Conservation of a symbiotic DNA region in soybean root nodule bacteria.

Bradyrhizobium japonicum USDA 3I1b110 contains a DNA region in which symbiotic genes and many repeated sequences are closely linked. Hybridization analysis revealed that this region was highly conserved in some B. japonicum strains (USDA 24, USDA 122, USDA 123, ATCC 10324, 61A24) but not in others (USDA 76, 61A76, 61A101). The genomic presence of multiple copies of one of the repeated sequences (RSalpha) appeared to be specifically characteristic for soybean root nodule bacteria, including the fast-growing Rhizobium fredii, which carries most of these RSalpha copies on the symbiotic plasmid.