Structural and Functional Analysis of Two Different nodD Genes in Bradyrhizobium japonicum USDA110. Michael Göttfert. Mikrobiologisches Institut, Eidgenössische Technische Hochschule, Schmelzbergstrasse 7, CH-8092 Zürich, Switzerland. Daisy Holzhäuser, Daniel Bäni, and Hauke Hennecke. Mikrobiologisches Institut, Eidgenössische Technische Hochschule, Schmelzbergstrasse 7, CH-8092 Zürich, Switzerland.. MPMI 5:257-265. Accepted 18 February 1992. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992.

Bradyrhizobium japonicum has two closely linked homologs of the nodulation regulatory gene, nodD; these homologs are located upstream of and in divergent orientation to the nodYABCSUIJ gene cluster. We report here the nucleotide sequence and mutational analyses of both nodD copies. The predicted NodD1 and NodD2 proteins shared 62% identical amino acid residues at corresponding positions and exhibited different degrees of homology with NodD proteins of other Bradyrhizobium, Azorhizobium, and Rhizobium strains. Induction of the nodYABCSUIJ operon, as measured by expression of a translational nodC'-'lacZ fusion, required the nodD1 gene, but not nodD2. A B. japonicum mutant deleted for both nodD copies (strain Δ1267) still showed residual nodulation activity; however, nodulation of soybean was significantly delayed, and nodulation of mung bean and siratro resulted in strongly reduced nodule numbers. Fully efficient nodulation of mung bean and siratro by strain Δ1267 was restored by genetic complementation with the nodD1 gene, but not with nodD2. We conclude from these data that nodD1 is the critical gene that contributes to maximal nodulation efficiency, whereas the nodD2 gene does not play any obvious role in nodulation of the host plants tested.