Correlation of Denitrifying Capability with the Existence of nap, nir, nor and nos Genes in Diverse Strains of Soybean Bradyrhizobia

To clarify the genetic basis to the diversity of denitrifying ability in soybean bradyrhizobia, we compared the end-products of denitrification (N₂, N₂O and NO₂^-) with the existence of denitrifying genes (napA, nirK, norCB and nosZ) of sixty phylogenetically diverse strains of Bradyrhizobium japonicum and B. elkanii. The results indicate that the existence of denitrifying genes directly determines phenotype (end-products) in most strains of B. japonicum. The denitrifying capability and gene set were reflected by phylogenetic position based on repeated sequences (RS)-fingerprints and 16S rRNA gene sequences. However, the denitrifying genes in HRS (highly repeated sequence-possessing) strains of B. japonicum, which were identified based on RS-fingerprints as having heavy hybridization, resulted in an inconsistent correlation probably because of genomic rearrangements. The evolutionary and ecological implications of the denitrifying genes and capability in soybean bradyrhizobia are discussed.