Identification and characterization of the nolYZ genes of Bradyrhizobium japonicum.

Characterization of an isoflavone-inducible locus closely linked to the common nod genes of Bradyrhizobium japonicum USDA110 led to the discovery of two open reading frames, designated nolY and nolZ. These open reading frames are preceded by a sequence with strong similarity to a consensus NodD-binding site (nod box). Studies utilizing a nolZ'-'lacZ fusion indicated that inducible expression is dependent upon both NodD1 and NodW, transcriptional regulators that are required for the expression of the common nodulation genes (e.g., nodYABC) of B. japonicum. A deletion mutation within nolY produced only slight defects in nodulation of soybeans, siratro, and cowpeas, but stronger defects were observed in nodulation of mung beans. An insertion mutation within nolZ showed no nodulation defects in the host plants tested. Competition assays for nodule occupancy in soybeans did not show any decrease in the competitiveness of a nolY mutant, nor did a nolY mutant show any detectable alteration in the production of lipooligosaccharide nodulation signals.