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Nitrogen fixation (nif) genes of Rhizobium leguminosarum were transferred to nif Klebsiella pneumoniae and E. coil by conjugation after partial heat induction of RP4 :: Mu cts in Rhizobium R+ transconjugant, and the hybrid plasmids in the transconjugant strains were isolated and characterized. In order to transfer the nif genes from Rhizobium, the hybrid plasmid RP4 :: Mu cts was transferred by conjugation from E. coil to the symbiotic nitrogen fixer, R. leguminosarum. After stabillity test, the RP4 :: Mu cts in Rhizobium R* transconjugant was subjected to partial heat induction by culturing it statically at 3810 for 16 hours, and then conjugated with the nif defective mutant strains of K. pneumoniae or nif mutant strains of E. coli having whole nif gene plasmid. Recombinant strains of K. pneumoniae, which could grow in a N-free medium and exhibit the nitrogenase activity were selected. However, in the case of E. coli, they could grow well in a NA medium containing antibiotices, but hardly grow in a N-free medium. The hybrid plasmids in these transcon- fugal strains were isolated by gel electrophoresis and compared their molecular sizes
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