AraC/XylS family. The DNA binding domain and the conse nsus motif, characteristic of the C-terminal region of the members of this family of transcriptional regulators, are present in the deduced ORF816 protein. Activation of nodA gene expression and nodulation of P . vulgaris by Rhizobium sp. NGR234nodD(1)::Omega (Nod(-)) complemented with the Rhizobium sp. BR816 nodD(3) gene were significantly increase d in the presence of orf816. This increased nodulation and nod gene in duction are mediated through positive regulation of nodD(3) expression by ORF816. Expression of orf816 itself is partially RpoN dependent. T he role of this transcriptional regulator in the complex cascade regul ation of the Rhizobium sp. nodD3 gene is discussed.