IS 1631 Occurrence in Bradyrhizobium japonicum Highly Reiterated Sequence-Possessing Strains with High Copy Numbers of Repeated Sequences RSα and RSβ

Abstract

ABSTRACT From Bradyrhizobium japonicum highly reiterated sequence-possessing (HRS) strains indigenous to Niigata and Tokachi in Japan with high copy numbers of the repeated sequences RSα and RSβ (K. Minamisawa, T. Isawa, Y. Nakatsuka, and N. Ichikawa, Appl. Environ. Microbiol. 64:1845–1851, 1998), several insertion sequence (IS)-like elements were isolated by using the formation of DNA duplexes by denaturation and renaturation of total DNA, followed by treatment with S1 nuclease. Most of these sequences showed structural features of bacterial IS elements, terminal inverted repeats, and homology with known IS elements and transposase genes. HRS and non-HRS strains of B. japonicum differed markedly in the profiles obtained after hybridization with all the elements tested. In particular, HRS strains of B. japonicum contained many copies of IS 1631 , whereas non-HRS strains completely lacked this element. This association remained true even when many field isolates of B. japonicum were examined. Consequently, IS 1631 occurrence was well correlated with B. japonicum HRS strains possessing high copy numbers of the repeated sequence RSα or RSβ. DNA sequence analysis indicated that IS 1631 is 2,712 bp long. In addition, IS 1631 belongs to the IS 21 family, as evidenced by its two open reading frames, which encode putative proteins homologous to IstA and IstB of IS 21 , and its terminal inverted repeat sequences with multiple short repeats.