Two strains of the soybean endosymbiont Bradyrhizobium japonicum, USDA 110 and 61 A101 C, were mutagenized with transposon Tn 5. After plant infection tests of a total of 6,926 kanamycin and streptomycin resistant transconjugants, 25 mutants were identified that are defective in nodule formation (Nod-) or nitrogen fixation (Fix-). Seven Nod- mutants were isolated from strain USDA 110 and from strain 61 A101 C, 4 Nod- mutants and 14 Fix- mutants were identified. Subsequent auxotrophic tests on these symbiotically defective mutants identified 4 His- Nod- mutants of USDA 110. Genomic Southern analysis of the 25 mutants revealed that each of them carried a single copy of Tn 5 integrated in the genome. Three 61 A101 C Fix- mutants were found to have vector DNA co-integrated along with Tn 5 in the genome. Two independent DNA regions flanking Tn 5 were cloned from the three nonauxotrophic Nod- mutants and one His-Nod- mutant of USDA 110. Homogenotization of the cloned fragments into wild-type strain USDA 110 and subsequent nodulation assay of the resulting homogenotes confirmed that the Tn 5 insertion was responsible for the Nod- phenotype. Partial EcoR1 restriction enzyme maps around the Tn 5 insertion sites were generated. Hybridization of these cloned regions to the previously cloned nod regions of R. meliloti and nif and nod regions of B. japonicum USDA 110 showed no homology, suggesting that these regions represent new symbiotic clusters of B. japonicum.
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