Interspecific plasmid and genomic DNA sequence homologies and localization of nif genes in effective and ineffective strains of Rhizobium japonicum.

Three strains of Rhizobium japonicum were examined for the presence of interspecific conserved plasmid-borne DNA sequences and the location of their nif DNA sequences. Strains 61A76 and 110, which both form effective (nitrogen fixing) nodules on soybeans show very low (24%) total DNA sequence homology with each other; strain 61A76 contains one plasmid, and strain 110 contains no identifiable plasmids. Strain 61A24 which forms ineffective nodules on soybeans shows relatively high (50%) sequence homology to strain 110 and contains two plasmids. While the total sequence homology between purified plasmid DNA isolated from strains 61A76 and 61A24 appears limited, heterologous hybridizations between plasmids and total DNAs from these strains and DNA from strain 110 suggest that several sequences which are plasmid-borne in one strain can occur in the chromosome or in very large plasmid(s) in the other strains. The plasmid isolated from strain 61A24 exhibits some homology with several SmaI fragments of an octopine Ti plasmid from Agrobacterium tumifaciens, whereas the 61A76 plasmid shows no homology to this Ti plasmid. DNA sequences from five strains of Rhizobium japonicum were found to hybridize with pSA30, a plasmid carrying the Klebsiella pneumoniae nif-KDH genes, but these sequences were not located on the isolated plasmids. The organization of these genes appears to be similar in strains 110 and 61A24, but is different in strain 61A76.