Fourteen randomly selected clones from cosmid libraries of Bradyrhizobium were used as hybridization probes in Southern blot analysis. Seven of the probes used were from strain USDA 83, a group II strain, and the other seven were from strain I-110, a group Ia strain. The 30 strains examined included 9 strains of Rj4-incompatible soybean bradyrhizobia. Considerable polynucleotide sequence dissimilarity between DNA homology groups was evidenced by striking differences in the number of hybridizing bands, except where the probe carried repetitive DNA. Predictable, simple restriction fragment length polymorphism differences were observed only within DNA homology groups. The previous description that 8 of 9 Rj4-incompatible strains belonged to DNA homology group II was verified. The new data, together with many previously documented differences, make it clear that the DNA homology group II organisms should be classified as a new species, for which the name Bradyrhizobium elkanii is proposed, and strain USDA 76 is designated the type strain. The ATCC number for the type strain is 49852. Key words: DNA:DNA hybridization, soybean, nodulation, bacteria, symbiosis, nitrogen fixation, host compatibility.

This is the first report identifying bacteriophages and documenting megaplasmids of Sinorhizobiumfredii. Plasmid DNA content and bacteriophage typing of eighteen strains of S. fredii were determined. S. fredii strains fell into ten plasmid profile groups containing 1 to 6 plasmids, some evidently larger than 1000 MDa. Twenty-three S. fredii lytic phages were isolated from soil, and they lysed six different S. fredii strains. The host range and plaque morphology of these phages were studied. Susceptibility to S. fredii phages was examined for S. meliloti; Rhizobium leguminosarum bvs. viceae, trifolii and phaseoli; R. loti; Bradyrhizobium japonicum; B. elkanii and Bradyrhizobium sp. (Arachis). Several phages that originally lysed S. fredii strain USDA 206 also lysed strains of all three S. fredii serogroups described originally by Sadowsky et al. Phages that infected S. fredii strains USDA 191 and USDA 257 were highly specific and lysed only serogroup 193 strains. S. meliloti strains L5-30 and USDA 1005 were lysed by three of the phages that lysed S. fredii strain USDA 217. No other Rhizobium or Bradyrhizobium strain tested was susceptible to lysis by any of the S. fredii phages. The present investigation indicates that phage susceptibility in conjunction with plasmid profile analysis may provide a rapid method for identification and characterization of strains of S. fredii.

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