IS1631 occurrence in Bradyrhizobium japonicum highly reiterated sequence-possessing strains with high copy numbers of repeated sequences RS alpha andRS beta

From Bradyrhizobium japonicum highly reiterated sequence-possessing (HRS) s trains indigenous to Niigata and Tokachi in Japan with high copy numbers of the repeated sequences RS alpha and RS beta (K, Minamisawa, T. Isawa, Y. N akatsuka, and N. Ichikawa, Appl. Environ. Microbiol. 64:1845-1851, 1998), s everal insertion sequence (IS)-like elements were isolated by using the for mation of DNA duplexes by denaturation and renaturation of total DNA, follo wed by treatment with S1 nuclease. Most of these sequences showed structura l features of bacterial IS elements, terminal inverted repeats, and homolog y with known IS elements and transposase genes. HRS and non-MRS strains of B. japonicum differed markedly in the profiles obtained after hybridization with all the elements tested. In particular, MRS strains of B. japonicum c ontained many copies of IS1631, whereas non-MRS strains completely lacked t his element. This association remained true even when many field isolates o f B. japonicum were examined. Consequently, IS1631 occurrence was well corr elated with B. japonicum HRS strains possessing high copy numbers of the re peated sequence RS(Y or RSP. DNA sequence analysis indicated that IS1631 is 2,712 bp long. In addition, IS1631 belongs to the IS21 family, as evidence d by its two open reading frames, which encode putative proteins homologous to IstA and IstB of IS21, and its terminal inverted repeat sequences with multiple short repeats.