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SUMMARY: Segments of the Klebsiella pneumoniae chromosome, which included the nitrogen fixation (nif) genes, were maintained as covalently closed circular molecules of DNA in Escherichia coli hybrids, C-M9 and C-L4. The R factor, R144drd3, which had been used to confer fertility on the donor, was also detected as covalently closed circular DNA of molecular weight (65·4) × 106 daltons according to sedimentation studies and electron microscopy.The nif genes in hybrid C-M9 were linked to his on a plasmid which had a molecular weight of 9.5 X 106 daltons. A third plasmid in this hybrid had a molecular weight of 118 X 106 daltons. The Nif plasmid in C-M9 was transferred efficiently to other Escherichia coli strains only when R1drd19 was present in the donor in addition to R144drd3.Presence of three electrophoretically distinguishable gluconate-6-phosphate dehydrogenases was evidence for duplication of gnd determinants in C-M9. Resistance of both hybrids to the rough-specific phage × 174 suggested that they inherited Klebsiella his-linked rfb genes.Four plasmids were isolated from hybrid C-L4, one of which was R144drd3. Two plasmids, one carrying his and nif and another met G, were mobilized by R144drd3. The molecular weights of the C-L4 plasmids, excluding R144drd3, were (105 ± 1) x 106, 46 X 106 and 12 X 106 daltons.

Plasmids Formed in Nitrogen-fixing Escherichia coli-Klebsiella pneumoniae Hybrids