Mapping of a Bradyrhizobium japonicum DNA Region Carrying Genes for Symbiosis and an Asymmetric Accumulation of Reiterated Sequences.

Spontaneous kanamycin-sensitive derivatives were obtained from Bradyrhizobium japonicum (strain 110) carrying Tn5 insertions in symbiotic gene cluster I; the derivatives were shown to have deletions of cluster I plus flanking DNA which was indicated by the absence of different copies of the repeated sequences RSalpha and RSbeta. The deletion endpoints were mapped using cloned wild-type DNA fragments containing RSalpha copies which also served as origins for overlapped cosmid cloning. The majority of the deletions resulted from recombinational fusion of two remote RSalpha copies. Novel types of repeated sequences (RSgamma, RSdelta, and RSepsilon) occurring in 12, 10, and 4 copies per genome were detected. Seven, nine, and three copies of RSgamma, RSdelta, and RSepsilon, respectively, were located near cluster I. It is concluded that the B. japonicum genome has an unusual DNA segment of >230 kilobase pairs characterized by the presence of repeated sequences and genes for symbiotic N(2) fixation.