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

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, R 144drd3, 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 × 106 daltons. A third plasmid in this hybrid had a molecular weight of ii8 × 106 daltons. The Nif plasmid in C-m9 was transferred efficiently to other Escherichia coli strains only when R 1drd19 was present in the donor in addition to R 144drd3. 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 øX 174 suggested that they inherited Klebsiella his-linked rfb genes. Four plasmids were isolated from hybrid c-14, one of which was R 144drd3. Two plasmids, one carrying his and nif and another met G, were mobilized by R 144drd3. The molecular weights of the c-l4 plasmids, excluding R 144drd3, were (105 ± 1) × 106, 46 × 106 and 12 × 106 daltons.