Diverse Flavonoids Stimulate NodD1 Binding tonodGene Promoters inSinorhizobium meliloti

NodD1 is a member of the NodD family of LysR-type transcriptional regulators that mediates the expression of nodulation (nod ) genes in the soil bacteriumSinorhizobium meliloti . Each species of rhizobia establishes a symbiosis with a limited set of leguminous plants. This host specificity results in part from a NodD-dependent upregulation ofnod genes in response to a cocktail of flavonoids in the host plant's root exudates. To demonstrate that NodD is a key determinant of host specificity, we expressednodD genes from different species of rhizobia in a strain ofS. meliloti lacking endogenous NodD activity. We observed thatnod gene expression was initiated in response to distinct sets of flavonoid inducers depending on the source of NodD. To better understand the effects of flavonoids on NodD, we assayed the DNA binding activity ofS. meliloti NodD1 treated with the flavonoid inducer luteolin. In the presence of luteolin, NodD1 exhibited increased binding tonod gene promoters compared to binding in the absence of luteolin. Surprisingly, although they do not stimulatenod gene expression inS. meliloti , the flavonoids naringenin, eriodictyol, and daidzein also stimulated an increase in the DNA binding affinity of NodD1 tonod gene promoters. In vivo competition assays demonstrate that noninducing flavonoids act as competitive inhibitors of luteolin, suggesting that both inducing and noninducing flavonoids are able to directly bind to NodD1 and mediate conformational changes atnod gene promoters but that only luteolin is capable of promoting the downstream changes necessary fornod gene induction.