We previously reported that extra-slow-growing Bradyrhizobium japonicum isolates obtained from three field sites in Japan, designated as HRS (highly reiterated sequence-possessing) strains, have high copy numbers of the insertion sequences RSα and RSβ. When strain collections in the USA, Japan, Korea, Thailand and China were examined by Southern hybridization using RSα, RSβ and IS1631 as probes, HRS strains were found in the Japanese, Chinese, and American collections, but not in the Korean and Thai ones. Copy number analyses of RSα and RSβ, calibrated with the copy number of rrs (16S rRNA gene), indicated that the HRS stains can be divided into two major groups. Group A is comprised of members with a high copy number of RSα (mean±S.D., 139±27), a low number of RSβ (mean±S.D., 30±13) sequences, and extremely slow growth rates (mean doubling time±S.D., 27±9 h). In contrast, group B is comprised of strains with a high copy number of RSβ (mean±S.D., 93±6) and a lower number of RSα. These groupings of HRS strains were well correlated with phylogenetic clusters based on rrs, gyrB and serogroups (110/122 and 123/135). Growth rate of B. japonicum strains was also correlated exclusively with RSα copy number. The ecological, evolutionary and biotechnological implications of the findings are discussed.
Bibliographical noteFunding Information:
We are grateful to M. Abe (Kagoshima University, Japan), U. Kang (National Yeongnam Agricultural Experimental Station, Korea) for kindly providing Bradyrhizobium strains, and to Dr. S. Harayama (Marine Biotechnology Institute, Japan) for his continuing interest and encouragement. We thank PROBRAIN (Japan) for supporting the research of K. Minamisawa. This work was supported in part to K.M. by grants from the Ministry of Education, Science, Sports and Culture of Japan (no. 10460028) and from the Joint Research Program of the Institute of Genetic Ecology, Tohoku University (no. 981002).
- Bradyrhizobium japonicum
- Insertion sequence
- Slow growth