Novel regulatory mechanisms for the nodulation genes of Bradyrhizobium japonicum

Genetic studies were carried out to ascertain the functions and mechanisms of regulation for a new flavonoid induced locus in Bradyrhizobium japonicum USDA110. These studies revealed that this locus is comprised of two open reading frames that have been designated no1Y and no1Z. These genes have been found to be regulated by NodD1 and NodW, a manner similar to the regulation of the nodD1 and nodYABC genes of USDA110. Mutational analysis of no1Y and no1Z showed that these genes were not essential for modulation of host plants. Studies were undertaken to analyze nod gene expression in USDA110. The use of different nod gene regulatory mutants for these studies produced several novel results. The nodD2 gene product might be important for regulation of nodYABC, nodD1, and no1YZ. Expression of nodYABC and nodD1, but not no1YZ, in a strain deleted of nodD1, nodD2, no1A, and two open reading frames downstream of no1A, was found to be similar to that of wild-type strains upon isoflavone induction. This result suggested that some type of repression unique to nodYABC and nodD1 had been relieved. The product of the genotype-specific modulation gene no1A is proposed to be a represser of nodD1 and nodYABC,/i>. The phenotype of a nodD1 nodD2 no1A deletion (i.e. poor modulation, wild-type levels of common nod gene expression) suggests that other unknown genes are necessary for efficient modulation of host plants.