Abstract
Several Rhizobium genes (designated nod genes) are involved in early steps in nodule formation. Here we present the results of DNA sequence and functional analysis of two nodD genes from the symbiotic plasmid of USDA 191, a fast-growing strain that forms nitrogen-fixing nodules on soybeans. Both genes encoded full-length nodD-related polypeptides, which were 69% homologous to each other. One of these genes, nodD1, complemented a Rhizobium trifolii nodD::Tn5 mutant for clover nodulation; the other gene, nodD2, did not. The nodD1 coding region was preceded by a conserved DNA sequence previously noted in other rhizobia, but no such sequence was found in front of nodD2. Plants inoculated with a nodD1 insertion mutant appeared to be nitrogen starved and had a greatly reduced nodule number. Plants inoculated with a nodD2 mutant had a partially nitrogen-starved appearance and normal nodule number, were slightly delayed in nodule formation, and formed nodules that contained reduced levels of nodulin-35 and had fewer bacteroids per infected plant cell. Thus, both of these genes are involved in symbiosis. USDA 191 carrying extra copies of nodD2 on a plasmid vector had an altered colony morphology that suggested inhibition of exopolysaccharide synthesis. The predicted gene products of nodD1 and nodD2 both showed homology to LysR, an E. coli regulatory protein. We conclude that nodD1 probably has the same function as nodD in temperate rhizobia, namely, activation of nodABC transcription in the presence of plant signals. nodD2 may be involved in regulation of exopolysaccharide synthetic genes.