The NodD proteins of Rhizobium sp. strain BR816 differ in their interactions with coinducers and in their activities for nodulation of different host plants.

The early steps of symbiotic nodule formation by Rhizobium spp. on plants require coordinate expression of several nod gene operons, which is accomplished by the activating protein NodD. Rhizobium sp. strain BR816, isolated from Leucaena leucocephala, contains four nodD genes which differ in their interaction with flavonoids. Two of the four NodD proteins, namely, NodD1 and NodD2, obey the LysR rule regarding the need of a coinducer. NodD3 shows hardly any inducing activity, and NodD4 contains a high basal activity and no response to any of the flavonoids tested. Complementation experiments with the NGR234 nodD mutant by the different nodD genes of BR816, as well as the analysis of the nodulation phenotype of different nodD mutants of BR816, revealed that all the nodD genes of BR816 are functional, but differences can be noticed when different host plants are tested. Whereas the nodD2 and nodD4 genes of BR816 have a great impact on the nodulation of L. leucocephala, nodD3 and nodD4 appear to be important for the nodulation of Phaseolus vulgaris. It appears that NodD1 of BR816 can function as a transcriptional activator in bean nodulation but not in nodulation of L. leucocephala.